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Planet Earth

Against All Odds, Sex Has Returned

A mite reevolves sex after hundreds of millions of years without it.

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Richard Dawkins said it was statistically improbable; Louis Dollo, a French paleontologist, famously developed a hypothesis stating that it could never happen. But a tiny soil mite smaller than a pinhead has reevolved the ability to mate, according to a study by evolutionary geneticists Katja Domes of the Technical University of Darmstadt, Germany, and Roy Norton of the SUNY College of Environmental Science and Forestry in Syracuse.

After analyzing various mites’ DNA, the duo concluded that the sexually reproducing Crotoniidae mite is a descendant of Camisiidae mites, which have reproduced asexually for hundreds of millions of years since they themselves evolved from a sexually reproducing ancestor. Through a process called parthenogenesis, Camisiidae females typically lay eggs that are exact copies of themselves. Although males are born every once in a while, they are always sterile. Or so it was thought.

“Those rare males may have enabled Crotoniidae to reevolve sex,” says Domes. Usually when traits fall into disuse, their corresponding genes quickly mutate to code for something else. So when something as complex as the ability to produce sex gametes is lost, it’s likely never to be developed again. Oddly, the Camisiidae mites seem to have retained that ability, despite surviving millions of years without using it.

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