When opposites don't attract.

The quirks of two kinds of European corn borers are giving researchers another way to study how a single species might split in two.

The classic scenario for forming a new species starts when a geographic barrier, such as a mountain range, emerges and divides a single species.

In recent decades, though, biologists have found populations that seem to be splitting even though they could in theory mingle geographically. For instance, two races within a corn borer species in France live side by side but generally attack different host plants, says Thibaut Malausa of Paul Sabatier University-Toulouse III in France. One race feeds and lays eggs mostly on corn, while the other prefers hops or mugwort.

The researchers used genetic markers and chemical indicators to measure the tendency of individuals to pick mates like themselves. Called assortative mating, this is a critical factor in maintaining genetic differences among geographically mingled races. At four study sites, mate-seeking borers found a partner within their own race about 95 percent of the time.

The researchers note that each race relies on its own pheromone to attract mates, but no one yet knows how these pheromone differences arose. The researchers report their findings in the April 8 Science.--S.M.