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

Abstract

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-killingWolbachiain natural populations ofDrosophila innubilafemales from the Chiricahua Mountains of Arizona. We find that there is substantial (20 000-fold) variation inWolbachiadensity among wild flies and that within-hostWolbachiadensity is positively correlated with both the efficacy of male killing and maternal transmission fidelity. MeanWolbachiadensity increases three- to five-fold from early to late in the season. This pattern suggests thatWolbachiadensity declines with fly age, a conclusion corroborated by a laboratory study ofWolbachiadensity as a function of age. Finally, we suggest three alternative hypotheses to account for the approximately lognormal distribution ofWolbachiadensity among wild flies.