PROJECT SUMMARY Sexual reproduction requires precise orchestration of expression of myriad genes in males and females. These genes mediate interactions between the sexes at molecular, cellular, neural and whole- organism levels, including the differential fertility success of sperm from two competing males that mate to a single female. In the current funding period, we used the tractable Drosophila model system to dissect the role of genes expressed in octopaminergic neurons in differential sperm use by a multiply-mated female. In this renewal application, we propose as Aim 1 a set of experiments that identify genes and quantify their effects on mating plug ejection, a proximal phenotype with a key role in sperm competition outcomes. Mating plug ejection timing allows females to exert control over paternity of their future offspring. Early ejection of a mating plug disadvantages the male by decreasing the number of his sperm that can be used; conversely longer retention of the plug gives the male a paternity advantage. Both males and females contribute to the mating plug and its ejection. After completing a GWAS for female and male determinants of mating plug ejection timing and validating the resulting genes, we will perform a grid cross to determine whether variation in mating plug timing is mostly determined by genetic variation in the male, or the female, or an interaction between the two. Follow-up perturbation of those genes will begin to unravel genetic pathways linking male quality or its perception with mating plug ejection. In Aim 2, we will pursue the implications of our recent discovery that mating induces differential exon or promoter use by a large suite of genes in the female brain. Intriguingly, two of these genes, desat1 and foraging, act in mating discrimination or other reproductive- relevant behaviors. Each strongly activates one of four alternative promoters in response to mating. We will survey the transcriptome to quantify how variation in male factors drive post-mating differential transcript use, how the latter varies across female genotypes, and how this regulation occurs. In Aim 3 we extend and generalize models designed to quantify and predict outcomes of pairwise interactions. The classical Bradley-Terry model, widely applied in predicting outcomes of sporting contests between teams that have not yet competed, has a direct analogy in sperm competition “contests.” Assessing fit to models of this class will test whether the males’ fitness can be rank-ordered, and whether those rank orders produce accurate outcome predictions. We will extend these models to the outcomes of the experiments in Aims 1 and 2, and those performed in this project over the years, using the results to assess the overall importance of male x female interactions in each case. Many of the processes that we will study show high levels of evolutionary conservation, implying that our results will expand our understanding of male x female interactions...