My research deals with questions at the interface of ecology and evolution, with a specific focus on host-parasite interactions, X-chromosome meiotic drive, and the contribution of Wolbachia to reproductive isolation and sex-ratio bias in natural populations of Drosophila. Most of this research focuses on various species of mycophagous Drosophila, as these are both amenable to laboratory study and ideal for ecological work in the field.
The endosymbiotic bacteria Wolbachia occur in a wide variety of insects and other arthropods. In several cases that have been examined in the laboratory, Wolbachiaenhance their own transmission by manipulating host reproduction or by causing cytoplasmic incompatibility (CI). We have recently discovered Wolbachia in two species of Drosophila, with the bacteria having very different effects in the two species. In D. recens, Wolbachia cause both intraspecific CI as well as CI in crosses with the sibling species D. subquinaria. We have recently found that the unidirectional interspecific CI caused by Wolbachia may have played a role in the evolution of reproductive isolation between these two species. Specifically, we have found an extraordinary level of reproductive character displacement (reinforcement) in sympatric populations of D. subquinaria, an effect consistent with the expected effects of interspecific CI.
In D. innubila, female flies that are infected with Wolbachia produce all-female progeny, as infection with Wolbachia results in the death of male embryos. In both of these Drosophila species, we are currently exploring the ecological and genetic factors that determine the prevalence of infection by these male-killing Wolbachia. In particular, we are studying how within-host density of Wolbachia affects the maternal transmission rate and penetrance of male killing, variables that play a major role in determining the dynamics of infection prevalence in host populations. The aim of these studies is to integrate within-host population dynamics of Wolbachia to the dynamics of infection prevalence at the level of host populations.
The ubiquitous occurrence of parasites in natural communities and the adverse effects they have on their hosts indicate that parasites can be of major importance in ecological and evolutionary processes. Our research on host-parasite interactions involves Drosophila and their nematode parasites, focusing on macroevolutionary patterns of host-parasite associations, coevolution of interacting hosts and parasites, and population dynamics and the community-level effects of host-parasite interactions.
X-chromosome meiotic drive is a form of segregation distortion that results in the production of female-biased sex ratios in the progeny of "Sex-ratio" males and at the level of whole populations. If unchecked by natural selection, Sex-ratio alleles will increase to fixation and may cause the extinction of entire species. We are interested in the selective factors that prevent the spread of these elements in nature, and the conditions under which such selective constraints break down. In addition, we are interested in the evolutionary interactions between driving X chromosomes and Y-linked or autosomal suppressors of drive. Such interactions provide an excellent opportunity to explore both intragenomic conflict and the operation of natural selection simultaneously at several hierarchical levels.
Offspring sex ratios of wild-caught D. innubila females. Flies infected with Wolbachia produce almost exclusively female offspring, because these Wolbachia cause embryonic male killing. Uninfected flies produce normal offspring sex ratios.
Proportion of females mating with either D. recens or D. subquinaria males in mass population cages. Black and gray circles on map indicate the ranges of these two species, showing a zone of sympatry in central Canada. Left two panels show mate preferences of D. subquinaria females from allopatric and sympatric populations, and right two panels show mate preferences of D. recens females from sympatric and allopatric populations. Within each panel, matings with conspecific males are shown on the left and matings with heterospecific males on the right.