The Role of Context in Investment into Reproductive Tissue and Implications for Mating

Abstract

Reproductive traits are often thought of as fixed, genetically determined properties. However, such traits are often dynamic, exhibiting different expression patterns depending on context. Both internal state and external environment can have a strong effect on how traits are expressed. Variation in these factors across the lifetime of an individual should select for flexibility in trait expression, rather than fixation.My dissertation work examines how mating behavior and testes size respond to several previously unexplored contextual factors, using Rhagoletis juglandis, the walnut fly, as a model system. For mating behavior, I predicted that differences in female reproductive state (egg load) and experience with host resource would impact mating decisions. For testes size, I predicted that social environment (sex ratio) and changes in resource environment would determine testes size.Behavioral observations of flies showed that a large egg load increased the likelihood of copulation, while prior experience with host fruit decreased copulation time. These results are the first to distinguish effects of experience on physiological state from other effects of experience in the context of mating behavior.Manipulation of the sex ratio revealed that males develop larger testes when reared in an environment with many potential competitors. This is the first study to show that that allocation to a male reproductive organ can change depending on the sex ratio. My studies showed that resource environment is also important in determining testes investment patterns. When adult males are deprived of protein, they develop smaller testes. A stable isotope analysis of testes further confirms that resource environment is important for testes development. Males rely more on nitrogen derived at the larval stage than at the adult stage, but adult carbon sources are a large component of testes mass.In sum, this dissertation demonstrates the importance of context in the expression of reproductive traits. Recent research has shown that such traits can respond more dynamically to context than previously thought, but this area of research is young. My results help provide a greater understanding of the processes shaping the evolution of reproductive traits.