Most people are aware that social insects, like honeybees, have three “sexes”: queens, drones and workers.
Drones are males. Their only job is to fly out and mate with the queen after which they drop dead.
Female larvae fed ‘royal jelly’ emerge as queens. After mating, the young queen takes a bunch of workers with her and sets up a new colony. She lives much longer than other bees and spends her life laying gazillions of eggs continuously around the clock, while being fed by workers.
Female larvae not fed the ‘royal jelly’ emerge as workers.
Workers perform a variety of jobs in the hive. Some are hive-cleaners, some are ‘nurses’ (they feed the larvae), some are queen’s chaperones (they feed the queen), some are guards (they defend the hive and attack potential enemies) and some are foragers (they collect nectar and pollen from flowers and bring it back to the hive).
What most people are not aware of, though, is that there is a regular progression of ‘jobs’ that each worker bee goes through. The workers rotate through the jobs in an orderly fashion. They all start out doing generalized jobs, e.g., cleaning the hive. Then they move up to doing a more specialized job, for instance being a nurse or taking care of the queen. Later, they become guards, and in the end, when they are older, they become foragers – the terminal phase.
This pattern of behavioral development is called “age polyethism” (poly = many, ethism = expression of behavior), or sometimes “temporal polyethism” (image from BeeSpotter):
This developmental progression in behavior is accompanied by changes in brain structure, patterns of neurotransmitter and hormone synthesis and secretion, and patterns of gene expression in the central nervous system.
Some years ago (as in “more than ten years ago”) Gene Robinson and his students started looking at daily patterns of activity in honeybees. The workers in their early stages are doing jobs inside the hive, where it is always dark. They clean the hive, take care of the eggs and pupae, and feed the larvae and the queen around the clock. Each individual bee sometimes works and sometimes sleeps, without any semblance of a 24-hour pattern. Different individuals work and sleep at different, apparently random times. The hive as a whole is thus constantly busy – there is always a large subset of workers performing their duties, day and night.
As they get older, they start doing the jobs, like being guards, that expose them to the outside of the hive, thus to the light-dark and temperature cycles of the outside world.
Finally, the foragers only go out during the daytime and have clear and distinct daily rhythms. Furthermore, the foragers have to consult an internal clock in order to orient towards the Sun in their travels, as well as to be able to communicate the distance and location of flowers to their mates in the hive using the ‘waggle dance’. As bees are social insects, it is difficult to keep individuals in isolation for longer periods of time, but it has been done successfully and, in such studies, foragers exhibit both freerunning (in constant darkness) and entrained (in light-dark cycles) circadian rhythms, while younger workers do not.
In the Robinson lab, then PhD student Dan Toma and postdoc Guy Bloch did much of the early and exciting work on figuring out how the rhythmicity develops in individual worker bees as they pass through the procession of ‘jobs’.
In an early study, they measured levels of expression of mRNA of the core clock gene Period (Per). The gene was expressed at low levels and no visible daily rhythm in early-stage workers, but at much higher levels and in a circadian fashion in foragers.
As the levels of expression were measured crudely – in entire bee brains – it was impossible at the time to be sure which of the two potential mechanisms were operating: 1) the celluular clock did not work until the bee became a forager, or 2) the cellular clocks were working, but different cells were not synchronized with each other, producing a collectively arrhythmic output: both as measured by gene expression of the entire brain and as measured by behavior of the live bee.
Either way, the study showed correlation: the appearance of the functional circadian clock coincided with other changes in the brain structure, brain chemistry and bee behavior. They could not say at the time what causes what, or even if the syncronicity of changes was purely coincidental. They needed to go beyond correlation and for that they needed to experimentally change the timing to see if various processes can be dissociated or if they are tightly bound to each other.
And there is a clever way to do this! First, they took some hives and removed all the foragers from it. This disrupted the harmony of the division of labor in the hive – too many cleaners and nurses, but nobody is bring the food home. When that happens, the behavioral development of other workers speeds up dramatically – in no time, some nurses and guards develop into foragers. And, lo and behold, the moment they became foragers, they developed rhythms in behavior and rhythms of the Per gene expression in the brain. So, as the development is accelerated, everything about it is accelerated at the same rate: gene expression, brain structure, neurochemistry, and behavioral rhythmicity.
Nice, but then they did something even better. They removed most of the cleaners and nurses from some hives. Again, the balance of the division of labor was disrupted – plenty of food is arriving into the hive but there is not enough bees inside to take care of that food, process it, feed the larvae, etc. What happened then? Well, some of the foragers went back into the hive and started performing the house-keeping duties instead of flying out and about. And, interestingly, their brain structure and chemistry reverted its development to resemble that of cleaners and nurses. They lost behavioral rhythmicity and started working randomly around the clock. And the rhythm of clock-gene expression disappeared as well.
So, genetic, neural, endocrine, circadian and behavioral changes all go together at all times. Social structure of the colony, through the patterns of pheromones present in the hive, affects the gene expression, brain development and function, and behavior of individual bees. Just like the gene expression and behavioral patterns, the patterns of melatonin synthesis and secretion in honeybee brains is low and arrhythmic in young workers and becomes greater and rhythmic in foragers. With the recent sequencing of the honeybee genome, the potential for future research in honeybee chronobiology looks promising and exciting.
But are these findings generalizable or are they specific to honeybees? How about other species of bees or other social insects, like wasps, ants and termites? Are they the same?
Other species of socials insects have been studied in terms of age polyethism as well. The earliest study I am aware of (let me know if there is an older one) studying behavioral rhytmicity in relation to behavioral development was a 2004 Naturwissenschaften paper by Sharma et al. on harvester ants. In that study, different castes of worker ants exhibited different patterns – some were strongly diurnal, some nocturnal, some had strange shifts in period, and some were arrhythmic. Those with rhythms could entrain to light-dark cycles as well as display freerunning rhythms in constant darkness.
