| Symbiotic microorganisms are widespread in nature and often form intimate associations with their hosts. These associations can serve as a source of evolutionary novelty for hosts and lead to ecological adaptations as well as rare, but major evolutionary transitions. It has been suggested that symbionts can also promote speciation—key evolutionary process by which one species divides into two. This view is controversial, as speciation typically proceeds through mutations in nuclear genes. Recent investigations into a genus of cytoplasmically inherited bacteria are, however, shedding new light on this controversy. Wolbachia are bacterial symbionts that occur within the cells of reproductive tissues in millions of insect species and nearly all filarial nematodes. Wolbachia are particularly good candidates for driving speciation since they can reduce gene exchange between host species by altering compatibility between egg and sperm of the host through a mechanism termed cytoplasmic incompatibility (CI). This thesis examines the link between these bacteria and speciation in three closely related wasp species. Two strains of Wolbachia infect each of the species, comprising a total of six strains in the genus. In this thesis, it is shown that crosses between the species with infected individuals yield few hybrid offspring, while crosses with uninfected individuals produce normal numbers of offspring. Thus, Wolbachia-induced CI can limit gene flow between these species. It was also found that one species pair is so closely related that it shows no other genetic incompatibilities, suggesting that CI can potentially play a causal role in barring gene flow between them. Results from experimental populations complement these findings by establishing that CI can severely reduce levels of gene exchange between populations over many generations. Studies also show that genome interactions between host and Wolbachia can occur. Introgression lines were generated that were comprised of the Wolbachia from one species and the genetic background of a different species. Levels of CI, as well as the type of CI, varied according to the species genetic background that the bacteria reside in. The findings suggest that coevolutionary interactions between symbiont and host may also accelerate divergence between species and promote speciation. |
