"Much of the morphological diversity in nature is due to the sexual dimorphism in both size and shape. While sexual size dimorphism (SSD) is exceedingly common and can be rather extreme, size differences between the sexes are often quite mild. The fact that males and females typically share most of a genome raises important questions about the ubiquity of SSD, namely about how males and females achieve such different phenotypes given a mostly shared genome. Historically, explanations to explain this trend have focused more heavily on evolutionary mechanisms than proximate and/or genetic mechanisms. Using the fruit fly, Drosophila melanogaster, as a model organism I explore the developmental, physiological, and genetic mechanisms that regulate both sexual size and shape dimorphism. Like most insects, Drosophila females are larger than males and differ in many key morphological features (such as shape). I will describe a series of experiments aimed to successively uncover the mechanistic underpinnings of sexually dimorphic growth that regulate sex-specific size and shape. To investigate the mechanisms of SSD, I first compared the relative contributions of proximate, developmental mechanisms to size in both males and females. Previous research has also implicated critical size, a hormone-mediated physiological checkpoint, in helping to regulate overall body size in holometabolous insects. Here I demonstrated that, while males and females had equal larval growth durations, females reached their critical size at a larger weight and also grew faster in their final larval instar than males. The resulting SSD was further attenuated by an increase in female weight loss in the period intervening the achievement of peak larval mass and pupariation. Next, I demonstrated that mutants of the IIS pathway (specifically InR) completely eliminated SSD. Since condition-dependence is a common explanation for the evolution of SSD, the next step was to investigate the effects of nutrition on SSD. After demonstrating a negative relationship between SSD and nutritional quality, I investigated the effects of several candidate genetic pathways, including nutrient-sensitive growth pathways, on whole body SSD. I then investigated the sex-limited effects of mutants in similar pathways on sexual size and shape dimorphism where effects are unknown. Finally, I examined the ability of my own candidate pathways to influence both sexual size dimorphism (SSD) and sexual shape dimorphism (SShD). Collectively these studies bring our understanding of the proximate mechanisms that regulate SSD and SShD to a new and more profound level. While much is known of the selective pressures that generate sexual dimorphism, we know very little about the specific genetic targets of these pressures, and how these targets may facilitate or hamper the evolution of sexual dimorphism. My study goes some way to filling this conspicuous gap."--Pages ii-iii.
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