Journal Club

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Honeybee queens influence offspring caste through egg size

A honeybee virgin queen emerges for the first time. Recent research suggests queens can influence egg size, affecting the distinct appearance of honeybee castes.  Image credit: Wei Liang Zhang and Zhi Jiang Zeng

A honeybee virgin queen emerges for the first time. Recent research suggests queens can influence egg size, affecting the distinct appearance of honeybee castes. Image credit: Wei Liang Zhang and Zhi Jiang Zeng

Queen bees have surprising sway over the future development of their eggs, a recent study reports. The reigning bee matriarch lays larger eggs in the wax honeycomb cells where workers raise new queens as compared with the eggs laid in other nursery cells, according to work published in Current Biology. Differences in egg size lead to differences in epigenetic gene expression, the research showed, which in turn influences the distinct appearance of honeybee castes.

That mothers can sway the look of their offspring through the maternal environment has been well studied since the 1940s, when quantitative geneticists termed it maternal effects. That influence has been demonstrated in other social insects, including ants. And it’s been hypothesized in honeybees, but never demonstrated until now.

Queen bees and worker bees are genetically identical, and previous research showed that differences in larval diet and rearing set offspring down different developmental paths. But this recent paper places new emphasis on conditions inside the egg that are determined by the mother. It also “indicates the queen has a more active role in the production of the next generation of queens than has been previously recognized,” says coauthor Zhi Jiang Zeng of the Honeybee Research Institute at Jiangxi Agricultural University in China.

Zeng’s team put pregnant queens and attendant workers with plastic honeycomb cells of two sizes: a larger size that queens are typically raised in, and a smaller size that workers are raised in. Monitoring three different colonies, the researchers collected and weighed all eggs after six hours and found consistently heavier and larger eggs in queen cells, implying a maternal effect.

To test that effect, the team moved some embryos around, putting eggs from worker cells into queen cells six hours after laying, and larvae from worker cells into queen cells two days later. Attendant workers raised all these babies as queens.

After 16 days, the natural-born queens, which came from big eggs laid in queen cells, weighed consistently more than queens reared from eggs or larvae laid in worker cells. Natural-born queens also had more egg-producing ovarioles in some cases. The researchers found differences in gene expression among the three groups, some of which were previously associated with caste differences. These trends were consistent with a maternal effect.

Queen honeybees are known to make another important choice about their eggs: whether to release stored sperm and fertilize them or to not. Fertilized eggs develop into daughters; unfertilized ones become sons. So perhaps it’s not surprising that the queen might also control egg size, says evolutionary biologist Timothy Linksvayer at the University of Pennsylvania in Philadelphia. Then again, he says, it’s also possible that the queens in this experiment laid a mix of egg sizes, but the workers destroyed any small ones in queen honeycomb cells right away. The authors did not see evidence of workers selectively destroying eggs, but Linksvayer would like to see future research that rules out this possibility.

He also notes that three colonies is a small sample size, suggesting the need for further study to confirm the effect and tease apart the importance of egg size versus other factors—such as larval diet and rearing—on caste development. “I think the critical question is, ‘How do all these factors go together?’” Linksvayer says.

This new finding could have important implications for social evolution, Zeng adds. “It suggests caste determination begins in vivo, and is not driven just by how workers treat larvae,” he says. “We’ve shown the queen has an active role in the development of royalty. This changes the dynamics for models of caste evolution.”

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