Sexual Selection and Condition Dependence in the Fruit Fly, Drosophila melanogaster

While natural selection acts to strengthen nonsexual fitness (i.e., survivorship and fecundity), the capacity for sexual selection to enhance this process is the source of much theoretical and empirical debate. The potential for sexual selection to reinforce natural selection in reducing the mutation load is rooted in condition dependence theory. This theory predicts that females select mates on the basis of condition, as indicated by condition-dependent sexual traits, and in doing so simultaneously sort them by genetic quality. The premise of condition-dependent mate choice is transferrable to non-female-choice components of selection; hence, sexual selection at its multiple stages is predicted to remove mutations deleterious to condition. Support for the notion that sexual selection in toto can reduce the mutation load is mounting; however, understanding which components of sexual selection are important in the removal of deleterious mutations remains an important challenge. To address this issue, I employed the Drosophila melanogaster model system. I demonstrated support for the capacity of sexual selection to remove deleterious mutations at multiple stages of selection (Chapters 2-4), before and after mating. Furthermore, I showed that the efficacy of sexual selection in eliminating deleterious mutations is dependent on the stage of selection (Chapter 4), and the source of mutation under selection (Chapters 2-4). I deviated slightly in research objective for my final thesis study (Chapter 5). There is mounting empirical support for precopulatory male mate choice across insect taxa, and emerging support for choice post-copulation (i.e., sperm and seminal fluid allocation). I manipulated perceived female mating status in D. melanogaster, and I demonstrated precopulatory male preference for females perceived as virgins, which suggests male choice for low ejaculate competition risk. At the postcopulatory level, males mated to perceived virgins outperformed those mated to perceived mated females in several key components of sperm competition defence; however, I did not detect a downstream fitness cost to this differential investment. Ultimately, my thesis has contributed valuable and novel results on the relative impacts of pre- and postcopulatory sexual selection on genetic quality in D. melanogaster, which collectively suggest that sexual selection can be effective against deleterious mutations.Ph.D