ern Mexican coastline. In the lab, researchers can coax successful conjugal visits between members of the two groups. But even under laboratory conditions hybrid crosses aren't always fruitful. D. mojavensis mothers typically produce healthy offspring after mating with D. arizonae males, but when D. arizonae females mate with D. mojavensis males, all of the resulting hybrid sons are sterile. This partial capacity for interbreeding, Reed says, suggests that these flies are on the verge of evolving to become completely separate species.

Another finding adds support to that notion. Researchers had previously reported that for one strain of D. mojavensis, from Catalina Island, off the southern California coast, mothers always produce sterile sons when crossed to D. arizonae males.

Because the hybrid male sterility trait depends on the mother's genetic heritage, Reed and Markow concluded that the genetic change--polymorphism, in evolutionary biology parlance--responsible for creating sterile sons must not yet be "fixed," or firmly established in D. mojavensis populations. And that is a telltale sign that the change was recent.

Reed wanted to know just how deeply the polymorphism causing male sterility had suffused Catalina Island D. mojavensis populations. In other words, do all or just some of the Catalina Island mothers produce sterile sons when mated to D. arizonae males? When she did the experiment, she found that only about half the crosses resulted in sterile sons. That result implies that only half the females in the Catalina Island population had the gene (or genes) for hybrid male sterility.

Surprisingly, when she tested D. mojavensis females from other geographic regions, she found that a small fraction of those populations also exhibited the hybrid male sterility polymorphism. "That polymorphism exists in every population I looked at," Reed said. "It just happens to be that whatever factors are causing sterility are at higher
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7-Jun-2004

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