Category Archives: Chronobiology

Cicadas, or how I Am Such A Scientist, or a demonstration of good editing

Charles Q. Choi runs a bi-weekly series on the Guest Blog over at Scienctific American – Too Hard for Science? In these posts, he asks scientists about experiments that cannot be or should not be done, for a variety of reasons, though it would be fun and informative it such experiments could get done.

For one of his posts, he interviewed me. What I came up with, inspired by the emergence of periodic cicadas in my neighborhood, was a traditional circadian experiment applied to a much longer cycle of 13 or 17 years.

Fortunetaly for me, Charles is a good editor. He took my long rant and turned it into a really nice blog post. Read his elegant version here – Too Hard for Science? Bora Zivkovic–Centuries to Solve the Secrets of Cicadas.

Now compare that to the original text I sent him, posted right here:

The scientist: Bora Zivkovic, Blog Editor at Scientific American and a chronobiologist.

The idea: Everything in living organisms cycles. Some processes repeat in miliseconds, others in seconds, minutes or hours, yet others in days, months or years. Biological cycles that are most studied and best understood by science are those that repeat approximately once a day – circadian rhythms.

One of the reasons why daily rhythms are best understood is that pioneers of the field came up with a metaphor of the ‘biological clock‘ which, in turn, prompted them to adapt oscillator theory (the stuff you learned in school about the pendulum) from physics to biology.

And while the clock metaphor sometimes breaks down, it has been a surprisingly useful and powerful idea in this line of research. Circadian researchers came up with all sorts of experimental protocols to study how daily rhythms get entrained (synchronized) to the environmental cycles (usually light-dark cycles of day and night), and how organisms use their internal clocks to measure other relevant environmental parameters, especially the changes in daylength (photoperiod) – information they use to precisely measure the time of year and thus migrate, molt or mate during an appropriate season.

These kinds of experiments – for example building Phase-Response Curves to a variety of environmental cues, or a variety of tests for photoperiodism (night-break protocol, skeleton photoperiods, resonance cycles, T-cycles, Nanda-Hamner protocol etc.) – take a long time to perform.

Each data point requires several weeks: measuring period and phase of the oscillation before and after the pulse (or a series of pulses) of an environmental cue in order to see how application of that cue at a particular phase of the cycle affects the biological rhythm (or the outcome of measuring daylength, e.g., reproductive response). It requires many data points, gathered from many individual organisms.

And all along the organisms need to be kept in constant conditions: not even the slightest fluctuations in light (usually constant darkness), temperature, air pressure, etc. are allowed.

It is not surprising that these kinds of experiments, though sometimes applied to shorter cycles (e.g., miliseconds-long brain cycles), are rarely applied to biological rhythms that are longer than a day, e.g., rhythms that evolved as adaptations to tidal, lunar and annual environmental cycles. It would take longer to do than a usual, five-year period of a grant, and some experiments may last an entire researcher’s career. Which is one of the reasons we know so little about these biological rhythms.


Living out in the country, in the South, just outside Chapel Hill, NC, every day I open the door I hear the deafening and ominous-sounding noise (often described as “horror movie soundtrack) coming from the woods surrounding the neighborhood. The cicadas have emerged! The 13-year periodic cicadas, that is. Brood XIX.

I was not paying attention ahead of time, so I did not know they were slated to appear this year in my neck of the woods. One morning last week, I saw a cicada on the back porch and noticed red eyes! A rule of thumb that is easy to remember: green eyes = annual cicadas, red eyes = periodic cicadas. I got excited! I was waiting for this all my life!

Fortunately, once they emerge, cicadas are out for a few weeks, so my busy travel schedule did not prevent me from going to find them (just follow the sound) to take a few pictures and short videos.

There are three species of periodic cicadas that emerge every 17 years – Magicicada septendecim, Magicicada cassini and Magicicada septendecula. Each of these species has a ‘sister species’ that emerges every 13 years: M.tredecim, M. tredecassini and M.tredecula. A newer species split produced another 13-year species: Magicicada neotredecim. The species differ in morphology and color, while the 13 and 17-year pairs of sister species are essentially indistinguishable from each other. M.tredecim and M.neotredecim, since they appear at the same time and place, differ in the pitch of their songs: M.neotredecim sings a higher tone.

So, how do they count to 13 or 17?

While under ground, they undergo metamorphosis four times and thus go through five larval instars. The 13 and 17-year cicadas only differ in the duration of the fifth instar. They emerge simultaneously, live as adults for a few weeks, climb up the trees, sing, mate, lay eggs and die.

When the eggs hatch, the newly emerged larvae fall from the trees to the ground, dig themselves deeper down, latch onto the tree roots to feed on the sap, and wait another 13 or 17 years to emerge again.

There are a number of hypotheses (and speculations) why periodic cicadas emerge every 13 or 17 years, including some that home in on the fact that these two numbers are prime numbers (pdf).

Perhaps that is a way to fool predators which cannot evolve the same periodicity (but predators are there anyway, and will gladly gorge on these defenseless insects when they appear, whenever that is, even though it may not be so good for them). Perhaps this is a speciation mechanism, lowering the risk of hybridization between recently split sister species?

Or perhaps that is all just crude adaptationist thinking and the strangeness of the prime-number cycles is in the eye of the beholder – the humans! After all, if an insect shows up every year, it is not very exciting. Numerous species of annual cicadas do that every year and it seems to be a perfectly adaptive strategy for them. But if an insect, especially one that is so large, noisy and numerous, shows up very rarely, this is an event that will get your attention.

Perhaps our fascination with them is due to their geographic distribution. Annual cicadas may also have very long developmental times, but all of their broods are in one place, thus the insects show up every year. In periodic cicadas, different broods appear in different parts of the country, which makes their appearance rare and unusual in each geographic spot.

In any case, I am more interested in the precision of their timing than in potential adaptive explanations for it. How do they get to be so exact? Is this just a by-product of their developmental biology? Is 13 or 17 years just a simple addition of the duration of five larval stages?

Or should we consider this cycle to be an output of a “clock” (or “calendar”) of sorts? Or perhaps a result of interactions between two or more biological timepieces, similarly to photoperiodism? In which case, we should use the experimental protocols from circadian research and apply them to cicada cycles.

Finally, it is possible that a ling developmental cycle is driven by one timing mechanism, but the synchronization of emergence in the last year is driven by another, perhaps some kind of clock that may be sensitive to sound made by other insects of the same species as they start digging their way up to the surface.
The problem: In order to apply the standard experiments (like construction of a Phase-Response Curve, or T-cycles), we need to bring the cicadas into the lab. And that is really difficult to do. Husbandry has been a big problem for research on these insect, which is why almost all of it was done out in the field.

When kept in the lab, the only way to feed them is to provide them with the trees so they can drink the sap from the roots. This makes it impossible to keep them in constant conditions – trees require light and will have their own rhythms that the cicadas can potentially pick up, as timing cues, from the sap. So, the first thing we need to do is figure out a way to feed them artificially, without reliance on living trees for food.

Also, we do not know which environmental cues are relevant. Is it light cycle? Photoperiod? Or something cycling in the tree-sap? Or temperature cycles? What are the roles of developmental hormones like Juvenile Hormone or Ecdysone? We would have to test all of them simultaneously, hoping that at least one of them turns out to be the correct one.

Second, more obvious problem, is time. These experiments would last hundreds of years, perhaps thousands! Some experiments rely on outcomes of previous experiments for the proper design. Who would do them? What funding agency would finance them? Why would anyone start such experiments while knowing full well that the results would not be known within one’s lifetime? Isn’t this too tantalizing for a scientist’s curiosity?

