The role of body size on the outcome of mating interactions in Drosophila melanogaster

Sexual selection is the process by which some individuals produce more and/or better quality offspring than others because they are better at securing mates. While this may be accomplished by defeating same-sex rivals (intrasexual selection), individuals of one sex (typically females) may also “decide” on the suitability of individuals of the opposite sex (typically males), resulting in intersexual selection on attractive traits. While a great deal of scrutiny has focused on how sexual selection influences male display traits, much less scrutiny has been directed toward the factors underlying female preference, including genetic variation, as well as the extent to which both sexes are involved in mate choice. In Drosophila melanogaster, a model species for the study of sexual selection, previous studies have examined the role of body size variation in a single sex on the behaviours and outcomes related to courtship and copulation. However, few studies have simultaneously varied both male and female body size. In my first study (Chapter 2), I experimentally paired male and female flies from across a wide spectrum of body size phenotypes and quantified several behavioural traits: time to courtship initiation, length of courtship and length of copulation. I found that absolute body size differences affected length of courtship and that relative body size differences affected time to courtship initiation. While Chapter 2 demonstrated how mate choice may be expressed within a single generation of individuals, whether individual preference variation in females had a genetic component had yet to be determined experimentally. In my second study (Chapter 3), I investigated if female body size preference had a genetic component by directly selecting on female preference over multiple generations. Using artificial selection, I “penalized” females that mated with males of certain body sizes over 20 generations and observed several significant differences in female preference behaviour. In all treatments, females tended to associate significantly more with males of body sizes different from those they were artificially selected against. These results not only suggest that body size in both sexes can significantly influence female preference behaviours, but that body size may be a trait possessing significant genetic variation with the potential to be strongly shaped by sexual selection