Reproduction and parasite-mediated selection

Sexual reproduction generates genetic diversity that can help hosts respond to selection by parasites, and in this thesis, I test three predictions on how reproduction impacts predictions by the parasite theory of sex and the Red Queen hypothesis. In Chapter one, using a meta-analysis, I found that asexuals (lower genetic diversity) have more parasites than sexuals (higher genetic diversity), but this difference can be heavily mediated by the mode and origin of asexuality. Further, hybridization but not polyploidy can blunt predicted differences in parasite loads among sexual and asexual hosts. In Chapter two, I flip the perspective of Red Queen predictions to examine the maintenance of sex in parasite species using an arrhenotokous spider mite. Using data on fecundity, mortality, and sex ratio differences on benign and toxic host plants, I show that female spider mites may alter sex ratios to compensate for high male mortality on toxic host plants—thus assuring genetic recombination despite high male mortality. Finally, Chapter three uses meta-analysis to synthesizes parasite loads on hybrid hosts to further explore how this mechanism impacts parasite-mediated selection on hosts. Although increased heterozygosity among hybrids is predicted to blunt parasitism, I found that F1 hybrids had greater parasite loads than their parental species; however, this finding was generalizable only among interspecific crosses and not intraspecific hybrid networks. In whole, my thesis indicates that predictions of the Red Queen hypothesis tend to be too narrow, and that theory needs to encompass a greater diversity of genetic systems to better predict parasite loads on hosts and thus opportunities for parasite-mediated selection