The solution? One obvious solution is to figure out ways to get to the same answers in shorter time-frames. Perhaps by sequencing the genome and figuring out what each gene does (perhaps by looking at equivalents in other species, like fruitflies, or inserting them into Drosophila and observing their effects), hoping to find out the way timing is regulated. This will probably not answer all our questions, but may be good enough.

Another way is to set aside space and funding for such experiments and place them into an unusual administrative framework – a longitudinal study guided by an organization, not a single researcher getting a grant to do this in his or her lab. This way the work will probably get done, and the papers will get published somewhere around 2835 A.D.


See? How long and complex my text is? Now go back to the post by Charles to see again how nicely he edited the story.


A “sixth sense” for earthquake prediction? Give me a break!

This post is a slightly edited version of my December 29, 2004, post written in reaction to media reports about a “sixth sense” in animals, that supposedly allows them to avoid a tsunami by climbing to higher ground.

Every time there is a major earthquake or a tsunami, various media reports are full of phrases like sixth sense and extrasensory perception, which no self-respecting science journalist should ever use.

Sixth sense? Really? The days of Aristotle and his five senses are long gone. Even humans have more than five sensory modalities. Other animals (and even plants) have many more. The original five are vision, audition, olfaction, gustation and touch.

Photoreception is not just vision (perception of images) and is not a unitary modality. There are animals with capabilities, sometimes served by a separate organ or at least cell-type, for ultraviolet light reception, infrared perception (which is also heat perception as infrared light is warm), perception of polarized light, not to mention the non-visual and extraretinal photoreception involved in circadian entrainment, photoperiodism, phototaxis/photokinesis, pupillary reflex and control of mood. The “third eye” (frontal organ in amphibians, or parapineal in reptiles) cannot form an image but detects shadows and apparently also color.

Audition (detection of sound) in many animals also includes ultrasound (e.g., in bats, insects, dolphins and some fish) and infrasound (in whales, elephants, giraffes, rhinos, crocodiles etc., mostly large animals). And do not forget that the sense of balance and movement is also located in the inner ear and operates on similar principles of mechanoreception.

Olfaction (detection of smells) is not alone – how about perception of pheromones by the vomero-nasal organ (and processed in the secondary olfactory bulb), and what about the nervus terminalis? Some animals have very specific senses for particular chemicals, e.g., water (hygroreceptors) and CO2. Gustation is fine, but how about the separate trigeminal capsaicin-sensitive system (the one that lets you sense the hot in hot peppers)? Chemoreceptors of various kinds can be found everywhere, in every organism, including bacteria.

Touch (somatoreception) is such a vaguely defined sense. In our skin, it encompasses separate types of receptors for light touch (including itch), pressure, pain, hot and cold. The pain receptor is a chemoreceptor (sensing chemicals released from the neighboring damaged cells), while the others are different types of mechanoreceptors. Inside our bodies, different types of receptors monitor the state of the internal organs, including stretch receptors, tendon receptors etc. Deep inside our bodies, we have baroreceptors (pressure, as in blood pressure) and chemoreceptors that detect changes in blood levels of O2 or CO2 or calcium etc. Animals with exoskeletons, such as arthropods, also possess tensoriceptors that sense angles between various elements of the exoskeleton, particularly in the legs, allowing the animals to control its locomotion.

Pit-vipers, Melanophila beetles and a couple of other insects (including bed bugs) have infrared detectors. While snakes use this sense to track down prey, the insects like Melanophila beetles use it to detect distant forest fires, as they breed in the flames and deposit their eggs in the still-glowing wood, thus ensuring they are there “first.” While infra-red waves are officially “light,” it is their high energy that is used to detect it. In case of the beetles, the energy is transformed into heat. Heated receptor cells expand and get misshapen. Their shape-change moves a hair-cell, thus translating heat energy into mechanical energy, which is then translated into the electrical energy of the nerve cell.

Several aquatic animals, including sharks and eels, as well as the platypus, are capable of sensing changes in the electric field – electroreception.

More and more organisms, from bacteria, through arthropods, to fish, amphibians, birds and mammals, are found to be quite capable of sensing the direction, inclination and intensity of the Earth’s magnetic field. Study of magnetoreception has recently been a very exciting and fast-growing field of biology (pdf).

On a more philosophical note, some people have proposed that the circadian clock, among other functions, serves as a sensory receptor of the passage of time. If that is the case, this would be a unique instance of a sensory organ that does not detect any form of energy, but a completely different aspect of the physical world.

Finally, many animals, from insects to tree-frogs to elephants, are capable of detecting vibrations of the substrate (and use it to communicate with each other by shaking the branches or stamping the ground). It is probably this sense that allowed many animals to detect the incoming tsunami, although the sound of the tsunami (described by humans as hissing and crackling, or even as similar to a sound of a really big fire) may have been a clue, too.

I am assuming that birds could also see an unusually large wave coming from a distance, although they would need the warning the least, considering they could fly up at the moment’s notice. The “sixth sense” reports (in 2006) were from Indonesia and Sri Lanka – places worst hit by high waters. It would be interesting to know how the animals fared farther from the epicenter of the earthquake.

Which leads me to the well-known idea that animals can predict earthquakes. While pet-owners swear their little preciouses get antsy before earthquakes, studies to date see absolutely no evidence of this. Animals get antsy at various times for various reasons, and next day get as surprised as we are when the “Big One” hits.

When a strong earthquake hit California in the 1980s, a chronobiology laboratory looked back at the records of their mice and hamsters. Those were wheel-running activity records, continuously recorded by computers over many weeks, including the moment of the earthquake. No changes in the normal patterns of activity were detected. I believe that this finding was never published, but just relayed from advisor to student, generation after generation, and mentioned in courses as an anecdote.

On the other hand, one study – “Mouse circadian rhythm before the Kobe earthquake in 1995” – described an increase, and another study – “Behavioral change related to Wenchuan devastating earthquake in mice” – a decrease in activity of some of the mice kept in isolation in the laboratories. With one study showing increase, one showing decrease, and one anecdotal account showing no change, the jury on this phenomenon is still out.

Mice (or the monitoring equipment) could have shown these patterns for causes unrelated to earthquakes. How much each of the three laboratories was isolated from outside cues (light, sound, substrate vibration, air pressure, radiation, etc.) is also not known but could have been quite variable – it is difficult to build a laboratory that is completely isolated from every possible environmental cue (and in circadian research light and temperature are key cues to isolate from, so many others are neglected).

The key difference here, of course, is between sensing the earthquake as it is happening somewhere far away (as the animals can certainly do), or the ability to sense small “foreshocks” that often precede the strong earthquakes, and the ability to predict earthquakes before they happen (which animals cannot do). So, I don’t think there is anything mysterious about the survival of animals in the tsunamis, and the sense they use is certainly not just “sixth”…perhaps 26th or 126th (based on whatever criterion one uses for counting them) depending on the species.

My new science post on the SciAm Observations blog – History of circadian genetics research

I wanted to write about this for years. Finally a good opportunity emerged: two new circadian papers provided the “news hook” for a blog post I wanted to write providing historical, philosophical, sociological, theoretical and methodological context for the findings in circadian genetics.

I also used the new tool – Dipity – to make timelines of key events in this history. The post is long, but serves as an Explainer, a “basics” post and a source of important references, so I hope people bookmark it for future reference.

I hope you have the time and patience to read it (perhaps save on Instapaper and read on your daily train commute):

Circadian clock without DNA–History and the power of metaphor

Then let me know what you think – comment there, share the link on social networking sites, respond on your own blogs, etc….

