October 29, 2007
Biologists’ work suggests ‘X-effect’
plays powerful role in evolution
Research by Daven Presgraves, professor of biology, suggest the female X chromosome plays a special role in the process of speciation, confirming a controversial theory of evolution.
“There is no more debate. The large X-effect is
real,” according to Daven Presgraves, professor of biology.
It’s a bold statement backed up by new research findings that
indicate the X chromosome is a strikingly powerful force in the origin of
new species, although the reason why may be nothing like what biologists
expected.
Biologists have argued for years whether the X
chromosome—the female chromosome in most animals—plays a
special role in the process of speciation. The new study in the journal PLoS Biology confirms
a controversial theory of evolution, showing that 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, 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 males—far higher than
the 18 percent for all the non-sex chromosomes.
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 simply “exposed,” meaning its complement,
the Y chromosome, doesn’t have the information needed to mask the
effects of changes on the X. We inherit a set of chromosomes from each
parent with each chromosome acting as a sort of backup for its complement.
It’s a bit like cross-referencing two encyclopedias for errors, says
Presgraves. In the case of X and Y, however, it’s like trying to
cross-reference an encyclopedia with a pamphlet.
But Presgraves believes it’s not a simple case
of the X chromosome being exposed. He believes there’s something
special about the X. Somehow, it attracts genes that disrupt the creation
of sperm in hybrid males—the main cause of the hybrid’s
infertility, he says.
“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.”
Presgraves is planning new tests to see if the X is,
in fact, refusing to shut down when it should. If the process that controls
normal X inactivation during spermatogenesis is particularly susceptible to
evolutionary change, says Presgraves, then it may be largely responsible
for the X chromosome’s unusually prominent role in the origin of new
species.
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