Genetic conflicts over sex ratio: mite-endosymbiont interactions.

Nucleocytoplasmic genetic conflicts arise as a result of asymmetric transmission of cytoplasmic and nuclear genes. Spread of a cytoplasmic element promoting female-biased sex ratios creates selection on nuclear genes for mechanisms that decrease the bias. Here we investigate the conflict over sex ratio between the cytoplasmic bacterium Wolbachia and the two-spotted spider mite Tetranychus urticae Koch. We show that, first, infected females produce significantly more female-biased sex ratios than uninfected (cured) females. Second, this effect is not due to parthenogenesis, male killing, or feminization, phenotypes commonly associated with infection by Wolbachia. Third, sex ratio is a trait with a heritable component in this species; thus, it can evolve under selection. Fourth, the sex ratio produced by uninfected (cured) females changes over time, approaching the sex ratio produced by females from the infected culture. On the basis of these results, we suggest that after sex ratio manipulation by Wolbachia, a host compensatory mechanism evolved that allows infected females to produce the sex ratio favored by nuclear genes. We discuss the evolution of "mutualism" with respect to the evolution of host mechanisms that compensate for effects induced by vertically transmitted "parasites."

On the evolution of cytoplasmic incompatibility in haplodiploid species.

The most enigmatic sexual manipulation by Wolbachia endosymbionts is cytoplasmic incompatibility (CI): infected males are reproductively incompatible with uninfected females. In this paper, we extend the theory on population dynamics and evolution of CI, with emphasis on haplodiploid species. First, we focus on the problem of the threshold to invasion of the Wolbachia infection in a population. Simulations of the dynamics of infection in small populations show that it does not suffice to assume invasion by drift alone (or demographic "accident"). We propose several promising alternatives that may facilitate invasion of Wolbachia in uninfected populations: sex-ratio effects, meta population structure, and other fitness-compensating effects. Including sex-ratio effects of Wolbachia allows invasion whenever infected females produce more infected daughters than uninfected females produce uninfected daughters. Several studies on haplodiploid species suggest the presence of such sex-ratio effects. The simple metapopulation model we analyzed predicts that, given that infecteds are better "invaders," uninfecteds must be better "colonizers" to maintain coexistence of infected and uninfected patches. This condition seems more feasible for species that suffer local extinction due to predation (or parasitization) than for species that suffer local extinction due to overexploiting their resource(s). Finally, we analyze the evolution of CI in haplodiploids once a population has been infected. Evolution does not depend on the type of CI (female mortality or male production), but hinges solely on decreasing the fitness cost and/or increasing the transmission efficiency. Our models offer new perspectives for increasing our understanding of the population and evolutionary dynamics of CI.

Induction of preference and performance after acclimation to novel hosts in a phytophagous spider mite: adaptive plasticity?

We examined induction of preference and performance on novel host plants for two laboratory populations of the polyphagous spider mite Tetranychus urticae, with one population adapted to bean and the other population adapted to tomato. We bred four isofemale lines of the bean population only and used them in all the assays. The bean population had a 30% lower fecundity on tomato than on bean, while the tomato population had equal fecundity on both host plants. Acclimation of adult females to the novel host plant for both populations increased acceptability of that novel host but did not increase rejection of the original host. The bean population experienced a 60% benefit and a 30% cost in terms of egg production for acclimating to tomato, thus exemplifying adaptive plasticity. The tomato population showed a 23% benefit for acclimating to bean but no cost. Mites from the bean population that were acclimated to tomato fed more on tomato than did mites that were not acclimated to tomato. When these mites were fed inhibitors of cytochrome P-450 detoxification enzymes, their performance was severely depressed (84%) on tomato but not on bean. However, mites that were fed inhibitors of P-450 enzymes did not reduce their acceptance of tomato as a host. Thus, performance on novel hosts (but not preference) in this species is likely correlated with the induction of detoxifying enzymes. Spider mites are known to form host races rapidly on novel hosts. Induction of preference and physiological acclimation via detoxification enzymes may enhance performance and, thus, strongly contribute to initial stages of host race formation.