Circadian clock without DNA–History and the power of metaphor Last week, two intriguing and excellent articles appeared in the journal Nature, demonstrating that the transcription and translation of genes, or even the presence of DNA in the cell, are not necessary for the daily (“circadian”) rhythms to occur (O’Neill & Reddy 2011, O’Neill et al., 2011). (Scientific American is part of Nature Publishing Group.)

The two papers received quite a lot of media coverage, and deservedly so, but very few science bloggers attempted to write in-depth blog posts about them, placing them in a broader historical, theoretical and methodological context. I had a feeling that everyone was waiting for me to do so. Which is why you are now reading this. I know it is a long “Explainer” (which is all the rage in science journalism these days) but I hope you have patience for it and that you find it informative and rewarding.

What I intend to do is to, first, briefly describe and explain the research in these two papers, though the press release and media coverage were quite accurate this time. Diana Gitig did the best job of it at Ars Technica – I highly recommend you read her piece for clear background information.

Then I will try to give you a historical perspective so you can get a feel for the context in which this research was performed. This look at the history will bring into sharp relief how powerful the scientific metaphors are in guiding the questions that researchers try to answer in their laboratories. Finally, a look at the media coverage will show that the lay audience (including journalists) is guided by other metaphors – not always the same ones that are used by researchers.

What did they do?

In each of the two papers, the researchers chose an unusual laboratory model for their study. What is common to both models is that a) they are both Eukaryotic cells and b) there is no DNA transcription or RNA translation going on inside the cells.

In the first study (O’Neill & Reddy 2011), they used human red blood cells (photo left) as these cells have no nucleus, thus no DNA at all.

In the second study (O’Neill et al. 2011), the model of choice was a small protist, Ostreococcus tauri which has an interesting property – when kept in constant darkness, there is no DNA transcription or RNA translation that can be detected.

The starting point of both studies was detection of peroxiredoxins in the cytoplasm. Peroxiredoxins are enzymes (thus chemically proteins) that protect the cells from damage from strongly oxidizing molecules (often refered to as “free radicals”). The process of neutralizing such oxidants temporarily changes the chemical structure of the peroxiredoxin, which then reverts to its native state again – thus the molecule is constantly switching between the two states. This oscillation between the two states follows a daily (~24h) cycle synchronized to the day-night cycle of the environment.

In both studies, peroxiredoxins were detected using antibodies (“immunoblotting”). One of the chemical states of the molecule can be detected with this method, while the other state is indirectly detected by the comparative lack of signal. Thus a circadian rhythm would be seen as an alternating series of rises and falls of the detected signal with a period close to 24 hours.

And this is exactly what they discovered in both cases – there was a clear circadian rhythm of peroxiredoxins state-switching both in cultured red blood cells (above right) and in the cultured Ostreococcus tauri (below).

Furthermore, in the protist study, they used measurement of light emitted by luciferase added to the sea-water solution as a marker of DNA transcription and translation. While the cells were kept in constant darkness no light emitted due to presence of luciferase could be detected.

But at the onset of environmental light, luciferase-induced light measurements indicated that the transcription started at the phase predicted from the state of the clock before the cells were placed in the dark. This means that the circadian rhythm of DNA transcription did not start at some “Phase Zero,” triggered by switching on the light, but that it was driven by a clock that was operating all along while the organism was kept in the dark – a clock that does not require DNA transcription and translation.

Detecting a 24-hour rhythm is not sufficient to ensure that the rhythm is actually circadian. For a biological cycle to be considered circadian, it has to satisfy a number of criteria, e.g., it has to be endogeonous (generated inside the cell, not forced onto it by the environment), it has to persist for several cycles, it has to be unaffected by temperature levels (i.e., the period of the rhythm should be the same regardless of the level of environmental temperature kept constant in the laboratory), it has to be entrainable (synchronizable) by imposed cycles in the environment, etc.

In both model systems, the researchers performed (either in these or prior studies) the entire battery of standard experimental protocols to demonstrate that yes, these are indeed circadian rhythms in both laboratory models.

Use of temperature cycles instead of light-dark cycles to demosntrate entrainment in the first experiment makes sense as human red blood cells do not experience (and cannot detect) light, while they are normally exposed to daily fluctuations of body temperature. The difference between the dawn minimum and evening maximum temperature inside the human body can be as large as 1 degree Celsius, more than sufficient for entrainment – some entire cold-blooded animals like lizards, insulated from the environment by skin and scales, can entrain their rhythms to temperature cycles with the high-low difference as small as 2 degrees Celsius and in some individuals as small as 0.1 degrees (Underwood and Calaban 1987).

In the red blood cell paper, the researchers went further. They also detected circadian rhythms in a few other biochemical processes. For example, hemoglobin, the molecule that transports oxygen from the lungs to the cells, and carbon dioxide from the cells to the lungs, can exist in two different forms inside red blood cells. It is a complex protein molecule, built of four almost-identical units. In this form, hemoglobin can perform its normal function. But there is also a two-unit form which cannot perform the function in gas exchange. Furthermore, the two-unit form produces the oxidizing small molecules – exactly the kinds of molecules that peroxiredoxins have evolved to scavenge and neutralize. It is not surprising that the switching between two-unit and four-unit forms of hemoglobin was also seen to be circadian – and in sync with the peroxiredoxin rhythm.

Likewise in the protist paper (see O.tauri in the photo at right), the researchers performed a whole suite of additional experiments designed to eliminate a variety of potential alternative hypotheses. For example, pharmacological suppression of DNA transcription and translation did not eliminate the circadian rhythm in peroxiredoxin chemistry, but instead demonstrated a complex interplay between the clock driven by transcription of genes and the clock driven by spontaneous biochemical reactions in the cytoplasm.

In summary, the two papers are very solid, the experiments are well designed and performed, the results are persuasive, and the names of authors give me confidence that the data can be trusted.

What does this all mean?

The results of both papers demonstrated that transcription of DNA and translation of RNA is not necessary for the generation of circadian rhythms in two different types of eukaryotic cells belonging to evolutionarily very distant relatives – protists and mammals.

In the case of red blood cells, the result is clear – there is no DNA or RNA in these cells. Thus, the circadian rhythms in these cells have to be generated in the cytoplasm.

In the case of O.tauri, the picture is a little bit more complex: the cells have a nucleus which has DNA. There is a clock driven by transcription and translation of canonical “clock genes.” Yet, when this mechanism is supressed – either by constant darkness or by chemicals – the cells still exhibit circadian rhythms generated by the molecules residing in the cytoplasm (and some of those molecules, at least during the first day or two, may be strands of RNA transcribed earlier).

Furthermore, the phase at which the DNA-centered clock starts its cycle is determined by the phase of the cytoplasmic clock, not the other way round, i.e., the cytoplasmic clock is dominant over the nuclear clock.

Why is this so exciting?

Depends who you ask!

When the articles were first published I did not yet have time to read them carefully. But I have e-mail notifications set up so every time Google detects a news article or blog post containing the word “circadian” I get a message. Thus I read a number of media articles about these studies before I read the studies themselves.

The media accounts (see some examples) tended to emphasize two reasons why these studies are important.

The first one was a surprise that both humans and protists have the same molecules doing the same thing. Their surprise was a surprise to me! Circadian clocks are found everywhere. Peroxiredoxins are found in almost all living organisms on Earth. Just like the structure of cell membrane, the processes of DNA transcription and RNA translation, the genetic code, or the use of ATP (adenosine triphosphate) as the energy currency of the cell, peroxiredoxins are ubiqutous molecules in almost all of life on this planet.

