What stops workers from being royal?

Italian or Ligurian honeybee (Apis mellifera ligustica) worker. Image taken by Ken Thomas; in the public domain

Italian or Ligurian honeybee (Apis mellifera ligustica) worker. Image taken by Ken Thomas; in the public domain

Social insects have fascinating societies. In a typical social insect colony there is one queen – but tens of thousands of workers (who are also female). The worker caste does not produce any offspring. Instead the workers help the queen, their mother, rear offspring. This reproductive division of labour is a key characteristic of social insects. But workers and queens are genetically identical, so why can’t workers aim for royalty?

Worker sterility has been well studied in the evolutionary literature but biologists have tended to ‘black box’ how worker sterility operates. In my PhD research I aim to answer how worker sterility operates at the genetic and mechanistic level.

For my research I use the honey bee, which is the social insect that we know the most about. The honey bee queen secretes a pheromone that stops workers reproducing. If the queen dies, due to an accident or old age, the workers are released from their pheromonal shackles and can lay eggs. I was lucky enough to have the opportunity to work on a mutant strain of honey bee in which the workers can lay eggs even if the queen is present. My supervisor, Professor Benjamin Oldroyd, identified this mutant and named it ‘Anarchistic’ due to the behaviour it exhibited. Studies of this mutant suggested a particular gene was involved in worker fertility. We (logically!) named this gene Anarchy.

I have made a few discoveries since I started investigating the Anarchy gene in the honey bee. We found that just the presence of the queen in the colony affects the amount of Anarchy produced in the ovaries of honey bee workers. We then used a high resolution microscope to show that Anarchy is found in degenerating eggs within the ovary. We found that Anarchy can trigger the programmed cell death pathway (more colloquially known as ‘cell suicide’). We also used a biochemical assay to show that high amounts of cell death occurs in the worker honey bee ovary. All these results led me to an amazing conclusion – honey bee workers abort their eggs when a queen is present.

My research has identified just one example of how social insect workers become sterile. In all social insects workers become sterile whilst still larvae – this is maintained throughout their lifetime. I have established that workers become sterile via a general process: an environmental cue (for example, the presence of queen pheromone) interacts with certain genetic pathways to cause the degeneration of the ovary of the worker, via the mechanism of cell death. I have named the different stages of this process ‘reproductive control points’. My findings have been a major breakthrough towards a mechanistic understanding of worker sterility.

How worker sterility evolved is one of the most perplexing mysteries of evolutionary biology. What is really interesting is that these reproductive control points make use of mechanisms found in other insects. For example, solitary insects such as the fruit fly can abort their eggs in the exact same way as honey bees. So the reproductive control points were probably involved in the evolution of worker sterility. I had fun working out a scenario for how worker sterility emerged in social insects from their solitary ancestors.

I have now studied social insects for over five years and they never cease to amaze me.

Isobel Ronai.

IsobelIsobel is a final year PhD student at the University of Sydney, Australia. Her PhD research project is on the genetic and mechanistic basis of worker sterility in the honey bee. Learn more about her here and connect with her on Twitter @IsobelRonai.

Isobel’s scientific articles for further reading:

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