Y chromosomes have a problem: as they cannot (usually) recombine with X chromosomes, they tend to degenerate due to progressive accumulation of deleterious mutations. Yet, in most lower vertebrates, sex chromosomes are undifferentiated (homomorphic). In other words, the expected degeneration of Y is not occurring. Why?
In a nice paper published on the pages of Evolution, Nicola Perrin puts forth a interesting hypothesis: occasional sex reversals known to occur e.g. in common frogs (Rana temporaria) might serve as fountains of youth for Y chromosomes. How?
Sex-specific recombination patterns appear to depend on phenotypic, rather than genotypic sex. Therefore, homomorphic X and Y chromosomes are expected to recombine in sex-reversed females. This could serve as mechanism to counteract Muller’s ratchet and prevent the evolutionary decay of Y chromosomes goes the argument.
And how does Kilpisjärvi fit the picture? Through the fact that sex reversal appear to occur in Kilpisjärvi common frogs (see here). In fact, Nicola uses data on Kilpisjärvi frogs to illustrate his arguments.
Perrin N (2009) Sex reversal: fountain of youth for sex chromosomes. Evolution, in press. 10.1111/j.1558-5646.2009.00837.