By Jeanna Bryner Staff writer
A population of butterflies has evolved in a flash on a South Pacific island to fend off a deadly parasite.
The proportion of male Blue Moon butterflies dropped to a precarious 1 percent as the parasite targeted males. Then, within the span of a mere 10 generations, the males evolved an immunity that allowed their population share to soar to nearly 40 percent — all in less than a year.
“We usually think of natural selection as acting slowly, over hundreds or thousands of years," said study team member Gregory Hurst, an evolutionary geneticist at the University College London. "But the example in this study happened in a blink of the eye, in terms of evolutionary time."
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The scientists think the males developed genes that hold a male-killing microbial parasite, called Wolbachia, at bay.
The results, detailed in the July 13 issue of the journal Science, illustrate the power of positive natural selection on “suppressor” genes that thwart the lethal bacteria, allowing the male butterflies to bounce back.
Male killers
Sylvain Charlat of the University of California, Berkeley, and the University College London, along with colleagues, studied the sex ratios of Hypolimnas bolina butterflies on the Samoan islands of Upolu and Savaii, where males had dwindled to 1 percent of the populations in 2001.
The likely culprit was a male-killing parasite, Wolbachia, which lives inside the butterfly’s reproductive cells, preferably female sex cells. With a female host, Wolbachia can hitch a ride to the next generation aboard the mother’s eggs. Since males are “useless” for the bacteria's survival, the parasite kills male embryos.
But the male butterflies found a way to stealthily overcome the parasites. At the beginning of 2006, the scientists found the males made up about 40 percent of Upolu’s butterfly population.
On Savaii, females still dominated the Blue Moon butterfly population (99 percent) at the start of 2006, but by the year’s end, males made up nearly 40 percent.
Secret weapon
The team ran genetic analyses to see if the parasite had somehow vanished. It hadn’t. Wolbachia was still present in butterflies from both islands. Other lab experiments indicated the males had evolved suppressor genes to shield against the parasite.
Unlike genetic tweaks that alter wing color or antennae length, mutations that affect a population’s sex ratio can have a significant impact on the biology of a species, the scientists say.
“The suppressor gene allows infected females to produce males,” Charlat said. “These males will mate with many, many females, and the suppressor gene will therefore be in more and more individuals over generations."
The study was funded by the U.S. National Science Foundation, the U.K. Natural Environment Research Council and the Natural Sciences and Engineering Research Council of Canada.
© 2007 LiveScience.com. All rights reserved.
''H. bolina'' is  known colloquially as the Blue Moon Butterfly or Great or Common Eggfly and is found mainly in the South Pacific.
Scientists reported that they had documented very fast evolution in the butterfly species ''Hypolimnas bolina''. After infection by ''Wolbachia'', the fraction of the population that was male dropped drastically to about one percent of the total population. However, after approximately ten generations (about a year) the male population had rebounded to about 39 percent of the overall population. 

In the case of ''H. bolina'', infected females were unable to have male offspring since the male embryos died early on. However, a gene arose which suppressed ''Wolbachia'''s ability to kill the male offspring, and this gene spread rapidly through the natural ''H. bolina'' population.

At this time, it is unknown if the novel gene was a mutation or a pre-existing gene. However, researchers said that regardless, the findings constituted strong evidence that parasites can drive and substantially alter evolution. The research was performed by scientists at the University of Berkeley lead by Sylvain Charlat, a post-doc at Berkeley.

Public Library of Science / Scott O'Neill

''Wolbachia'' is a bacterium that is very effective at jumping from species to species, generally infecting arthropods such as insects. Since the bacterium resides in the cytoplasm of cells, males cannot pass ''Wolbachia'' onto their offspring because sperms only pass on their nucleus with the DNA, but females can. ''Wolbachia'' has thus adapted a number of strategies, such as killing male embryos of infected females or preventing infected males from reproducing with uninfected females.

''Wolbachia'' is dangerous to hosts because it is able to easily jump species barriers and so has little reason to restrain itself -if an infectious organism kills its host, it generally dies along with it. The bacterium is believed to be responsible for some extinction events as well as some speciation events.