Those are life’s universals, something that is expected as we have understood the unity of life even before Darwin. These universals are, to a biologist, usually deemed pretty useless and boring – the background. What gets a biologist excited is variation – the exceptions to the universals. If most organisms use a particular molecule for a particular function but one organism does not – now that is exciting! Why is this? How and why did this organism evolve this switch in function? What was the initial mutation, what was the selective pressure? Those are useful questions in biology that help us understand evolution.

My hunch is that the journalists, either by being lay audience themselves or by targeting their articles to the lay audience as they understand it, focus on the universals – the “Unity Of Life” – as something that in their minds is evidence for evolution and a counter-argument against non-scientific ideas about life (e.g. intelligent design creationism). While biologists find surprise and delight in exceptions, which are useful entries into detailed studies of evolutionary mechanisms, many in the lay audience are still surprised by the plain fact of the unity of life as it evolved from a single common ancestor.

The second reason given in MSM (mainstream media) articles as to why these studies are important is their novelty. For example, Chemical & Engineering News states that this “…involves a previously unknown cycle of posttranslational modifications, in addition to the transcription of a well-known handful of clock genes…”

There is a sense, reading all the coverage, that this is so novel, creative and revolutionary, it must have been the very first time anyone has ever thought of this! And even better – this is a great “conflict” story, in which a single study puts into question an entire field! A small group of young geniuses proved the entire old establishment wrong!

Not so fast!

Both of these studies have been done before. Several times.

I quickly went to the PDFs of the two papers and yes, the references to the old studies are there. The authors are aware of the history of the field, the giants on whose shoulders they climbed in order to see further. Of course, these are Nature papers with severe constraints on space and on the number of references. With so many experiments and so much to explain about their methods and results, they could not spend enough time on the history of the idea and on the work of their predecessors. And they could not cite all of the preceeding studies. But they chose the key ones and noted them briefly in the text – not prominently, but they are there.

So why did the MSM articles not pick up on this? First, by stressing the novelty – and the “conflict” story of geniuses proving the establishment wrong – they make the new studies seem more newsworthy, thus more likely to be approved by the editors to get into print.

Second, the MSM articles are limited by space and there is only so much one can put in 500 words. Thus, as usual, it is the context that gets left out and the novelty-factor that remains in the piece.

Finally, even if a dilligent journalist wanted to follow up on the background he or she would bump into the dreaded paywalls.

Nobody expects a journalist to know as much about the field as its practitioners do. I knew exactly what to look for and even I needed a few days to collect all the papers and read them – which is why you are reading this post now instead of last week.

As a member of the Society for Research on Biological Rhythms I have free access to the Journal of Biological Rhythms, the premier journal in the field. Once logged in, I knew exactly which five articles to look for. Those five articles (all cited below) contained the references to all the other papers I was interested in. As none of those are Open Access, I had to go to Twitter and, by using the hashtag #ICanHazPDF ask my followers to find me and send me PDFs of all of these papers. I got most of them (though some are not available or not even digitized yet as many of them are quite old). Then I had to spend some time reading them.

This all takes time, and I had the advantage of knowing where to start…perhaps even the advantage of being aware, to begin with, that such studies exist. Even Allison Brager who is in the field of chronobiology, did not note on her blog Dormivigilia the existence of prior research and exclaimed with excitement that “….the clinical and scientific relevance of this work are HUGE!!!!….” It is not always stressed hard enough to graduate students how important it is to read the historical literature of one’s field.

So, let’s quickly go through some of the aspects of the history of circadian research that are most relevant to the understanding of the context in which these two papers appeared.

Brief history of clock research

While the observations of daily rhythms in plants and animals go back to the antiquity, the first experiment in the field was performed in 1729 by Jean-Jacques d’Ortous de Mairan who observed the rise and fall of leaves of a Mimosa plant kept in constant darkness. Much early research was done in the 19th century, mostly on plants, but also a little bit on insects and humans.

In the early 20th century a number of people started studying rhythmic phenomena in living organisms. They came from very different scientific disciplines, e.g., botany (Bunning), ecology (DeCoursey), animal behavior (Kramer, Beling, Sauer), protozoology (Hastings), evolutionary biology (Pittendrigh), mammalian physiology (Richter), human biology and medicine (Aschoff, Halberg), and agriculture (Garner and Allard). It took them a few decades to discover each other’s work and to recognize that they are dealing with the same phenomenon regardless of the organism they were studying, be it fiddler crabs, starlings, tobacco or humans.

The founding moment of the field was the 1960 meeting at Cold Spring Harbor. The book of Proceedings from the Meeting (Symposia on Quantitative Biology, Vol.XXV) is a founding document of the field: I own three copies, strategically placed in three different spots in the house so at least one copy has a chance to be saved in case of fire. And you can bet I have read it over and over again during my 10 years in grad school (and after).

In 1960, structure of DNA was very new, and comparatively little was known about the inner workings of a cell. But most researchers were eager to break into the ‘black box’ and start investigating how the circadian clock ticks inside of the cells. A number of conceptual models were proposed, some focusing on cell membranes, others on DNA. Thus, experiments were started to separate the two parts of the cell and to test the role of nucleus and DNA in the clock mechanism.

Roenneberg and Merrow (2005) provide an excellent timeline of the research since then, especially the emergence of molecular and genetic techniques and subsequent findings. But briefly, over the next two decades or so, circadian mutants were discovered in a protist (Chlamidomonas), insect (Drosophila) and mammal (Golden hamster). The first canonical clock gene – the Drosophila period gene – was sequenced shrtly after.

Around 1995, clock genetics explodes. New clock genes were discovered left and right in several different organisms, from cyanobacteria to humans. There was a sudden influx of people into the field from other areas of genetics, and they required a few years to catch up on the field’s history and theory before they stopped making beginners’s mistakes in their experimental designs (though see Dunlap 2008 for different perspective on that history). But there were many of them, they had plenty of funding, they were excited and creative, had powerful new techniques, and they worked fast, so every week, or so it seemed at the time, there was a new discovery of genes involved in circadian rhytms.

Here is a timeline of the key events in this aspect of the history of the field:

What emerged from all of this activity is the transcription/translation model (Hardin, Hall & Rosbash 1990) for the circadian mechanism: a suite of canonical clock genes get transcribed and translated, and their protein products, after some delay, inhibit the transcription and translation of those same genes. Day in and day out. Those genes and their products then also regulate expression of all the other genes that the cell uses in its daily function.

But not all were happy with it.

If you read Dunlap 2008 you will certainly detect a tension between researchers who studied whole organisms (always with evolution in the backs of their minds) and treated the clock as a ‘black box’ for decades before the genetic explosion, and the geneticists who came into the field in the mid-1990s. The former regarded the latter as arrogant and simplistic in their complete focus on DNA. The latter regarded the former as out-dated holists who don’t understand the magnificient power of DNA and treated them like Lysenko by those who do not understand Lysenko.

You need to remember that this was in the middle of the Human Genome Project hype, when there was a huge overselling of crude genocentric and gene-deterministic ideas, many of which were fully embraced by the geneticists at the time (they have learned better since then).

I do not want to exaggerate the tension – it was mostly muted. Geneticists were mainly welcome into the field. After all, they were bringing in their tools and skills to do what the field was hoping to do all along – crack open the “black box” and peer inside. The two groups treated each other with respect, and soon started collaborating. The old guard of chronobiology was impressed by the speed and capability of the genetics labs, the rate at which new techniques were developed and improved, and the rapidity of discoveries. They learned (or sent their students to learn) the techniques, and started thinking how to employ them to study circadian phenomena that prior research has already shown occured at higher levels of organization.

