The formation of a species means that a group of organisms splits into two populations that cannot reproduce with one another. The reigning model of reproductive isolation holds that genetic differences accumulate between populations that render their hybrid offspring dead or sterile, like the mule, an infertile child of the donkey and horse. Many researchers have long assumed that this isolation must be the result of changes in gene sequence that introduce an incompatibility between groups, such as new sperm that do not recognize the old eggs. But examples of such speciation genes are very few.
Hoping to uncover a fresh example, researchers hybridized two species of fly, Drosophila melanogaster and Drosophila simulans, which would normally produce sterile males but were modified to yield modestly fertile females capable of breeding with a regular male melanogaster. Prior studies had shown that a melanogaster becomes sterile from immobile sperm if it incorporates a small part of the simulans genome--specifically, if it has a pair of fourth chromosomes from simulans instead of its own pair. By breeding hybrid flies for several generations, the group re-created such minimal hybrids. To discover which gene on the fourth chromosome of the simulans is responsible for neutralizing sperm, the group mated them to melanogaster flies missing different genes from their fourth chromosomes. Most deletions would spare the essential gene and lead to fertile offspring; only a fly lacking that gene would sire sterile children again. The group found the crucial gene, JYAlpha, but to their great surprise, the corresponding simulans gene is not on the fly's fourth chromosome but on its third. "It was completely unexpected," says John Masly of the University of Rochester, lead author of the report published in the September 8 Science.
Because each species carries the gene on a different chromosome, hybrids can end up without any copy of the essential gene. The result shows that reproductive isolation can begin to evolve without a change in the function of a gene, says Masly. Some take satisfaction in the finding. "For me it was a great result," says Michael Lynch of Indiana University, who recently revived the mechanism. "The fly people have been extremely skeptical of this model. It's probably going to dramatically change people's views of speciation."