New Paper: Evolution of the egg-yolk protein in honey bees

By , October 27, 2011

I am very happy to announce the first paper from the Zayed lab, recently published by journal Molecular Ecology, on the evolution of the egg-yolk protein in honey bees. As the name implies, egg-yolk proteins are used to provision animal eggs with the necessary proteins and fats.  The egg-yolk protein in honey bees is very interesting.  Queen honey bees are egg laying machines; they have massive ovaries and can lay 1,000+ eggs per day!  Queens make a lot of the egg-yolk protein to provision these eggs.  But workers bees are effectively sterile, so they do not need to make egg-yorlk proteins, right? Well, as it turns out, worker bees also synthesize egg-yolk protein but use it in a different way.  They use it to make food secretions which they spit out to feed younger bees.  Also, the egg-yolk protein interacts with an important hormone in workers to affect the way workers behave.  So, the egg-yolk protein affects both queen bee traits as well as worker bee traits. Now, imagine a new mutation in the egg-yolk protein… Can this mutation spread in honey bee populations given that the egg yolk protein is used differently in queens and workers? In other words, if a new mutation is good for the queen, is it also good (or neutral) for workers?  If this is the case, then we should see lots of signs of positive selection on the egg-york protein.  You can also imagine an alternative scenario – if a new mutation is good for workers but bad for queens (or vice versa), then we would expect the gene to show signs of ‘constraint’ – the protein is ‘stuck’ in evolutionary terms because it is being pulled in opposite directions by queens and workers.

Egg yolk protein (Vg) adaptively evolves in the bee, Y>0 indicates positive selection; (reproduced from Kent et al 2011)

We decided to test this idea by sequencing the egg-yolk protein, along with seven other genes (controls) in honey bees from Africa and Europe.  Two undergraduate Research At York students carried out the molecular biology work (Amer Issa and Alexandra Bunting, both co-authors on the paper), while NSERC Postdoctoral Fellow Dr. Clement Kent carried out the population genetic analyses. We found very high levels of adaptive evolution acting on the egg-yolk protein, supporting the idea that what is good for the Queen is also good (or at least neutral) for Workers!

Dr. Kent cleverly mapped all the functional mutations to the egg-yolk protein’s 3D structure.

He found that almost all of them line the internal cavity of the egg-yolk protein where lipids are ‘bound’.  This is very exciting as it suggest that mutations in the egg-yolk protein affect fitness in honey bees by changing the protein’s capacity to interact with lipids.  We’ll be following up on this in future studies.

Functional mutations in the egg yolk protein are associated with lipid binding (reproduced from Kent et al 2011)

As it turns out, many genes in the honey bee genome are active in both workers and queens, and our work suggests that this may not constrain adaptation in the bee. From a practical point of view, our results show that the activation of genes in both queens and workers need not hinder breeding efforts.  Beekeepers often breed colonies with good worker traits (e.g. good foraging, low aggression, good hygiene, etc.).  Our study suggests that mutations which benefit one caste do not often disadvantage the other.

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