The hope was that the genetics and molecular approaches will quickly discover all the core clock genes and the way they interract with each other so the focus can shift back to explaining the things that really matter – properties of ensembles of clock cells and the behaviors of whole organisms (after all, DNA is invisible to selection – phenotype of the whole organism is what ecology and evolution can see to act upon).

The discovery of clock genes required the use of a limited number of laboratory models that are amenable to genetic dissection: mouse as the model for all vertebrates, fruitfly representing all invertebrates, Neurospora crassa standing in for all fungi, Arabidopsis being “the plant” and Synechococcus being used as the only bacteirum known (at the time) to possess a circadian clock. In the early years, a few protists were also used – Paramecium, Euglena, Gonyalax, Acetabularia and Chlamydomonas, but they were later largely abandoned, while new models, like Xenopus and zebrafish entered the arena.

This was a highly unusual state for the field – chronobiology was always extremely comparative, with thousands of species of organisms being studied over the years. The hope was that, once the genes are discovered in model organisms, the findings can be applied to other creatures for a more comprehensive and comparative research program.

Likewise, the focus on genes was also seen as temporary, something to be “waited out” until the findings can be applied to other levels of organization in a more integrative approach. Thus, entire lines of research were reduced or have essentially stopped – tidal, lunar and circannual rhythms, Sun-compass orientation, photoperiodism, development, ecology and evolution – waiting for new techniques and new findings that will enable them to re-start.

And that is exactly what happened – a decade or so later, the field concluded that all the core clock genes were discovered and that the transcription/translation feedback loop model is good enough. The study of previously semi-abandoned topics (and organisms) started with new zeal and gusto.

Persisting problems

At the time of the Cold Spring Harbor symposium in 1960, there were two main lines of thinking about the cellular mechanism of the circadian clock. One focused on the nucleus and the DNA (Ehret and Trucco 1966). The other focused on the cell membrane (Njus et al. 1974).

How does one go about figuring out which one of the two models is right, using techniques available at the time?

One approach is to use cells that do not normally possess a nucleus or any DNA – like mammalian red blood cells – to see if they have circadian rhythms. If yes – nuclues is not important, membrane (or cytoplasm) is. Studies were difficult and results not always clear, but most could detect rhythms in red blood cells (Cornelius and Rensing 1976, Mabood et al. 1978, Ohm-Schradera et al. 1980, Peleg et al. 1990a,b)

Second approach is to use very large cells that can survive long enough once the nucleus is removed – in comes the protist Acetabularia (Sweeney and Haxo 1961, Schweiger et al. 1964, Vanden Driesche 1966,Terborgh and McLeod 1967, Vanden Driesche and Bonotto 1969, Sweeney 1974, Mergenhagen and Schweiger 1975a,b, Hartwig et al. 1985, Woolum 1991, Runft, Linda and Mandoli 1996). These studies showed that clock operates after the nucleus is removed, and, once the nucleus is reintroduced, it is the clock in the cytoplasm that determines the phase, entraining the nuclear clock.

The third approach is to pharmacologically block DNA transcription and RNA translation. This was, over the years, performed in a number of organisms, including Acetabularia (photo on the right) and, much more recently, the sea-slug Bulla gouldiana (Page 2000). Again, rhythms persisted in the absence of DNA transcription.

Fourth approach is to find single-cell organisms that reproduce or divide more often than once a day and see if the circadian phase is preserved during the process – there is no DNA transcription during cell division. This was initially done in the protist Paramecium (Barnett 1966), but later it was cyanobacteria that were used in this approach (Mori et al. 1996, Kondo et al. 1997). Circadian phase is preserved during reproduction in Paramecium and cell-division in bacteria.

Fifth approach is to find organisms that have circadian rhythms but do not have clock genes. Yeast (Saccharomyces cerevisiae) is one such organism. In the nematode Caenorhabditis elegans, which shows circadian rhythms, the genes usually used for circadian timing are instead used for developmental timing (so-called heterochronic genes).

Sixth approach is to study the rhythms in either the cell membranes (for example in the protist Gonyalax polyedra, Adamich et al. 1976, or fruiftly, Nitabach et al. 2005) or elements of the cytoplasm directly, in a dish (using bacterial clock proteins, Tomita et al. 2004, Mehra et al. 2006, Mori et al, 2007). Again, the isolated cell membrane cycles, and blocking the membrane processes also blocks overt rhythms in whole organisms. Bacterial clock proteins (not DNA) kaiA, kaiB and kaiC, when placed in a test tube, spontaneously oscillate in a circadian fashion.

Finally, one can genetically affect the clock: mutating, deleting, shutting down or overexpressing (forcing expression at high levels at all times with no cycling) canonical clock genes and see if any residual rhytmicity remains. This was done in the fruitfly (Helfrich-Förster 2000), where morning peak of activity is eliminated when the clock gene cycling is stopped, but the evening bout of activity in male flies persists nonetheless. Sometimes a genetics paper would triumphantly state that a deletion of a gene rendered half of the flies arrhythmic, just to be met with a question “so, how do the other half of the flies still cycle without it?”

This research program started with enthusiasm immediatelly after the symposium, yielding troves of interesting data over the years. But, once the geneticists entered the fray, these results were forgotten or ignored. They did not conform to the DNA-based model. The easiest way to make a circadian geneticist in the mid-1990s angry at a conference was to utter the word “Acetabularia” – this was “noise” to be ignored and swept under the rug.

You can see the timeline of the history of this “shadow research program” here:

Why did this research persist despite the victorious run of the transcription/translation model?

The earliest studies in this area were a direct outgrowth of the ideas discussed at Cold Spring Harbor. They all yielded the data suggesting that DNA is not the only part of the clock mechanism. Yet, once genetics work took off, these results were ignored. At least some of the people in the field were worried that genetic work is ignoring something potentially important.

Who in the field was worried about this depended on their own background and experience? First, people who worked on organisms that yielded unusual experimental data throughout the history of circadian research, including the fungi and the protists (especially Gonyaulax polyedra, recently renamed Lingulodinium polyedrum but you are unlikely to find many circadian papers using the new name, and the systematics may still be in flux) were one such group.

People working on non-mammalian vertebrates (fish, amphibians, reptiles and birds) were cognisant of the complexity of circadian organization – same clock genes, expressed in different tissues, resulted in clocks of different properties. The clock in the pineal organ, the clock in the retina, the clock in the SCN (suprachiasmatic nucleus of the hypothalamus), the peripheral clocks in all the other tissues – each of those behaved differently despite using the exact same molecular machinery. So the properties must have been modified by something else in the cell, or by the interactions between cells in the tissue.

On top of that, many phenomena, e.g., photoperiodism or sleep, are not properties of individual cells but of interactions between ensembles of cells in the tissues, or even interactions between the organism and its environment. The simplistic “this gene is for clocks” model just could not explain the complexity of observed reality.

Once all the clock genes were deemed discovered, the critiques started popping up (Roenneberg and Merrow 1998, 1999, 2005, Lakin-Thomas 2000, 2006), trying to move the circadian research up the levels of organization to the interplay between cells, tissues, organs and organisms. Most of these calls for the return to the organism were reviews of all the studies showing that DNA is not enough – somewhat like this article is. The two Nature articles last week are just the latest research results in this tradition.

The power of metaphor

Where does this fundamental misunderstanding between molecular and organismal biologists come from? They are both biologists, right? So they should be expected to operate from the same basic principles.

But they don’t. Geneticists come from a tradition starting with Schroedinger’s 1944 book What is Life? This is a linear, hierarchical view of life, with upward causation: genes cause and control everything else. Also, gene is the only level on which natural selection acts (Dawkins 1976). The reigning metaphor of this worldview is the “program.”

