Alien Wolbachia terrorise law-abiding insects shock!

 John Brookfiled

 The population genetics of many species of insects has been interestingly complicated by the presence of Wolbachia - a Rickettsial bacterium. Entirely maternally inherited, these intracellular bacteria are capable of spreading through populations as a result of a phenomenon called cytoplasmic incompatibility (CI) which has been described in numerous insects. CI is the death of embryos that occurs when eggs from females lacking Wolbachia are fertilised by sperm from males that possess the bacterium. Protection of their offspring from CI gives a fertility advantage to females with the bacterium. The population dynamics of the expected spread are unusual - the rate of spread of the parasite increases with its frequency. The prevalence of Wolbachia-driven CI is expected to have rather interesting consequences for the population genetics of the host species. A mitochondrial DNA variant, for example, associated with the same female cytoplasm as the Wolbachia might spread deterministically, in a way suggesting that the mitochondrial DNA itself had a selectively advantageous mutation. Two recent papers in Heredity throw further light on the biology of the interaction between Wolbachia and their hosts. Poinsot et al. (Heredity 85: 191-198) report studies on the dynamics of the bi-infection of Drosophila simulans by two strains of Wolbachia, belonging to the two major divisions A and B of the insect Wolbachia tree, and estimated to share common ancestry sixty million years ago. In D. simulans the strains have been called wHa and wNo, and wild individuals have been shown to be of three types, uninfected, those bearing wHa alone, and those bearing both strains. By successive crosses of infected females to uninfected males, the authors were able to measure the rate of random loss of the Wolbachia of the two strains, both when they were together and separate. The experiment was carried out crossing infected females to different male genotypes, with significantly different outcomes. Unlike the situation in the wild, flies with only the wNo strain are generated in these experimental crosses, showing that there is no requirement for wHa in order for the wNo strain to survive. Measurement of rates of loss of the bacteria show, however, that wHa is lost less frequently than is wNo, both when on its own and in bi-infections. What, then, is the explanation for the absence of pure wNo strains in the wild? Modelling implies that, if different D. simulans are infected by different strains of Wolbachia, then if one strain has both a higher rate of stochastic loss, and a lower induction of CI (which is true of the wNo strain here) only the wHa should persist at equilibrium. However, in a bi-infection, wNo-bearing females suffer no CI when crossed to wHa males, since their wHa Wolbachia protect them from the CI induced by wHa bearing males. Thus, Poinsot et al. suggest that the wNo strain can perhaps be viewed as a parasite of the wHa strain, which itself is a parasite of the flies. Another area of mystery in Wolbachia biology is the mechanism of determinism of the level of CI. In some species (such as Drosophila melanogaster) the level of CI induced by Wolbachia is low, while in others, such as D.simulans, it is high. Since tetracycline treatment will remove the parasites, and microinjection can reintroduce them, one can straightforwardly, by moving parasites between hosts, establish whether the level of CI is determined by the host or the parasite genotype. Experiments in Drosophila have been inconsistent, however, sometimes implicating the bacterium in the determination of CI level, sometimes implicating the host. Now, Sasaki and Ishikawa (Heredity 85: 130-135) report a similar experiment in the flour moth Ephestia kuehniella. Comparing two different strains of host from different Japanese locations, with differing rates of CI, they show clearly that, in this case, the strength of CI depends on the bacterial genotype and not that of the host. Clearly, the more consistently CI depends on the bacterium the more efficacious Wolbachia might be as a mechanism for experimentally driving novel cytoplasms through populations of naive hosts.