Wolbachia as populations within individual insects: causes and consequences of density variation in natural populations

The population-level dynamics of maternally transmitted endosymbionts, including reproductive parasites, depends primarily on the fitness effects and transmission fidelity of these infections. Although experimental laboratory studies have shown that within-host endosymbiont density can affect both of these factors, the existence of such effects in natural populations has not yet been documented. Using quantitative PCR, we survey the density of male-killing Wolbachia in natural populations of Drosophila innubila females from the Chiricahua Mountains of Arizona. We find that there is substantial (20 000-fold) variation in Wolbachia density among wild flies and that within-host Wolbachia density is positively correlated with both the efficacy of male killing and maternal transmission fidelity. Mean Wolbachia density increases three- to five-fold from early to late in the season. This pattern suggests that Wolbachia density declines with fly age, a conclusion corroborated by a laboratory study of Wolbachia density as a function of age. Finally, we suggest three alternative hypotheses to account for the approximately lognormal distribution of Wolbachia density among wild flies

Parasitic elements, extinction and adaptation : theoretical and empirical investigations

Thesis (Ph. D.)--University of Rochester. Dept. of Biology, 2011. Chapters 1-2 were co-authored with H. Allen Orr. Chapter 3 was co-authored with John Jaenike, Jeremy K. Herren, and Lisa M. Boelio. Chapter 4 was co-authored with John Jaenike. John Jaenike assisted with writing chapter 5.My dissertation research uses several different approaches to investigate evolutionary and evolutionary ecology questions. In Chapter 1, I investigate the likelihood that a population can save itself from extinction via spread of a rare beneficial allele after an environmental change. Although populations can, in principle, save themselves using new beneficial mutations or beneficial mutations from the standing genetic variation, I found, using a population genetic approach, that it is much more likely that they use standing genetic variation. This is particularly true if there are several copies of the beneficial mutation in the standing variation, making it unlikely that all are lost due to chance. In Chapter 2, I derive analytical solutions to predict the trajectory of evolutionary change under natural selection. Given that a population is predominated by a suboptimal genotype and that there are several beneficial mutations available, the analytical solutions in chapter 2 give the probability that the population fixes a particular set of beneficial mutations and the probability that it does so in a particular order. Generally, evolutionary trajectories are more predictable when there is greater variation in selection coefficients of beneficial mutations. My attention turns to empirical work on Drosophila innubila in chapters 3 through 5. About 1/3 of female D. innubila are infected with maternally-transmitted male-killing Wolbachia. In Chapter 3 I find that there is considerable seasonal variation in Wolbachia density in natural populations and that density correlates with both penetrance of male-killing and efficiency of transmission in the wild. Several alternatives for the distribution of densities among flies are discussed. In Chapter 4 I investigate what factors help maintain the Wolbachia infection. To persist, the Wolbachia must either supplement vertical transmission with horizontal transmission or benefit its host through the act of killing males or in some way unrelated to male-killing. I found that neither horizontal transmission nor male-killing-dependent benefits could explain the prevalence of infection. Instead, a combination of male-killing-dependent and -independent factors, including virus protection and nutritional provisioning, likely maintain the infection. Since Wolbachia in D. innubila protect against RNA viruses, as shown in Chapter 4, in Chapter 5 I attempted to determine what viruses are important in natural populations. I begin with a metagenomic survey to discover viruses. The most common novel virus was a DNA virus related to the Nudiviruses. This virus is phylogenetically widespread and is associated with significant fitness costs to its host