On the other hand, biologists coming from the study of evolution, ecology and animal behavior have a “systems” view of life in which many interacting elements, none of them with a primacy, determine the behavior of the entire system. There is no single element in control. The phenomena are a result of interactions, not of dominance of any particular actor.

The causation is downward (natural selection). DNA is just one of the elements in the system. Selection acts simultanously at several levels, including whole organisms and groups (Brandon 1996, Gannett 1976, Godfrey-Smith 1999, Griffiths and Gray 1994, Hubbard and Wald 1993, Keller 1995, Lewontin 1992, Nijhout 1990, Nelkin and Lindee 2004, Kitcher, P. 1999, Rose et al. 1990, are just a tip of the iceberg of the literature analyzing and criticizing the hierarchical DNA-first worldview). The reigning metaphor of this worldview is “the tangled bank”.

Circadian field is not the only area of biology in which these two worldviews clashed. But it is worth noting here that the studies of clock genes ignored everything else, while the studies that questioned DNA supremacy never just shifted the control to some other element – all of those studies say that DNA is not sufficient, not that it is replaced by another controller.

Let’s look at the “program” as a metaphor. A program is a term from information theory. It is a deterministic algorithm leading to a particular result. But what is reading that program? What is the “computer” that runs it? The cell?

And where is the person using the computer, the one who decides to run the program and decides if the program is useful or not? Where is natural selection?

Look at all the terminology of molecular biology: transcription, translation…those are all terms from information theory, which is linear, deterministic and hierarchical – there is a cause that controls the effect.

Even the “News and Views” article accompanying last week’s two papers (Bass and Takahashi 2011) re-frames the results of the papers into information theory metaphor. All the stuff that is happening in the cytoplasm is referred to as “post-translational” as if it was just something more that DNA “caused,” perhaps a little further downstream than usual.

But it is not. The cycles in the cytoplasm are not caused by anything any piece of DNA did. When in sync, the genetic feedback loops and cytoplasmic clocks work synergistically. But when placed in opposition, the cytoplasmic clock dominates (e.g., determines the phase, period, etc.).

The centrality of the gene in much of biological thinking led to another error that these two papers in Nature just fixed. Because different kingdoms of life (bacteria, protista, plants, fungi and animals) have different clock genes, it was assumed, despite the identical mechanistic logic of the mechanism, that the clock evolved independently several times. Identity of players trumped the mechanism of interaction between them.

But if, as the papers show, all organisms have cytoplasmic clocks based on anti-oxidant enzymes, then this cytoplasmic clock is the scaffolding, the base which allows evolution and replacement of all sorts of clock genes in different groups. As clock genes come and go, they can always latch onto the ever-present cytoplasmic clock. And the organism can keep on ticking regardless of the evolving stage in which any particular clock gene may be. This argues for a single origin of the circadian clock, due to universally adaptive nature of the clock, as postulated by Colin Pittendrigh decades ago.

The clock metaphor

The theory of biological rhythms has benefited immensely from the use of the clock as metaphor. Thinking of biological rhythms in terms of oscillatory theory (borrowed from physics) has allowed us to understand how the biological clock works, how it gets synchronized with the environment (entrainment), and how systems with multiple clocks can act together to produce higher-order phenomena (e.g., photoperiodism – measurement of seasonal changes in daylength).

The clock metaphor was also a key for understanding the mechanism as a collection of interacting cogs and wheels. This was crucial for the discovery of clock genes later on.

But once the number of clock genes was determined to be very small, and the interlocking feedback loops between them became the dominant paradigm for the mechanism, the meaning of the clock metaphor shifted – instead of looking at all the potential cogs and wheels, only those made of or from DNA counted. There is nothing wrong with counting everything – genes, and cytoplasmic elements, and the cell membrane, and the interactions between clock cells in a tissue – as cogs and wheels of the biological clock, but somehow, somewhere, we forgot that and settled for a DNA-only view.

Every metaphor that scientists invent has a heuristic value. The information theoretical thinking about genes sped up the research in genetics and molecular biology. The clock metaphor sped up the circadian research.

But it is always a good idea to sometimes step back and consider if the dominant metaphor is constraining in some ways, if it limits the imagination. I have argued before that an occasional switch to a different circadian metaphor – perhaps player-piano, or endless tape recorder, or Rube-Goldberg Machine, or camshaft, or Moebius strip – can be a good way to look at the problem from a new angle. This can be a very productive endeavor, opening one’s eyes to new angles, starting new avenues of research. Every field of science has its metaphors, and it is always a good idea to sometimes analyze them, and sometimes replace them once they outlive their usefulness.

What metaphors are used by lay audience and the media?

There is a difference between metaphors used by scientists to guide their research programs, and metaphors used by journalists to explain research to lay audiences.

The clock metaphor, for example, means ‘interlocking cogs and wheels to study’ for researchers, but ‘timepiece in your brain that tells you when to wake up and when to fall asleep’ for the audience.

Likewise, the gene-control metaphor is something that is easy to understand for the audience that may be used to a hierachical worldview of top-down control (in society, family, religion, politics, or simplistic mechanics of everyday life). A systems-worldview requires a little bit more tolerance for ambiguity (which not everyone has) and a little more sophisticated understanding as to how complex systems work (i.e., how complex behaviors emerge out of interactions between multiple elements, in which the nature of interactions is more important than the identity and behavior of individual elements).

This is probably why the media reports could not capture the complexity of the findings. It provided an or option instead of an and option – the lay reader is probably going to think that DNA has nothing to do with the clock, instead of understanding that both DNA and other elements of the cell are partners. Still, in the media saturated with “gene for X” stories, an occasional “not in our genes” story is a positive event.

On the other hand, since the early 2000s (once the hype over Human Genome Project died down a little bit), the geneticists have moved away from the gene-control metaphor to some extent. Yes, they still sometimes slip up to old habits of mind (and their terminology shows it) – like when they use the term “post-translational” for everything that does not involve DNA in the cell – but the results of their own studies, from quantitative genetics to bumping into walls in some areas of research, have moved them to a more systems-like thinking. They are reinventing Physiology and calling it Systems Biology. And we are all better off for it. It is a more complete Physiology, with the ‘black box’ now wide open.

Another way that gene-primacy seeps into coverage of science is when new studies using molecular techniques are said to have confirmed the old studies using more traditional methods. For a recent example, see how the hypothesis of butterfly migration and speciation by Nabokov was said to have been confirmed by a recent molecular study. But molecular techniques are new, still being tested, calibrated and evaluated. The Nabokov story is really about well done work from the past using tried and tested old reliable techniques, that was strengthened by the new study and in turn validated the molecular method. Comparative anatomy is what validated the genetic method, not the other way round.

Likewise, in this example in clocks, it is very nice that new techniques repeated the old results. Each strengthens the other. The new study does not confirm the old as much as they all confirm each other. But for those enamored with molecules (or those who always think that new is better than old), this duo of papers will seal the deal if the old papers did not.


To summarize, the publication of these two studies in Nature last week is, in my opinion, quite a milestone in the field. First, it showed how it was possible for the clock to originate only once on Earth yet evolve a number of different molecular elements – the cytoplasmic clock was there all along, keeping time while the genes swapped.

Second, it re-framed the discussion of the mechanism. It forcefully demonstrated what many prior studies did in small increments, but this time with modern techniques we love and with enormous power. By reminding the people in the field that DNA is an important but not sufficient element of the clock, it will hopefully guide future research in a new direction, with a more complete view of the clock, and perhaps may even allow some people to venture out and try other productive metaphors instead.


