By Dr David Whitehouse

BBC News Online science editor



Fruit flies are used to test ideas in biology

Biologists Laura Reed and Prof Therese Markow made the discovery by observing breeding patterns of fruit flies that live on rotting cacti in deserts.

The work could help scientists identify the genetic changes that lead one species to evolve into two species.

The research is published in the Proceedings of the National Academy of Sciences.

One becomes two

Whether the two closely related fruit fly populations the scientists studied - Drosophila mojavensis and Drosophila arizonae - represent one species or two is still debated by biologists.

However, the University of Arizona researchers believe the insects are in the early stages of diverging into separate species.

The emergence of a new species - speciation - occurs when distinct populations of a species stop reproducing with one another.

Graduate student Laura Reed

Though speciation is a crucial element of understanding how evolution works, biologists have not been able to discover the factors that initiate the process.

In fruit flies there are several examples of mutant genes that prevent different species from breeding but scientists do not know if they are the cause or just a consequence of speciation.

Sterile males

In the wild, Drosophila mojavensis and Drosophila arizonae rarely, if ever, interbreed - even though their geographical ranges overlap.

In the lab, researchers can coax successful breeding but there are complications.

Drosophila mojavensis mothers typically produce healthy offspring after mating with Drosophila arizonae males, but when Drosophila arizonae females mate with Drosphila mojavensis males, the resulting males are sterile.

Laura Reed maintains that such limited capacity for interbreeding indicates that the two groups are on the verge of becoming completely separate species.

Another finding that adds support to that idea is that in a strain of Drosophila mojavensis from southern California's Catalina Island, mothers always produce sterile males when mated with Drosophila arizonae males.

Because the hybrid male's sterility depends on the mother's genes, the researchers say the genetic change must be recent.

Reed has also discovered that only about half the females in the Catalina Island population had the gene (or genes) that confer sterility in the hybrid male offspring.

However, when she looked at the Drosophila mojavensis females from other geographic regions, she found that a small fraction of those populations also exhibited the hybrid male sterility.

The newly begun Drosophila mojavensis genome sequencing project, which will provide a complete roadmap of every gene in the species, will help scientists pin down which genes are involved in speciation.