Worms and flies share a sexy secret.

From the creation of the overall body plan to the formation of specific organs, widely divergent animals depend on similar developmental genes. This unexpectedly close correspondence, revealed over the last decade, has highlighted a genetic ancestry shared by worms, insects, and mammals--including people.

Yet when it comes to the genes that make an animal male or female, it appeared that those same groups of animals picked different sets of genes. For example, the genetic cascade that transforms an asexual embryo into a male or female fruit fly showed no similarity to the sex-determination cascade in the worm Caenorhabditis elegans.

Now, David Zarkower of the University of Minnesota Medical School in Minneapolis and his coworkers have identified a gene called mab-3 that governs aspects of male development for C elegans, which is either male or hermaphroditic. Mutations in this gene eliminate sensory bristles called V rays, which a male needs to mate with a hermaphrodite.

To the surprise of Zarkower and his colleagues, mab-3 closely resembles doublesex, a gene intimately involved in the sex determination of the fruit fly. Both genes encode transcription factors--proteins that turn other genes on and off.

"There are clearly fundamental differences between the two systems, but it's interesting to think there are at least some shared remnants, of which we may now have a hint," says Nipam H. Patel of the University of Chicago.

Neither gene is at the beginning of the sex-determination cascade, but their similarity suggests that additional genes in the cascade may also be similar, says Zarkower. He has begun to look at other animal groups to determine whether they have their own version of mab-3.

Other investigators are examining whether C. elegans contains a gene similar to a fruit fly gene called fruitless, which is also involved in sex determination.