You probably already know that social insects, such as bees, ants and wasps, have sterile workers and one queen that does all the hard work when it comes to laying eggs.
But this presents something of an evolutionary conundrum. After all, why would a female worker give up her opportunity to reproduce and instead help her mother, the queen, produce more offspring?
It was only in the 1960s that this evolutionary conundrum was solved. Biologist Bill Hamilton used mathematical modelling to show that genes that reduce the fertility of workers can spread in a population, provided that they enhance the reproductive fitness of their relatives that share the same gene.
However, even half a century after Hamilton, no one had figured out the genetic basis of honey bee worker sterility. It was a bit like the hunt for the Higgs-Boson: we knew that these genetic pathways had to exist, but identifying one was proving difficult.
But now, in research we’ve recently published in the Molecular Biology and Evolution and Journal of Insect Physiology, we think we have found an answer.
Anarchy in the hive
The story goes back more than 20 years, when one of us (Ben), through his work with local beekeepers, found a mutant anarchistic strain of honey bee where the workers were reproductive, even though the queen was present.
Ben used a DNA fingerprinting technique to show that the workers were reproductive and that they all had the same father. Ben’s research showed that worker reproduction has a genetic basis.
We realised we could use this mutant honey bee to uncover the underlying genetics of worker fertility, by comparing the mutant workers to normal workers. When we did so, we found a gene that differed between the two strains of honey bee, and which seemed to be the strongest candidate for influencing whether the workers were reproductive or not.
We called the gene Anarchy, after the mutant from which it was identified.
We found that the expression of the gene Anarchy is more than twice as high in workers with deactivated ovaries (i.e. no mature eggs present in ovary) compared to workers with activated ovaries (i.e. mature eggs present in ovary). In addition, we found that the expression of the Anarchy gene predicts the ovary state of workers with almost 90% accuracy.
Interestingly, expression of the Anarchy gene goes up if a worker is exposed to a queen. This discovery proved to be the linchpin.
In a normal honey bee colony, the queen secretes a pheromone that inhibits ovary activation in the worker. And if the queen dies, the workers can activate their ovaries and lay eggs.
Even though workers cannot mate or store sperm, they can produce viable male offspring due to the sex determination system of honey bees (haplodiploidy).
So it appears that the queen controls reproduction in her workers by manipulating the expression of Anarchy via her pheromone.
The Buffy connection
While the Anarchy gene is found in many different organisms, very little is known about it. What was exciting for us is that Anarchy belongs to a protein family that is associated with a process termed programmed cell death (more colloquially called “cell suicide”). We already knew that programmed cell death causes the eggs to degenerate in the ovaries of workers exposed to queen pheromone.
To prove that the gene Anarchy interacts with the programmed cell death pathway, we manipulated its expression using a technique called RNA interference. When we did so, we found that Anarchy interacts with the programmed cell death regulator gene named Buffy (yes, this gene is named after the TV heroine).
A further experiment revealed that expression of Anarchy localises to the degenerating eggs in the worker’s ovaries. Therefore the Anarchy gene is in the right place to cause the death of the eggs.
Bees on the pill
So a honey bee worker in the presence of the queen is similar to a woman who is on the contraceptive pill. In both these situations, the female is not irreversibly sterile and the process is under external control.
Workers continually produce eggs. But when queen pheromone is present, the eggs are aborted early in their development.
Our findings have also helped us to understand how worker sterility evolved. Queen pheromone seems to have co-opted a mechanism that is already present in many insects. For example, starve a female fruit fly and she aborts her eggs in the exact same way as a honey bee worker does in the presence of a queen.
Social insects are remarkable creatures because of their massively social and highly altruistic behaviour. With the discovery of the role that Anarchy plays in worker sterility, we finally have a better mechanistic understanding of what makes these insects so unique.