Hybrid gene forms clue to human, ape origins. (Evolution's DNA Fusion).

A gene of mixed evolutionary pedigree may have transformed mammalian reproduction, leading to the evolution of apes and humans.

Analyses of genetic data from a variety of mammals show that this gene, called Tre2, occurs only in apes and people, say graduate student Charles A. Paulding and geneticist Daniel A. Haber, both of Massachusetts General Hospital Cancer Center in Charlestown, and anthropologist Maryellen Ruvolo of Harvard University.

Although other investigators first identified Tre2 about 10 years ago, the gene's evolutionary origins were unknown. Tre2 represents a hybrid, or so-called chimeric version, of two genes that fused together, Paulding and his coworkers assert in an upcoming Proceedings of the National Academy of Sciences. The DNA sequence of roughly half of Tre2 closely corresponds to an evolutionarily ancient gene still possessed by many species of mammals, the scientists hold. The rest of Tre2's sequence matches a more recently evolved gene found only in monkeys, apes, and people.

Fusion of the two genes must have occurred after the arrival of a common ancestor of apes and humans, between 21 million and 33 million years ago, the scientists theorize.

Although Tre2's two precursor genes both translate into proteins that act on many tissues, Tre2's corresponding protein affects only the testes, Paulding's group finds. If further research implicates Tre2 in sperm function, it will support the possibility that the gene's emergence created reproductive barriers between ancient creatures that did and didn't have it.

In other words, Tre2 may have influenced the evolution of species ancestral to modern apes and humans.

"Tre2 by itself isn't a magic bullet that explains the evolution of ape and human ancestors," says Ruvolo. "This is the beginning of a new line of research into many chimeric genes that characterize different primate species."

Chimeric genes apparently form as part of a DNA-reshuffling process. Many genes contain two or more segments that produce specific proteins.

For instance, unlike its genetic precursors, part of Tre2 codes for a protein that influences cell proliferation, even in animals that don't possess Tre2.

"Many genes have hybrid histories," comments geneticist Pascal Gagneux of the University of California, San Diego. "This interesting new finding is the beginning of an avalanche of information from different laboratories searching for genes specific to humans and apes." When the complete sequence of the chimp genome becomes available later this year, scientists will be able to expand their hunt for hybrid genes, Ruvolo adds.