MBL scientists confirm evolutionary exception
Marine Biological Laboratory
Biologists at the Josephine Bay Paul Center for Comparative Molecular
Biology and Evolution at the Marine Biological
Laboratory (MBL) have confirmed that a group of
microscopic animals has evolved for tens of millions
of years without sexual reproduction. Their results
demonstrate a radical exception to the biological
rule that abandonment of sexual reproduction is
an evolutionary dead end.
While almost all multicellular organisms reproduce sexually, this form
of reproduction is much less efficient than asexual reproduction (or mitosis)
whereby females effectively make clones of themselves. Although asexual
organisms often enjoy short-term success against their sexual ancestors,
they are rarely found as higher-order taxa, implying that they cannot
survive in evolutionary time.
While many hypotheses have addressed this problem, the paradox raises
one of the most perplexing questions in biology: If asexual reproduction
is more efficient than sexual reproduction, why does sexual reproduction
predominate so thoroughly? New research from MBL evolutionary biologists
may help scientists come closer to an answer.
In a paper to be published in next week's Proceedings of the National
Academy of Sciences (PNAS), MBL scientists Jessica Mark Welch and her
colleagues David Mark Welch and Matthew Meselson provide the strongest
evidence to date that a higher-ranking taxon has evolved without sexual
reproduction.
The researchers studied the bdelloid rotifer, a microscopic animal found
throughout the world in almost all aquatic habitats. Bdelloids appear
to have given up sex 50 million years ago, yet the organism has evolved
into 370 described species. While the researchers previously demonstrated
that bdelloid genomes contain two or more divergent gene copies, an observation
consistent with long-term asexual reproduction, a significant shortcoming
of their approach was the inability to detect nearly identical gene pairs,
as might result from inbreeding or other rare forms of sexual reproduction.
To overcome this methodical shortcoming and conclusively demonstrate
that bdelloids are, in fact, completely asexual, Mark Welch and her colleagues
painstakingly analyzed the genome of the bdelloid species, Philodina roseola.
Using a method called fluorescent in situ hybridization, they scoured
the genome, looking for chromosome partners, also called homologous pairs.
Identification of these would be a clear indication of sexual reproduction
as each member of the chromosome pair is derived from a different parent.
The researchers identified four copies of a target P. roseola marker
gene, however each gene was on a separate chromosome and all were quite
a bit different from each other. These results, consistent with asexual
reproduction, eliminate the possibility that bdelloids reproduce sexually
and thus confirm that the organism has evolved without sexual reproduction
or genetic exchange for tens of millions of years.
What drives early extinction, and why it can be averted by sex, remains
one of the central mysteries of biology, the resolution of which is likely
to have far-reaching impact on scientists' understanding of basic biological
and evolutionary processes. "Sex and genetic recombination are
obviously tremendously important for life," says Jessica Mark
Welch, "but we don't understand why they are so important. When
we do eventually understand, it could have practical consequences we can't
yet imagine."
Mark Welch and her colleagues will continue to study bdelloids as they
offer an ideal model system in which to explore the effects of asexual
reproduction. Their hope is to better understand how the animals have
evolved without sexual reproduction and escaped extinction. "We
can now use belloid rotifers to test the theories about why sex is important,"
says Mark Welch. "Any good theory will now have to account for
why the bdelloids are an exception."
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