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Tremough biologist charts rise of male butterflies

A biologist on the Tremough Campus in Penryn has helped to discover a remarkable example of natural selection in a tropical butterfly. Along with scientists from London, Toronto and California, Dr Nina Wedell of the University of Exeter's Tremough Campus has tracked the amazingly rapid recovery of male butterflies in the species Hypolimnas bolina, commonly known as the 'blue moon' butterfly.

The blue moon butterfly, which lives on the South Pacific island of Savaii, has been affected by the Wolbachia bacteria, which is passed down from the mother and kills males before they have a chance to hatch. Males had been reduced to just one percent of the population of the species, but this has risen to 39 percent in less than ten years and within just ten generations. The researchers credited this increase to the rise of a suppressor gene that holds bacteria in check.

"To my knowledge, this is the fastest evolutionary change that has ever been observed in the wild," said Dr Nina Wedell of the University of Exeter's Tremough Campus. "This study shows that when a population experiences very intense selective pressures, such as an extremely skewed sex ratio, evolution can happen very fast."

It is not yet clear whether the suppressor gene emerged from a chance mutation from within the local population, or if it was introduced by migratory Southeast Asian butterflies in which the mutation had already been established. Bacteria that selectively kill male offspring are found among a range of arthropods, so what was seen in this study may not be unusual, despite the fact that it has never before been observed.

"In essence, organisms must evolve to stay in the same place, whether it's a predator-prey relationship, or a parasite-host interaction," said Dr Wedell. "In the case of H. bolina, we're witnessing an evolutionary arms race between the parasite and the host. This strengthens the view that parasites can be major drivers in evolution."

The researchers tested for the continued presence of Wolbachia in the butterflies. By mating infected females with males from a different island that did not have the suppressor gene, they also confirmed that the bacteria were still effective at killing male embryos. The male-killing ability of the bacteria emerged again after three generations. Thus, they could rule out a change in the bacteria as an explanation for the resurgence of the males in the butterfly populations studied.

This study has been published in leading journal Science. The US National Science Foundation, the UK Natural Environment Research Council and the Natural Sciences and Engineering Research Council of Canada helped support this research.

The £100 million Tremough campus is a Combined Universities in Cornwall initiative of which the University of Exeter and University College Falmouth are two of the founding partners. It is funded mainly by the European Union (Objective One), the South West Regional Development Agency, and the Higher Education Funding Council for England, with support from Cornwall County Council. Set in 70 acres of countryside, but close to the waterside towns of Penryn and Falmouth, the campus offers a lively student community. The University of Exeter now offers degrees in Biology, Cornish Studies, English, Geology, Geography, History, Law, Mining Engineering, Politics and Renewable Energy on its Tremough Campus, which has expanded rapidly as part of the Combined Universities in Cornwall initiative.

For further information contact Sarah Hoyle, Press Officer, University of Exeter on 01392 262062/07989 446920 or email

The Objective One Programme for Cornwall and the Isles of Scilly has invested in the Combined Universities in Cornwall (CUC) project, both Phase 1 and Phase 2, through the European Regional Development Fund (ERDF) and the European Social Fund (ESF). University of Exeter is a partner of the CUC.


Editor's notes:



Clare Morgan
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