Hornett, E.A.; (2007) Interactions between the inherited bacterium Wolbachia and the butterfly Hypolimnas bolina. Doctoral thesis , University of London.

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Abstract

Many species of arthropod are infected by matrilineally inherited bacteria. One of these bacteria, Wolbachia, has received much attention because it is common and also has a variety of interactions with its host. Wolbachia may therefore be an important driver of host evolution and ecology. However, the dynamics of Wolbachia-host interactions are poorly understood, except in theoretical terms. In this thesis, I demonstrate that the presence of a male-killing strain of Wolbachia in the butterfly Hypolimnas bolina has selected for a host modifier that suppresses the male- killing action in Southeast Asian populations. I further demonstrate that suppression of male-killing reveals the Wolbachia to have a second phenotype, namely induction of cytoplasmic incompatibility. This new phenotype explains the persistence of the infection following the evolution of male-killer suppression. Mathematical simulations of the dynamics of suppression indicate that novel suppressors spread rapidly - agreeing with historical data that suggest the presence of male-killing in Southeast Asia in the recent past. Ancient H. bolina DNA is utilised to gain further information about historical processes in this system. Flux in the Wolbachia-H. bolina interaction is demonstrated - with the infection rising and falling in prevalence in different host populations. A search for a third phenotype, a direct benefit, provides no clear evidence of the existence of such an effect in this system. The thesis concludes with a general discussion in which it is argued that Wolbachia-bost interactions may be more dynamic than previously recognised, with strong selection driving the spread of new mutations through the host population, modifying the phenotype produced by the bacterium. The system has in the past experienced high sex ratio bias due to the presence of male-killing Wolbachia, with concomitant effects on host ecology. My thesis predicts this phenotype will become largely lost in the next 100 years from this species.

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