The risks of sex (well, one of them, anyway)

That got your attention, didn't it?

Being male can be a risky business. In a butterfly called Hypolimna bolina, there's a rather nifty bacterial parasite called Wolbachia that has unfortunate effects on male butterflies. It's carried down the maternal line and kills off male embryos. Just the males. This can result in some pretty skewed sex ratios. (And presumably an interesting, albeit exhausting, time for the surviving males.)

In 2001, scientists working in the Solomons found that on several islands the H. bolina populations were 99% female. But to their surprise, a survey only 4 years later found that males had become much more common on some islands, while on others they were still vanishingly rare. What was going on? This year the researchers published the results of an investigation into the butterflies and their bacterial parasite (S. Charlat et al. [2007] "Extraordinary flux in sex ratio". Science 317:214).  All the females they collected from an island where males had become more common, produced sons – and the sex ratio in their offspring was close to 1:1. Yet all the butterflies, male and female, were still carrying the Wolbachia parasite. Had the butterflies somehow acquired an immunity to the parasite? Or had the parasite itself changed?

PCR analysis showed that the strain of the bacterium was the same in all the butterfly populations, regardless of whether or not the males were surviving. And this strain hadn't changed from the earlier study. Further experiments indicated that it was the butterflies that had evolved, with a mutation that suppressed the effects of the parasite. Any butterfly with a mutation suppressing Wolbachia's effects would be at a very strong selective advantage – producing more sons who would in turn would successfully produce sons. This seems to have driven a rapid evolutionary change in the population – from <1% males to almost 50% males, in only 10 generations. Evolution isn't necessarily a glacially slow process!

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