Yeast sex: Only for certain partners.

Two independent studies reveal that Candida albicans, the fungus responsible for oral thrush and vaginal yeast infections, can recombine sexually. Although other members of the budding-yeast family have sexual cycles, C. albicans has been classified for 80 years as asexual.

In two papers in the July 14 SCIENCE, researchers at the University of California, San Francisco and the University of Minnesota in St. Paul report that C. albicans can mate, but only under limited circumstances.

C. albicans possesses genes that control mating in related species, but scientists had thought that these genes are inactive in this fungus. The Minnesota researchers, Beatrice B. Magee and Paul T. Magee, altered the growth conditions of the yeast to produce strains lacking one version or the other of a mating gene, MTL. Placed on lab plates, the yeast mated across, but not within, the strains.

The San Francisco team, led by Alexander D. Johnson, demonstrated crossbreeding of MTLa and MTL[Alpha] strains that had been injected into mice. Although it's too early to claim that MTLa and MTL[Alpha] are the only strains of C. albicans that mate, the team says, no evidence exists for any other combinations.

The restriction of mating to these laboratory-created strains, the San Francisco researchers conclude, may explain why sexual behavior has never been observed in wild yeast. Such strains spontaneously arise only rarely and may not exist in the natural population long enough for members of one to come into contact with those of the other.