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Image credits: Red blood cells, Wikimedia Commons; Ostreococcus tauri, The Joint Genome Institute; Acetabularia crenulata, The College of Exploration; graphs – from O’Neill JS, & Reddy AB (2011) and O’Neill JS. et al. 2011; and Transcription-translation Feedback Loop model, Nature Reviews Neuroscience

Postscript to Pittendrigh’s Pet Project – Phototaxis, Photoperiodism and Precise Projectile Parabolas of Pilobolus on Pasture Poop

ResearchBlogging.orgPostscript to Pittendrigh's Pet Project - Phototaxis, Photoperiodism and Precise Projectile Parabolas of Pilobolus on Pasture PoopThis is an edited, expanded, updated, revised and (hopefully) improved version of an old post. You can see the original here (or click on the “From The Archives” icon as usual).

Have you ever been out in the country visiting a farm? If so, you must have seen piles of manure, either stashed somewhere or just lying around the paddocks. And if that manure was a little older and starting to dry out and decompose, you likely saw some fine, white fuzz on its surface. Have you seen that? That fuzz is Pilobolus (not the dance troupe, but the fungus), one of a number of species in the genus. If you had a strong magnifying glass with you, and you trained it at the fuzz, you would have seen something like this:

Pilobolus has a portion of its life-cycle in which it has to pass through the digestive tract of a large herbivorous mammal. Since large mammals roam far and wide, this is a great way for the fungus to disperse. There is one problem, though: once excreted out with the feces, how do fungal spores get back into a large mammal again?

Unlike rabbits and some rodents, large mammals do not tend to eat their own manure. Actually, if you observe a field with a properly kept cow herd – a relatively small number of animals in a relatively large area, and rotated regularly between fields – you will notice that all the cows poop in one spot and no cow ever comes close to that spot to graze. So, what is a poor Pilobolus to do?

It gets ready, it aims, and it shoots!


Pilobolus assumes the position, builds a weapon, fills it with ammunition, aims and shoots. The position is on top of the pile of manure. The ammunition are spores, packaged tightly at the very tip of the filament. The weapon is the sporangiophore, a large swelled organ right below the tip.

The sporangiophore fills up with sap – osmotically active compounds – which builds up pressure until it is about 7 kilograms per square centimeter (100 pounds per square inch). There is also a line of weakness where the cap – the spore package – joins the sporangiophore vesicle. In the end, the pressure causes the sporangiophore to explode which sends the package of spores far, far away – if the wind is in the right direction, as far as 12 feet.

The goo from the sporangiophore goes with the spore package. It is very sticky, so wherever the spores land they tend to stay put. Ideally, that is on a blade of grass which is far enough from the manure pile to have a chance of getting eaten by a cow.

Here is a pretty picture of Pilobolus and a photomicrograph of the spore mass (crushed by the slide and slipcover):

[images from BioImages]

This is very cool (though wait for more coolness below), but also has an economic and environmental impact. Pilobolus spores themselves do not cause harm to their mammalian hosts, but some parasitic worms have evolved a neat trick – hitchiking on the Pilobolus spores right into the digestive tracts of large mammals.

While domestic cattle is regularly dewormed, the real problem is with wild ruminants, especially in places in which they do not have large areas to roam in, as in the elk in the Yellowstone Park. Here is a photograph of a Pilobolus harboring the Dyctiocaulus larvae:


So, Pilobolus shoots its spores really far away, by exerting enormous pressure on the ‘cap’. But, anyone who’s been in an artillery unit in the military will tell you that the distance is determined by angle. Soldiers manning the cannons know that an approximately 45 degree angle of the cannon will result in the greatest distance for the projectile. But a cannon projectile is a large, heavy object (also smooth and aerodynamic), so air resistance plays almost no part in this calculation – the force of gravity is the only force that the projectile needs to overcome.

A fungal spore is a microscopic object. At the small scale (pdf), physics works a little differently – gravity effects are minimal and the air resistance (drag) is the main determinant of maximal distance. Thus, 45 degrees is not neccessarily the optimal angle for achieving the greatest distance.

Frances Trail and Iffa Gaffoor, working with Steven Vogel at Duke University, made some calculations (which I have not seen and I do not think they got published, but I heard them from Dr.Vogel some years ago), looking at the shape and size of spore-caps of several species of Pilobolus (they published data on some other shooting fungi, though – you can read the paper here if you have access, sorry – not OA). The optimal angle for maximal distance ranges, in different species, between 9 and 30 degrees, the most common fuzz found on cow dung requiring about 15 degrees. The maximal distance, without wind, is about 6-7 feet. Quite right. Six feet is about as close as cows will come to a cowpie in well managed cattle establishments.

But does Pilobolus really shoot at 15 degrees? Well, what it does is it shoots towards the Sun. The way Pilobolus aims is through positive phototaxis. Like a sunflower, it follows the Sun in the sky and shoots at the Sun in the morning.

If you place Pilobolus in a box with light coming in only through a pinhole, all the fungi will shoot their spores at the pinhole:

How does Pilobolus see the light? Beneath the sporangium is a lens-like subsporangial vesicle, with a light-sensitive `retina’. It controls the growth and shape of the sporangiophore quite precisely. Thus, the packet of spores is always aimed at a light source:

So, the Pilobolus spores are found 6-12 feet away from the manure and they reproduce quite nicely even in the best managed cattle herds. So, they are probably shot at their optimal 15-degree angle. But they shoot at the Sun. Ergo, they shoot at the Sun when the Sun is about 15 degrees above the horizon.

One can think of two possible ways this can be achieved. One would be a mechanical sensor that triggers the explosion when the angle between the stalk and the cap is 15 degrees. This would work only if each individual was always standing upright on a flat surface, which is not the case on the rough relief of a manure pile.

The other strategy is to time the release so it coincides with the time when the Sun is about 15 degrees above the horizon. But, the trajectory of the Sun differs at different times of year.
In the middle of the summer in a high latitude, when the daylength is, let’s say, 18 hours, the Sun shoots straight up from the East and reaches the zenith right above exactly at noon. Thus, the Sun is around 15 degrees above the horizon about 90 minutes after dawn.

In winter, when the day may be only 6 hours long, the Sun traverses the sky low above the horizon from East to South to West, and may reach 15 degrees much slower (some Earth scientist in the audience can make a quick calculation here), e.g., 2 or even 3 hours after dawn.

How does the Pilobolus adjust to seasonal differences in Sun’s trajectory? By using its circadian clock, which entrains to different photoperiods with a systematically different phase:

Actually, the Pilobolus was the first fungus in which a clock was discovered. The effects of daylength on timing of spore-release was discovered back in 1948. The endogenous rhythmicity – meaning that the spores get shot every day even if there is no light present (in continous darkness) – was discovered in 1951. The major breakthrough was provided by (pdf) Esther-Ruth Uebelmesser in her dissertation:

At the same time that Schmidle published his findings, Esther-Ruth Uebelmesser (1954) dedicated her thesis work to the same subject. Her thesis is remarkable in many ways. Many of her experiments anticipated circadian protocols, frequently used in later years (different T-cycles and photoperiods, reciprocity, night interruption experiments, entrainment by temperature cycles, etc.). Although she did not fully exploit the richness of her experimental approaches in her interpretations, she must be considered a pioneer of the field and has certainly inspired Colin Pittendrigh to use Pilobolus as a circadian model system (Bruce et al., 1960). Probably, Pittendrigh abandoned this model system because of the unbearable smell penetrating the laboratory when the bovine dung media was prepared (Michael Menaker and Gene Block, personal communication, December 2000).


