Asexual Reproduction Catches On

Asexual reproduction is usually considered a way of life--an evolutionary choice a species makes when the drawbacks of sex outweigh its long-term benefits. But recent research has shown that in some insects, parthenogenesis--in which females give rise only to daughters and no males are born--is more of a sickness than a strategy. Now, in a paper in the latest Royal Society Proceedings: Biological Sciences, two Dutch entomologists claim that parthenogenesis may literally be contagious.

The culprit is Wolbachia, a bacterium that lives within the ovaries and testes of many arthropod species, playing havoc with sex lives and gender ratios. In some wasp species, Wolbachia has eliminated males altogether, by causing the egg to develop as a female. In wasps, the infection seemed to be inborn; in the lab, bacteria couldn't be transferred between wasps. That led researchers to speculate that the wasp and its parasite might have cospeciated: Whenever a wasp lineage diverged into two species, a new Wolbachia strain would then develop in each isolated wasp species.

Richard Stouthamer and Menno Schilthuizen of Wageningen Agricultural University in the Netherlands collected 20 species of Trichogramma, a genus of minuscule wasps condemned by Wolbachia to perpetual asexual reproduction. After sequencing specific DNA regions in each, they drew evolutionary family trees for the insects and for the microbe. Because they found almost identical Wolbachia in distantly related wasp species, the researchers concluded that Wolbachia must have leaped from one Trichogramma species to another many times, carrying the parthenogenetic lifestyle with it. "We knew parthenogenesis was curable; now we know it's contagious too," says Stouthamer. He suggests that the infection spreads in butterfly eggs, Trichogramma's favorite place to breed. When infected and uninfected wasp species share an egg, the infection may jump between species.

All this may have practical use. Says Wolbachia researcher Henk Braig of Yale University School of Medicine, "If there is horizontal transfer, there's hope we might be able in the future to make insects parthenogenetic at our will." Certain Trichogramma species are already used as effective parasitic weapons against insect pests, and all-female strains would be even better, because only the female wasps parasitize pests by laying eggs--and an all-female strain wouldn't waste any time on sex.