Whether froglets switch sexes distinguishes ‘sex races’

Amphibian starts all female in one region of Europe; in another, with gonads of either sex

common frog

OFF TO THE RACES  What’s called the common frog in Europe (shown here  in Switzerland) is one widespread species even though different races vary in their potentially sex-reversing lifestyles.

Andreas Meyer

Sex races
SEHKS REHY-sez pl. n.

Groups of organisms within a single 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 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 relies on environmental cues that create males and females, with no consistent genetic differences between the sexes. 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 is more complicated.  “That’s what makes it interesting,” Rodrigues says.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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