Just last month, a new paper on harvester ants came out in BMC Ecology (Open Access). In it, Ingram et al. show that foragers have circadian rhythms (both in constant darkness and entrained to LD cycles) in expression of Period gene (as well as behavioral rhythms), while ants working on tasks inside the hive do not exhibit any rhythms either in clock-gene expression or in behavior, suggesting that the connection between age polyethism and the development of the circadian clock may be a universal property of all social insects.
We know that in humans, night-shift and rotating-shift schedules are bad for health as the body is in the perpetual state of jet-lag: the numerous clocks in our bodies are not synchronized with each other. We have evolved to be diurnal animals, entrained to environmental light cycles and not traveling over many time zones within hours, or working around the clock. Social insects have evolved a different strategy to deal with the potentially ill effects of shift-work: switch off the clock entirely until one develops far enough that time-keeping becomes a requirement.
Yang, L., Qin, Y., Li, X., Song, D., & Qi, M. (2007). Brain melatonin content and polyethism in adult workers of Apis mellifera and Apis cerana (Hym., Apidae) Journal of Applied Entomology, 131 (9-10), 734-739 DOI: 10.1111/j.1439-0418.2007.01229.x
Sharma, V., Lone, S., Goel, A., & Chandrashekaran, M. (2004). Circadian consequences of social organization in the ant species Camponotus compressus Naturwissenschaften, 91 (8) DOI: 10.1007/s00114-004-0544-6
Ingram, K., Krummey, S., & LeRoux, M. (2009). Expression patterns of a circadian clock gene are associated with age-related polyethism in harvester ants, Pogonomyrmex occidentalis BMC Ecology, 9 (1) DOI: 10.1186/1472-6785-9-7
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Nice post, Coturnix. Further evidence that the social insects are a rich scientific vein indeed.
it was impossible at the time to be sure which of the two potential mechanisms were operating
Still can’t tell from this research, right? They have better correlation, but still correlation only. Without finer mapping of clock cells we still can’t tell whether it’s function or synchronization that can be “turned on and off” in response to social structure/pheromones. Or am I missing something?
I was cautious, as I am not sure if they actually did the relevant work on this. The two alternative hypotheses were around for a long time, in a bunch of organisms, but now we know in mammals, for instance, that it is desynchronization, not damping of individual cellular clocks, in cases when organism as a whole is arrhythmic (e.g., in constant light). I’ll look around to see if anyone’s done this in bees (or at least Drosophila) before I make any stronger statements.
But in mammals, as you pointed out, desynch is problematic (jet lag etc). Insects don’t seem to suffer any ill effects — though I guess that’s an open question, maybe they feel like shit until they get to forage… Is there a way to look for jet lag in bees? Supposing that they don’t suffer any ill effects, does that argue against desynch?
I wish an insect person would come here and tell us more – I bet there is something in Drosophila.
In human jet-lag, the desynch is between the pacemaker in the SCN and peripheral cells in different tissues, so hormone is secreted at the time when the target cell is not producing receptors, etc. But the behavioral rhythms are strong, as the SCN itself is not desynchronized internally – the cells (of at least one half of the SCN, there is this crazy structure to it in two parts) are strongly coupled and drive behavior. The SCN shifts to the new time-zone within a couple of days, while, for example, the liver clock may take weeks to shift.
On the other hand, organisms that become arrhythmic in various conditions (e.g., in constant bright light) seem to have their pacemaker cells uncoupled from each other. Who knows what’s going on in the periphery?! Keeping quail in LL for years does not alter their health or longevity (though in females, the continuous egg-laying may shorten their lives somewhat – we have never tested the fecundity/longevity trade-off hypothesis).
I’ll have to look at the actual numbers, but if there is a significant difference in overall levels of Per expression, i.e., the levels of Per mRNA are much lower in young bees than old ones at every time-point of the cycle, that would argue that each cell is damped oscillator: either with much smaller amplitude, or completely arrhythmic.
I had thought that drones manage the temperature and humidity of the hive. A critical job when you are making honey.
There are too few of them to do so, and they are not always there, once they die and before the new cohort emerges. The entire hive thermoregulates.
Surely a quick FISH for Per RNA in brains from 1, 2, 3 and 4 weeks…? Of course, we all know how “quick” experiments work out.
Just a quick question: is there any circadian or any other rythmic in human embryos?
Thanks Bora. That was really interesting.
Matthias – I wrote about human development before.
Thanks, very interesting blog!
Thanks for this nice post! Working on circadian rhythms in bumblebees myself, I was quite relieved to see that other people find it interesting as well.
The interesting thing with honeybees is, that their circadian clock is more similar to the mammalian one than to the Drosophila one. This might also be true for bumblebees, but their genome has not yet been sequenced.
For example, honeybees have only the mammalian type Cryptochrome, and do not encode the Drosophila type of Timeless. Their cryptochrome is not sensible to light and therefore honeybees (and bumblebees) show free-running rhythms in constant light, in contrast to Drosophila, which get arrhythmic under LL conditions.
In contrast to honeybees, where division of labour is correlated with age-related development of circadian rhythms, age doesn’t play a role in bumblebees but the size of the workers does.
Small workers stay in the nest and tend the brood around the clock with no or only weak diurnal rhythms, while bigger workers leave the nest to forage and show strong rhythms. Using immunoreactivity a recent study has shown that brains of large bumblebee workers have a significant higher number of PDF-ir neurons (Pigment Dispersing Factor) than the brains of smaller workers.
So, circadian rhythms in social insects are really a very interesting field.