While in Neurospora accumulation of conidia (conidial bands) appears to be driven in these protocols with a constant phase angle in reference to lights-off (Fig. 2A), the phase angle of the spore-shooting rhythm in Pilobolus was systematically different with changing cycle lengths (Fig. 2B), possibly reflecting circadian entrainment. Closer investigation, however, revealed that the Pilobolus sporulation rhythm is also driven by the LD cycle, but unlike in Neurospora, by lights-on. Sporulation in Pilobolus is triggered by light, and the spores mature for approximately 28 h before they are shot (see arrows in Fig. 2B and C). The maturation time represents a kind of memory capacity for prior events. This is seen in experiments in which the fungi were released to DD (e.g., from LD 4:4 shown in Fig. 2C). The rhythm, synchronized to a given light cycle, persists for another 28 h until the endogenous circadian control takes over. Thus, depending on conditions, the production of asexual spores in Pilobolus is controlled both by the clock (phase angle) and by light (a driven spore release once per LD cycle).

[images from Roenneberg and Merrow 2001]

What this all means is that a circadian clock in this fungus is entrained by the dawn (not dusk) and it integrates photoperiodic information in a manner that is consistent with the need to shoot spores towards the Sun at the time of the morning when the Sun first reaches 15 degrees (actually, the tracking movement of the spore lags the Sun by about 20 minutes – fungi are slow to move – but even that is probably compensated for by the circadian clock).

Moreover, Pittendrigh’s students discovered that the Pilobolus clock is extremely sensitive to light (both intensity and duration of light). Its clock requires only a millisecond of light to be completely reset.


In a more recent paper, the explosive ejection of the spores was filmed with an ultra-high-speed video camera and in their subsequent calculations derived from the images, the “launch speeds ranged from 2 to 25 m s−1 and corresponding accelerations of 20,000 to 180,000 g propelled spores over distances of up to 2.5 meters.” You can see the video (turn on the volume – it is set to music) here:

What next?

This is where the story ends, for the time being. But there are still gaps.

For instance, I am not sure if it was ever tested in the laboratory that Pilobolus actually shoots at 15 degrees. This is, according to Dr.Vogel, relatively easy to do, by placing the fungi on a manure-based medium at the center of one of those transparent semi-spheres used by exhibitors at various product fairs (e.g., technology fairs). The ejected spores stick to the inside of the transparent plastic and can be seen from the outside. Measuring the length of the arc from the desk to the spore (and knowing the radius) is all one needs to calculate the angle.

Second, we still do not know for sure if the Pilobolus cues in to the season-specific photoperiod (more likely) or the season-specific Sun trajectory (less likely). One can, in the laboratory, dissociate these two factors by exposing groups of fungi to summer-specific photoperiod and winter-specific trajectory (using a strong flashlight attached to a string and mounted on an arc-shaped wire, attached to a little motor) or vice-versa, as well as season-specific photoperiod with diffuse (instead of focused) light source.

Finally, an evolutionary question. Horses are not as picky as cows concerning the distance from the manure at which they will graze. Pilobolus lives in our horses and shows up in the manure all the time. Is there relaxed selection for the populations (species?) that live in horses? Is their timing off? Is their angle-determination lousy? This would be an easy head-to-head test in the lab (and field) as well. And if there is such a difference between species, looking at molecules – dynamics of gene expression patterns and protein-protein interactions – can perhaps teach us something more about the ways simple parts can accomplish complex tasks in these organisms.

But, if you’d rather learn all of the above in a Dr.Seuss-like poem, go ahead, it’s right here.


Bruce, V., Weight, F., & Pittendrigh, C. (1960). Resetting the Sporulation Rhythm in Pilobolus with Short Light Flashes of High Intensity Science, 131 (3402), 728-730 DOI: 10.1126/science.131.3402.728

TRAIL, F., GAFFOOR, I., & VOGEL, S. (2005). Ejection mechanics and trajectory of the ascospores of Gibberella zeae (anamorph Fuarium graminearum) Fungal Genetics and Biology, 42 (6), 528-533 DOI: 10.1016/j.fgb.2005.03.008

Fischer, M., Stolze-Rybczynski, J., Cui, Y., & Money, N. (2010). How far and how fast can mushroom spores fly? Physical limits on ballistospore size and discharge distance in the Basidiomycota Fungal Biology, 114 (8), 669-675 DOI: 10.1016/j.funbio.2010.06.002

Roenneberg, T., & Merrow, M. (2001). Seasonality and Photoperiodism in Fungi Journal of Biological Rhythms, 16 (4), 403-414 DOI: 10.1177/074873001129001999

Uebelmesser E-R (1954) Über den endogenen Tagesrhythmus der Sporangienbildung von Pilobolus. Arch Mikrobiol 20:1-33.

Yafetto, L., Carroll, L., Cui, Y., Davis, D., Fischer, M., Henterly, A., Kessler, J., Kilroy, H., Shidler, J., Stolze-Rybczynski, J., Sugawara, Z., & Money, N. (2008). The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi PLoS ONE, 3 (9) DOI: 10.1371/journal.pone.0003237

Seven Questions….with Yours Truly

Last week, my SciBling Jason Goldman interviewed me for his blog. The questions were not so much about blogging, journalism, Open Access and PLoS (except a little bit at the end) but more about science – how I got into it, what are my grad school experiences, what I think about doing research on animals, and such stuff. Jason posted the interview here, on his blog, on Friday, and he also let me repost it here on my blog as well, under the fold:

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Are Zombies nocturnal?

day of the dead.jpgBlame ‘Night of the Living Dead’ for this, but many people mistakenly think that zombies are nocturnal, going around their business of walking around town with stilted gaits, looking for people whose brains they can eat, only at night.
You think you are safe during the day? You are dangerously wrong!
Zombies are on the prowl at all times of day and night! They are not nocturnal, they are arrhythmic! And insomniac. They never sleep!
Remember how one becomes a zombie in the first place? Through death, or Intercision, or, since this is a science blog and we need to explain this scientifically, through the effects of tetrodotoxin. In any case, the process incurs some permanent brain damage.
One of the brain centers that is thus permanently damaged is the circadian clock. But importantly, it is not just not ticking any more, it is in a permanent “day” state. What does that mean practically?
When the clock is in its “day” phase, it is very difficult to fall asleep. Thus insomnia.
When the clock is in its “day” phase, metabolism is high (higher than at night), thus zombies require a lot of energy all the time and quickly burn through all of it. Thus constant hunger for high-calory foods, like brains.
Insomnia, in turn, affects some hormones, like ghrelin and leptin, which control appetite. If you have a sleepless night or chronic insomnia, you also tend to eat more at night.
But at night the digestive function is high. As zombies’ clock is in the day state, their digestion is not as efficient. They have huge appetite, they eat a lot, but they do not digest it well, and what they digest they immediately burn. Which explains why they tend not to get fat, while living humans with insomnia do.
Finally, they have problems with wounds, you may have noticed. Healing of wounds requires growth hormone. But growth hormone is secreted only during sleep (actually, during slow sleep phases) and is likewise affected by ghrelin.
In short, a lot of the zombies’ physiology and behavior can be traced back to their loss of circadian function and having their clock being in a permanent “day” state.
But the real take-home message of this is…. don’t let your guard down during the day!
Picture of me as a Zombie (as well as of all my Sciblings – go around the blogs today to see them) drawn by Joseph Hewitt of Ataraxia Theatre whose latest project, GearHead RPG, is a sci-fi rogue-like game with giant robots and a random story generator – check it out.