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"Long before it's in the papers"
August 03, 2010

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Sex, cleaner of genomes

Feb. 16, 2006
Courtesy Indiana University
and World Science staff

Scientists have wondered for decades why sex evolved. It’s a costly process that produces less offspring than the alternative, asexual reproduction. 

The water flea Daphnia pulex is a commonly used model organism among ecologists and other environmental scientists. (© P.D.N. Hebert, University of Guelph)

Now, researchers say they have found evidence to back up one leading theory: sex occurs because it helps prevent the buildup of harmful mutations.

The two biologists, from Indiana University Bloomington, found that when sexual species reproduce asexually, they accumulate bad mutations more quickly. They used the water flea, Daphnia pulex, as a model species for their studies. 

The finding supports a theory that sex is a sort of evolutionary housekeeper that reorders genes and cleans out harmful genetic mutations, the researchers wrote in the Feb. 17 issue of the research journal Science. Thus sexual reproduction would maintain its existence by punishing, in a sense, members 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 make them function less efficiently.

Sexual reproduction is biologically costly. In mammals, it usually goes along with intricate mating behaviors. It requires the compatibility of sexual structures, a sex act, fertile eggs and sperm, and the successful combining of egg and sperm.

Meanwhile, asexual species continually evolve, and they don’t have to pair up to reproduce. It seems at first glance that they should outcompete their sexual brethren with their more efficient reproduction. Scientists have pondered, what is it about sex that justifies its big energy investment? 

A key idea that goes into the sex-as-genetic-housekeeper proposal is that sex helps evolution occur more efficiently. Evolution takes place when organisms with the worst genes for a given environment die out, leaving their fitter peers to survive and spread their better genes. This process, called natural selection, leads the species’ gene pool to slowly change, gradually producing major changes in the organisms themselves. 

Sometimes, though, it can be hard for evolution to erase a bad gene, simply because it’s sitting next to a good gene on a chromosome. Sex helps alleviate this problem by shuffling genes every generation. Thus, biologists suppose sex may make natural selection more efficient.

“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,” study co-author Michael Lynch said. 

Different lines of Daphnia pulex reproduce either sexually or asexually. Lynch and Paland studied genes from specialized cellular compartments called mitochondria in sexual and asexual lines. They compared the rates at which the proteins, key molecules inside living cells, mutated. They found that asexual lines have accumulated mutations that are considered probably harmful, four times faster than sexual lines have.

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

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WORLD SCIENCE

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