A gene that keeps species apart

Nearly 150 years after Darwin published On the Origin of Species, biologists are still debating how new species emerge from old–and even the definition of species itself. Darwin demurred from offering a hard and fast definition, suggesting that such a thing was “undiscoverable.”

In this issue of PLoS Biology, Daniel Barbash and colleagues identify a true speciation gene in the fruitfly Drosophila.

One of the more enduring definitions characterizes organisms as distinct reproductive units and species as groups of individuals that can interbreed and produce viable, fertile offspring. The lack of genetic exchange between species, called reproductive isolation, lies at the heart of this definition.

At the heart of reproductive isolation is a phenomenon called hybrid incompatibility, in which closely related species are capable of mating but produce inviable or sterile offspring.

The classic example of hybrid incompatibility is the male donkeyfemale horse cross, which yields a sterile mule, but many other cases have been documented among mammals, and thousands of plant crosses produce infertile offspring.

To elucidate the molecular mechanisms of reproductive isolation, biologists must first identify candidate hybrid incompatibility genes.

Species- or lineage- specific functional divergence is an essential trait of these genes. (That is, the genes evolve different functions after the species diverge from their common ancestor.) While several such candidate genes have been identified in the fruitfly Drosophila melanogaster, none has been shown to display this functional divergence.

Now, working with D. melanogaster and its sibling species D. simulans and D. mauritiana, Daniel Barbash, Philip Awadalla, and Aaron Tarone establish the functional divergence of a candidate hybrid compatibility gene and confirm its status as a true speciation gene.

Citation: Barbash DA, Awadalla P, Tarone AM (2004) Functional divergence caused by ancient positive selection of a Drosophila hybrid incompatibility locus. PLoS Biol 2(6):e142. DOI: 10.1371/journal.pbio.0020142.

CONTACT:
Daniel Barbash
University of California, Davis
Davis, CA, U.S.A.
530-752-4253
dabarbash@ucdavis.edu