Sex races \SEHKS REHY-sez\ pl.n.

Groups within a species that differ dramatically in how gonads develop.

The best-studied examples are the three sex races of Rana temporaria frogs, a species found from Spain to Norway. In the milder southern climates, virtually all new froglets emerge from tadpolehood with ovaries. Only later do about half of them replace their ovaries with testes. In the north, however, another sex race hits the transition to four-legged life with gonads of males or females already in place in about equal proportions. And between these geographic extremes hops a race that varies considerably in proportions of the sexes among new froglets.

A new look at the genetics of these sex races suggests they differ fundamentally in mechanisms that determine an individual's final sex, says Nicolas Rodrigues of the University of Lausanne in Switzerland. The ovaries-first population shows no consistent differences between adult males and females in sex-determining genes. So these frogs may be relying on environmental cues to differentiate the sexes. In contrast, the straight-to-final-gonads froglets do sort out genetically, with males consistently different from females, Rodrigues and colleagues say in the May 7 Proceedings of the Royal Society B. What happens in between the extremes is more complicated. "That's what makes it interesting," Rodrigues says.

Caption: Races of the frog Rana temporaria vary In their potentially sex-reversing lifestyles.

----------

Please note: Illustration(s) are not available due to copyright restrictions.