Have you ever considered the size of the eggs laid by the queen which drives your honey bee colony? Do you think they are all created equal? Not so, and you might be surprised what influences the range in egg sizes laid by not only different queens, but the same queen within her lifetime, and what that means for the emerging bee. Science writer Dave Black takes out the tape measure and explains why ‘biggest, is best’.
When we think about queens laying eggs it’s always about quantity, not quality. Probably that is simply because it’s much more difficult to evaluate or express ‘quality’, whereas counting eggs is pretty straightforward. Singling out one aspect means we could be missing something though.
Size matters. ‘Size’ and ‘quality’ are not synonymous, but for eggs there is enough scientific work to know that size is an awfully good proxy for quality in almost every case. Larger eggs develop faster, survive better, and produce offspring more likely to reach maturity.
In most organisms we understand there is a choice made when it comes to the amount of resources dedicated to offspring and insect eggs have been studied a lot in this respect. We would expect a trade-off between the number of eggs produced and the size of the eggs (physiology says you could have a lot of small ones or a few big ones) but it also depends on other things, like the environmental conditions at the time, or the physical fitness and age of the mother.
In honey bees and other social insects that modify their nest environment, share food, and co-operate to care for their young, what this relationship could be is not obvious. Maybe resource allocation to an individual like a colony queen is not important.
Previous studies of egg size in honey bees observed a lot of things; laying worker’s eggs are bigger, fertilised and unfertilised eggs (from a queen) are the same size, eggs can be longer but weigh less, eggs size varies with sub-species, old queens lay smaller eggs, and so on. A lot of numbers, but no coherent explanation. More recently, the science is beginning to tidy some of this up (Amiri et al, 2020).
First, there is a significant and systematic difference in egg size within different strains, and between individual queens even from the same line, so comparisons need to be made taking genetics into account. Next, using a pollen trap to reduce the colony’s pollen intake and artificially creating a protein deficit increases the size of eggs the queen lays, and the evidence is that the survival at larva, pupa, and adult stage is better for individuals that come from larger eggs in any circumstance. The counter-intuitive observation that pollen limitation increases egg size is one indication that it is actively ‘managed’ by queens and not merely the passive, fixed, result of just ‘getting out what you put in’. It’s what biologists call a ‘plastic’ response (as in bendy, or, malleable).
Another discovery published last year (Han, Wei, Amiri, 2022) that supports this idea is that egg size changes with colony size, but maybe not as you might expect. Queens in large colonies lay smaller eggs than queens in smaller colonies. Swapping sister queens around in big or small colonies resulted in a predictable and reversible increase or decrease in egg size laid by the same queen. Oddly, the change in egg size was not linked to the number of eggs being laid, but to colony size, and the same thing has been observed in termites. It was even sufficient to connect a small colony to a larger colony with a screened tube to cause the smaller colony’s queen to reduce her egg size, and that didn’t happen when she was connected to an empty box.
It’s not known how the queens were able to sense the size of the colony, but this change in egg size was accompanied by a change in ovary size in the opposite direction, and a change in the relative abundance of 290 different proteins (about 10% of those measured!). Among these, one particular protein seemed to stand out as central to the system regulating egg size, and masking the action of the particular protein decreased egg size in experimental queens regardless of what colony they were in. This protein (called Rho1) may turn out to be key to the regulation of egg size in all insects.
So, however the starting point is determined by genetic, developmental, or environmental factors, it seems queens are still able to deliberately adjust their egg size in response to unfavourable or unpredictable circumstances (or vice-versa). Perhaps it’s because in a small colony the survival of each egg is more important and because brood care or food supply is more precarious. Maybe, in a large colony, the queen can reduce her investment in the next generation because she knows other colony members can increase theirs. In honey bees, egg size is not the passive result of resource availability.
Ultimately, there must be a physiological limit on a queen such that there can be lots of small eggs, or a few big eggs, but it looks like social honey bees have managed to set that aside. If these studies are right, we’d expect to see Rho1 expression linked to queen genotype and age, and further work to explain how a social cue about colony state modifies its presence, but there is another reason they may be significant.
If honey bee queens exercise choice over what size of egg to lay, something we’ll call ‘maternal choice’, we need to think more about other potential choices they make. We also have evidence that they differentiate between worker, drone, and queen cells and lay larger eggs in queen cells (Wei et al, 2019). While larval diet (‘royal jelly’) certainly has a role in producing queens so, it seems, does maternal choice. That is not something we have really appreciated previously.
There is also an interesting connection to a study from 2021 (AL Kahtani, Bienefeld 2021). We have presumed that the most important factor deciding which larvae were selected to be raised as replacement queens, a very important selection in evolutionary terms, was family relationships – plainly, nepotism. This study found that in fact it’s more likely to be egg size. Biggest is best.
Dave Black is a commercial-beekeeper-turned-hobbyist, now working in the kiwifruit industry. He is a regular science writer providing commentary on “what the books don't tell you”, via his Substack Beyond Bee Books, to which you can subscribe here
References
Amiri et al, (2020) Egg-size plasticity in Apis mellifera: Honey bee queens alter egg size in response to both genetic and environmental factors. J Evol Biol. 33:534–543. doi.org/:10.1111/jeb.13589
Han, Wei, Amiri, et al. (2022) The molecular basis of socially induced egg-size plasticity in honey bees. eLife 11:e80499. doi.org/10.7554/eLife.80499
AL-Kahtani SN., Bienefeld K., (2021) Strength surpasses relatedness - queen larva selection in honeybees. PLoS ONE 16(8):e0255151. https://doi.org/10.1371/journal.pone.0255151
Wei et al, (2019) A Maternal Effect on Queen Production in Honeybees, Current Biology 29, 2208–2213 https://doi.org/10.1016/j.cub.2019.05.059
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