X-effect: female chromosome confirmed a prime driver of ...( Researchers at the University of Roc...)

Researchers at the University of Rochester believe they have just confirmed a controversial theory of evolution. The X chromosome is a strikingly powerful force in the origin of new species.

Biologists have argued for years whether the X chromosomethe female chromosome in most animalsplays a special role in the process of speciation. In a new study in the journal PLoS Biology, Daven Presgraves, professor of biology at the University of Rochester, has confirmed that the X chromosome is indeed heavily influentialand the reason may be nothing like what biologists expected.

When one species splits into two, interbreeding between the two daughter species is much more likely to produce infertile hybrids when the species exchange X chromosomes than when they exchange any other chromosomes, says Presgraves. The process, dubbed the large X-effect, acts as a wedge between the two newly formed species, pushing them onto divergent evolutionary paths.

Over the course of a year, Presgraves and research associate J. P. Masly interbred fruit flies for 15 generations. The team painstakingly substituted individual genes of one fly species with the genes of a closely related species, and tracked which genes caused infertility in hybrids. The Rochester team showed that 60 percent of X-chromosome genes cause infertility in hybrid malesfar higher than the 18 percent for all the non-sex chromosomes.

There is no more debate, says Presgraves. The large X-effect is real.

But in solving one mystery, the findings give rise to another.

Scientists expect evolutionary changes in DNA to accumulate in random locations across a genome, but Presgraves instead found that most changes causing hybrid infertility cluster inexplicably on the X chromosome.

Presgraves is now looking into why the X is a hotspot for speciation genes, that prevent genetic exchanges between closely related species.

The traditional notion of the large X-effect is that the X chromosome is s
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