Keywords: n. genotype to phenotype; h. sex-specific traits and molecules
In sexual selection, male trait evolution is often driven by female preference in a way that can generate exaggerated male phenotypes via a Fisherian runaway process. This male-female co-evolution requires a genetic correlation between the male trait and female preference, but the genetic architecture behind this correlation is typically unknown. In Drosophila, sperm are extremely long and are stored within the long, coiled female sperm storage organ, the seminal receptacle, or SR. SR length is a mechanism of cryptic female choice for sperm length, such that when females mate with multiple males, long SRs favor long sperm, and short SRs favor short sperm. Thus, sperm length and SR length are a post-copulatory male trait-female preference system. Consistent with this, sperm length and SR length are coevolving across the Drosophila lineage, driven by a genetic correlation between the two traits. Here, we have used the Drosophila Synthetic Population Resource (DSPR) to characterize the genetic architecture underlying sperm-SR coevolution and to identify candidate genes underlying each trait. We found that sperm and SR length are each associated with a single QTL peak that are closely linked on chromosome 3R, leading us to conclude that the genetic mechanism of male-female co-evolution in this case is linkage rather than pleiotropy. Within the sperm QTL is the gene scotti (soti), which encodes a testis-specific protein that localizes to late-stage spermatids and regulates the onset of individualization. We examined soti mutants and found that spermatid cysts are 350 microns shorter than wild type controls. Within the SR QTL are two developmental regulators of epithelial cell migration, heartless (htl) and stripe (sr). Both of these genes are expressed in the female reproductive tract and enriched in SR tissue, which is epithelial in origin. Ongoing experiments are further characterizing the roles of htl and sr in SR development.