How many species are infected with Wolbachia? Cryptic sex ratio distorters revealed to be common by intensive sampling

Inherited bacterial symbionts from the genus Wolbachia have attracted much attention by virtue of their ability to manipulate the reproduction of their arthropod hosts. The potential importance of these bacteria has been underlined by surveys, which have estimated that 17% of insect species are infected. We examined whether these surveys have systematically underestimated the proportion of infected species through failing to detect the low–prevalence infections that are expected when Wolbachia distorts the sex ratio. We estimated the proportion of species infected with Wolbachia within Acraea butterflies by testing large collections of each species for infection. Seven out of 24 species of Acraea were infected with Wolbachia. Four of these were infected with Wolbachia at high prevalence, a figure compatible with previous broad–scale surveys, whilst three carried low–prevalence infections that would have had a very low likelihood of being detected by previous sampling methods. This led us to conclude that sex–ratio–distorting Wolbachia may be common in insects that have an ecology and/or genetics that permit the invasion of these parasites and that previous surveys may have seriously underestimated the proportion of species that are infected.