The Molecules That Keep Species From Mingling

Mate a crocodile and an abalone, the old joke goes, and you'll get a crock-a-baloney. But seriously, scientists know very little about the specific genes that keep flings between more closely related species--say an orangutan and a chimp--from fruition. In today's Science, researchers describe a gene that seems to make fruit fly hybrids sterile. The finding could offer clues to how genes keep animal species from intermingling.

Typically, when flies of two species mate, the male offspring are sterile. Previous work had implicated several genetic regions, including one on the X chromosome called Odysseus, which is expressed in the testis. To home in on the gene responsible, evolutionary biologists Chung-I Wu and Chau-Ti Ting at the University of Chicago inserted progressively shorter pieces of DNA from one species, Drosophila simulans, into a fly of another species, Drosophila mauritiana, and tested them for fertility. The researchers knew to look for their gene in the region between the shortest insert that caused infertility and the longest one that left flies fertile.

The gene they found, dubbed Odysseus, contains a so-called homeodomain, a stretch of DNA that usually changes very little even in creatures as distantly related as worms and mice. But the researchers discovered that in the fly, Odysseus has been accumulating mutations at up to 1000 times the normal rate. Because the gene may help sperm production, the researchers speculate that the rapid change might be driven by the intense competition between fruit fly males to gain an edge on one another in the mating game. This fast mutation rate means that one male's Odysseus may not work when combined with the corresponding DNA from a female of another species, making the male offspring unable to produce sperm.

Experts praise the work for providing hard data on the troublesome topic of speciation. "A lot of speculative papers have been written in the last 50 years," says Masatoshi Nei at Pennsylvania State University, University Park, "but there's been very little real data." But he points out that Wu and Ting must still discover what genes Odysseus might help regulate in order to know the whole picture.