The study also suggests sexual reproduction maintains its own existence by punishing, in a sense, individuals of a species that meander into asexuality.

"It is known that sex is common in plants and animals, and that asexual species are typically short-lived, but why this should hold throughout evolutionary time is a great mystery," said Susanne Paland, who led the study. "Our results show that asexual deviants are burdened by an ever-increasing number of genetic changes that negatively affect the function of their proteins. It appears sex is important because it rids genomes of harmful mutations."

Coauthor Michael Lynch added, "Although there has been solid theory on the matter for quite some time, these results provide the first definitive proof at the molecular level that sexual reproduction magnifies the efficiency of natural selection in eliminating deleterious mutations from populations."

Sexual reproduction is biologically costly and at times complicated. In mammals, sex is usually preceded by intricate mating behaviors. It requires the compatibility of sexual structures, an insertion event, fertile eggs and sperm, and the successful unification of egg and sperm into a viable zygote. All of this adds up to a big energy investment -- energy an organism might have used for other purposes. Scientists have long been left to ponder, what is it about sex that justifies its big energy investment?

Biologists have come up with a wide variety of competing (and, in some cases, complementary) hypotheses to explain why sex continues to exist in the midst of recurrently evolving asexual competitors. The most widely accepted explanation has been that sexual reproduction confers the benefit of "unlinking" genes, meaning bad versions of genes won't always get to ride the coattails of good versions, and vice versa. In essence, the theory holds that natural selection operates best when parts of the genome are free to shuffle.

The present report provides evidence this is so. In the case of Daphnia pulex, sex appears to have enabled the separation of beneficial and deleterious versions of genes, so natural selection could act more efficiently in favoring the good and weeding out the bad.

The scientists used mitochondrial genome data to compose a phylogenetic tree depicting relationships among sexual and asexual strains of Daphnia pulex sampled from 75 ponds as far west as Illinois and as far east as Nova Scotia, Canada. This family tree reveals that sexual populations have recently and repeatedly spun off asexual strains.

The scientists sequenced the entire mitochondrial genomes for a subset of these sexual and asexual lines of Daphnia pulex, and by comparing rates of protein evolution, they found the asexual lines have accumulated bad mutations four times faster than sexual lines.

Paland and Lynch reason that if a switch to asexuality causes a big increase in the number of protein defects, a mechanism for removing those defects must somehow be missing when sex, too, is missing. The present report supports the notion that it is sex -- or genetic recombination that is a component of sexual reproduction -- which is the purifying force that helps get rid of genetic mishaps that harm the overall evolutionary health of a population.

The ability to reproduce asexually may be useful to organisms that can't get mates, but its long-term benefits are questionable.

"Ultimately, we would like to know how long a species can abstain from sex without going extinct," Lynch said.

The research was supported by grants from the National Science Foundation, the National Institutes of Health, and a German Academic Exchange Service Hochschulsonderprogramm III doctoral fellowship. Computer support was provided by IU University Information Technology Services, which are funded in part by the Indiana Genomics Initiative and the Lilly Endowment.

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