Biologists have argued for years whether the X chromosome -- the female chromosome in most animals -- plays a special role in the process of speciation. In a new study, Professor Daven Presgraves and colleagues at the University of Rochester have confirmed the X chromosome is indeed heavily influential.

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, said Presgraves. The process, dubbed the “large X-effect,” acts as a wedge between the two newly formed species, pushing them onto divergent evolutionary paths.

Presgraves, research associate J. P. Masly and colleagues interbred fruit flies for 15 generations. The team showed that 60 percent of X-chromosome genes cause infertility in hybrid males -- far higher than the 18 percent for all the non-sex chromosomes.

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

The research is reported in the current issue of the journal PLoS Biology.