The study of size covariation between traits has a long history of describing morphological variation. For over a century, scientists have recognized variation in the proportional size of traits, and have searched to explain the patterns from an evolutionary perspective. Research on the scaling relationships between traits, called allometries, has established the interaction of traits within an organism plays a crucial role in the adaptation of species to their environment. The evolutionary forces that give rise to changes in the proportional size of traits have been more difficult to elucidate. Using the model organism, Drosophila melanogaster, I have focused my research on the scaling of male genitalia in relation to overall body size to explore proximate and ultimate causes of allometries in general. Most traits scale at or near a 1:1 ratio to overall body size, called isometry. In contrast, the male genitalia of many groups scale hypoallometrically to body size, remaining a constant size across a range of body sizes. Determining the factors that drive the atypical allometric relationship of the male genitalia promises to reveal principles of size control across all traits. To investigate the developmental mechanisms underlying genital hypoallometry, I first compared the effects of genetic variation on genital traits (hypoallometric) to somatic traits (isometric). Previous research has shown that genital traits are less sensitive to environmental variation than somatic traits and here I demonstrate that genital traits are also less sensitive to variation in genetic factors that affect trait size. I also showed that genitalia have low levels of developmental stability than somatic traits, measured as the response in trait size to stochastic developmental errors. Next, I used targeted gene expression of insulin-signaling genes in developing genital tissues of Drosophila to allometrically engineer male flies with extreme genital sizes. Females were exposed to males with different genital sizes, and demonstrated a preference for copulating, and fertilizing progeny, with males that had larger genitalia. To expand the scope of these results, a stochastic mathematical model of allometry evolution was designed that incorporated the developmental regulation of size. Results of simulated allometry evolution showed that the underlying factors controlling final trait size largely determine how scaling relationships respond to selection and evolve. Collectively, my dissertation represents a significant step forward in our understanding of trait size regulation between covarying traits. Additionally, my research demonstrates the novel use of Drosophila melanogaster to modify existing levels of trait variation to test selection hypotheses. Scaling relationships between traits are an important component of morphological evolution that we can continue learning about only via multifaceted research as demonstrated here.
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