The story of X -- evolution of a sex ...( Berkeley -- Move over Y chromosome ...)

Berkeley -- Move over, Y chromosome it's time X got some attention.

In the first evolutionary study of the chromosome associated with being female, University of California, Berkeley, biologist Doris Bachtrog and her colleagues show that the history of the X chromosome is every bit as interesting as the much-studied, male-determining Y chromosome, and offers important clues to the origins and benefits of sexual reproduction.

"Contrary to the traditional view of being a passive player, the X chromosome has a very active role in the evolutionary process of sex chromosome differentiation," said Bachtrog, an assistant professor of integrative biology and a member of UC Berkeley's Center for Theoretical Evolutionary Genomics.

Bachtrog, UC Berkeley post-doctoral fellow Jeffrey D. Jensen and former UC San Diego post-doc Zhi Zhang, now at the University of Munich, detail their findings in this week's edition of the open-access journal PLoS Biology.

"In our manuscript, we demonstrate for the first time the flip side of the sex chromosome evolution puzzle: The X chromosome undergoes periods of intense adaptation in the evolutionary process of creating new sections of the genome that govern sexual differentiation in many species, including our own," she said.

Not all animals and plants employ genes to determine if an embryo becomes male or female. Many reptiles, for example, rely on environmental cues such as temperature to specify male or female.

But in life forms that do set aside a pair of chromosomes to specify sex from fruit flies to mammals and some plants the two X chromosomes inherited by females look nearly identical to the other non-sex chromosomes, so-called autosomes, Bachtrog said. The Y chromosome, however, which is inherited by males in concert with one X chromosome, is a withered version of the X, having lost many genes since it stopped recombining with the X chromosome.

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