Female Chromosome ‘heavily influences’ origin of new species

November 14th, 2007 - 2:28 am ICT by admin  
Writing about the new findings in the journal PLoS Biology, Biology Professor Daven Presgraves has said 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.

The process, which is known as the “large X-effect”, acts as a wedge between the two newly formed species, pushing them onto divergent evolutionary paths, says the researcher.

Presgrave and his research associate J. P. Masly have interbred fruit flies for 15 generations over the course of a year. During their experiments, the researchers substituted individual genes of one fly species with the genes of a closely related species, and tracked which genes caused infertility in hybrids.

It was found that 60 per cent of X-chromosome genes cause infertility in hybrid males, which is far higher than the 18 per cent for all the non-sex chromosomes.

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

He also said that as his team solved one mystery, the findings gave rise to another.

Although the scientists were expecting evolutionary changes in DNA to accumulate in random locations across a genome, they 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 researcher believes that the X chromosome somehow attracts genes that disrupt the creation of sperm in hybrid males, the main cause of the hybrid’s infertility.

“When I look at this, I think the X is not behaving normally during spermatogenesis (sperm creation),” says Presgraves.

“I think it may be that in the production of sperm, when the fly’s genome is shut down and compacted to fit into the sperm head, the X is not shutting down and is wrecking the process,” he added.

The researchers are planning to carry out further experiments to test whether the X is, in fact, refusing to shut down when it should.

Presgraves says if the process that controls normal X inactivation during spermatogenesis is particularly susceptible to evolutionary change, it may be largely responsible for the X chromosome’s unusually prominent role in the origin of new species. (ANI)

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