Butterfly shows fast-track survival skill in evolutionary 'arms race'

A butterfly in the South Pacific has displayed the fastest known rate of evolution in a dramatic "arms race" with a microscopic parasite that kills only males of the species.

Biologists have witnessed how the butterfly has fought back against the parasite by spreading a gene that confers resistance against a bacterium that kills male embryos.

The scientists said the rapid spread of the gene exemplified the Red Queen principle of evolution - named after the character in Lewis Carroll's Through the Looking Glass who ran faster and faster only to stay in the same place.

Normally, the sex ratio of the blue moon butterfly - Hypolimnas bolina - on the Samoan islands of Savaii and Upolu is 50:50. But because of attacks by the bacterial parasite, the proportion of males fell to below 1 per cent, with females making up more than 99 per cent.

In 2006, however, field workers carried out a survey of the butterfly and found that the sex ratio had bounced back to near normal, according to Dr Sylvain Charlat of University College London and University of California, Berkeley. "To my knowledge, this is the fastest evolutionary change ever observed. This study shows that when a population experiences very intense selective pressures, such as an extremely skewed sex ratio, evolution can happen very fast," Dr Charlat said in a report published in the journal, Science.

Scientists found that the parasite was the bacterium Wolbachia which is passed from the mother and selectively kills male embryos before they hatch.

The researchers hatched eggs from females captured during 2006 and found that the males could survive Wolbachia infection to produce sex ratios that were almost normal. They believe the resistance was the result of a "suppressor" gene that had spread among the population, which had allowed males on the island of Savii to increase from 1 per cent to about 40 per cent in less than a year.

The suppressor gene allows infected females to produce males, these males will mate with many, many females and the suppressor gene will therefore be in more and more individuals over generations," Dr Charlet said.

Biologists have known about heavily skewed sex ratios in certain butterfly species since the 1920s, but it was not until 2002, when scientists led by Gregory Hurst of University College London first identified Wolbachia as the culprit.

When sex ratios are altered significantly from the 50:50 norm, natural selection can exert intense pressure on individuals to bring the ratio back to normal. This is why the blue moon butterfly evolved so quickly to get back to the same place, as the Red Queen principle dictates.

"In the case of H. bolina, we're witnessing an evolutionary arms race between the parasite and the host," Dr Charlet said.

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