A new study of the European common frog, Rana temporaria, published recently in Molecular Biology and Evolution, offers some fresh insights on sex-chromosome evolution.
The prevailing view is that the Y chromosome is shrinking in males because XY recombination is suppressed by either chromosomal inversions or nonhomologous regions. In frogs however, genetic control over sex determination can vary both within and among populations, resulting in the occurrence of regular sex reversals.
Now, Nicolas Perrin from the University of Lausanne, says he has found the first direct evidence that these sex reversals help maintain the Y over evolutionary time. One of the keys to the study was finding among the breeding adults not only a few sex-reversed XX males, but also an extremely rare, XY sex-reversed female.
Using 16 sex-linked genetic markers, the team measured the number of recombination events, or chromosome crossovers that occurred in their progeny. They found that recombination in sex chromosomes depends on the phenotypic sex, but not on the genetic sex. All that seems to matter is being male or female.
"As we show here by including sex-reversed males and females in the analysis, recombination patterns are entirely explained by phenotypic sex, with no detectable effect of genotypic sex, either alone or in interaction. Males, in particular, had strongly reduced recombination independent of sex-chromosome differentiation," said Perrin.
"More importantly (as far as evolutionary consequences are concerned), our present results provide the first direct field evidence that X and Y chromosomes recombine in XY females," said lead author Nicolas Rodrigues.
The researchers concluded that occurrence of XY-sex reversed female followed by recombination between X and Y chromosomes may be involved in maintaining the state of sex chromosomes in this species and others as well.
"Our results, therefore, bring strong support for the fountain-of-youth model, by showing that X and Y chromosomes recombine in naturally-occurring sex-reversed XY females," said Perrin. "This result has the potential to account for the lack of sex-chromosome differentiation in Rana temporaria, and by extension in other lineages of amphibians, fishes and non-avian reptiles where sex reversal events have also been documented."
Image: A new study of the European common frog, Rana temporaria, offers some fresh clues that challenge the conventional scientific wisdom on sex-chromosome evolution. Image courtesy of Nicholas Perrin.