Scientists have found the most convincing evidence yet that a parasite can contribute to splitting a species in two, thanks to a phenomenon where a wasps damaged sperm can be rescued or fixed only by mating with particular females. A bacterium called Wolbachia prevents the successful development of embryos in matings between two very closely related wasp species that could otherwise produce viable offspring. In the February 8th issue of Nature, University of Rochester researchers show that the bacteriums species-splitting effect came before any other in the wasps, strongly suggesting that the parasite accelerated the natural evolution of the insect. The research was conducted by two graduate students, Seth Bordenstein and Patrick OHara, and led by John Werren, professor of biology.
Wolbachia propagates itself in an unusual manner: Instead of merely helping its host compete against non-infected hosts as many parasites do, Wolbachia actively seeks to eliminate non-infected hosts by stopping them from reproducing. To do this, the parasite alters the sperm of its male host, rendering it infertile when paired with an uninfected female. If, however, the male mates with an infected female, the damaged reproductive cells are rescued by the females parasite. Its as if the bacterium encodes the sperm cell, rendering it useless unless it encounters the de-coding bacterium from another infected wasp. The result is that infected males can only impregnate other infected females, not uninfected ones, and makes it difficult for uninfected females to find a compatible mate.
The trickery is part of a wider system that assures that as many wasps as possible will pass Wolbachia on to the next generation. Infected males can have offspring only with infected females, and infected females automatically pass the infection on to all their offspring. The only possibility for producing uninfected offspring is for two uninfected wasps to mate.