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January 21, 2004
An Evolutionary Exception Confirmed
MBL Researchers Provide Strong Evidence that an Ancient Animal Thrives and Evolves Without Sex
WOODS HOLE, MA - 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 this 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 methodological 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."
Note to Editors: The paper entitled "Cytogenetic evidence for asexual evolution of bdelloid rotifers" by Mark Welch, et al. is scheduled to be published in the PNAS Online Early Edition the week of 1/19/2004 to 1/23/2004. Related research entitled " When did gene copies in the asexual class Bdelloidea diverge?" by David Mark Welch, et al. will also appear in this issue.
The Marine Biological Laboratory is an internationally known, independent, nonprofit institution dedicated to improving the human condition through creative research and education in the biological, biomedical and environmental sciences. Founded in 1888, the MBL is the oldest private marine laboratory in the Western Hemisphere.
The Josephine Bay Paul Center for Comparative Molecular Biology and Evolution explores the evolution and interaction of genomes of diverse organisms that play significant roles in environmental biology and human health. This dynamic research program integrates the powerful tools of genome science, molecular phylogenetics, and molecular ecology to advance our understanding of how living organisms are related to each other, to provide the tools to quantify and assess biodiversity, and to identify genes and underlying mechanisms of biomedical importance. Projects span all evolutionary time scales, ranging from deep phylogenetic divergence of ancient eukaryotic and prokaryotic lineages, to ecological analyses of how members of diverse communities contribute and respond to environmental change. Three interlocking programs define the scope of research in the Bay Paul Center: the Program in Global Infectious Diseases, the Program in Molecular Evolution, and the Program in Molecular Microbial Diversity.