Scientists trace evolution of butterflies infected with deadly bacteria

10 September 2009: Scientists at the University of Liverpool have traced the evolution of a species of tropical butterfly, infected with a bacterium that kills males, by comparing current butterfly populations with more than 200 museum specimens. The bacteria, called Wolbachia, are a parasitic microbe and are known to significantly alter the reproductive capabilities of a high proportion of insect species, including wasps and fruit fly. The team found that the male-to-female ratio in butterfly populations fluctuated rapidly due to the interaction between the species and the bacteria over time and geographical space.

In the butterfly, Hypolimnas bolina, these bacteria are passed to offspring through the mother and kill the males as they develop in the egg. This results in the butterfly population having a biased sex-ratio, with a high number of females reproducing with very few males.

Using forensic DNA techniques to analyse specimens from the 1800s and 1900s, small samples of DNA were taken from the legs of both female and male butterflies and tested for the presence of the bacterium. The frequency and activity of the bacteria in the population was estimated and the results were compared to modern day samples so that the evolution in each population could be directly observed.

The team found that in some butterfly populations a gene had evolved that suppresses the effects of the bacteria so that infected males survive, but in other populations in the South Pacific for example, the frequency of the bacterium varied greatly and could rise to extreme levels. The result was that the sex-ratio of a population changed rapidly from a balanced male/female population to female-biased, which can alter the butterfly's behaviour and reproductive biology.

Professor Greg Hurst, from the University's School of Biological Sciences, said: "The butterfly has fascinated scientists for more than a century and was heavily collected and studied during the Victorian period and early 20th century. The butterfly, with its many colour patterns, illustrated variety within a species and was therefore a good model for 19th century scientists studying Darwin's theory of evolution. Today we can benefit from this early interest through museum collections, where we can now use the latest DNA technology to understand how species have evolved across time and geographical space."

Dr Emily Hornett added: "Before the 1990s it was practically impossible to extract DNA from historical specimens. Recent developments in forensic science, however, have allowed us to utilise new technologies to study museum collections and build a detailed picture of a species' evolution over a long period of time. By analysing DNA samples from the legs of butterflies living a century ago, and comparing these with contemporary samples, we were able to directly observe evolution between a butterfly and the bacteria that infects them."

Scientists are now working to locate the 'suppressor gene' in the genome in order to understand how the butterfly has evolved in response to the harmful effects of the bacteria.

Source: University of Liverpool