examine alleles from different populations."

In the research, Zhang and coworkers from U-M, Michigan State University and the University of Kansas sequenced csd genes from individuals in three closely related species of honey bee: the familiar backyard denizen Apis mellifera and the Asian honey bees Apis dorsata and Apis cerana. The group also sequenced six so-called neutral regions of the genome which, unlike genes, do not carry codes telling cells how to make proteins. Then, the researchers constructed gene genealogies—family trees for both the csd gene and the neutral regions.

Their results showed that csd is about seven times more variable than neutral regions of the honey bee genome. In addition, many csd variants are shared among the three species, evidence that the many different alleles have been preserved in these lineages for a very long time.

Such a pattern supports the idea that an evolutionary mechanism known as balancing selection has been at work. Evolution works through the process of natural selection, in which genetic mutations that offer some advantage are favored, and those that have harmful effects are weeded out. Typically, this results in one version of a gene becoming very common and other versions becoming rare or disappearing altogether. When balancing selection operates, however, natural selection favors a diverse mix of alleles, as seen with csd in honey bees.

The research also showed just how long the csd alleles have been around.

"We estimated the age of the alleles at about 14 million years," said Zhang. "We don't know for sure when the species formed, but it's thought to be about six to eight million years ago, so the alleles are even older than the species."

Zhang collaborated on the research with postdoctoral fellow Soochin Cho and undergraduate student Daniel Green of the University of Michigan, Zachary Huang of Michigan State University and Deborah Smith of the Uni
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