Assessing the Impact of Secondary Sexual Traits on Sexual Selection in Drosophila

The role of sexual selection in driving speciation has been a major question in evolutionary biology since Charles Darwin. Sexual selection occurs when there is mating competition within species and certain traits are chosen during mating over others by the opposite sex. Sexual isolation is a mechanism of reproductive isolation that causes mating discrimination against different species. This process inhibits gene flow, ultimately resulting in speciation. However, the link between sexual selection within species and sexual isolation between species is unclear. For instance, do females target the same traits in both processes when assessing their mating partners? Alternatively, females may first screen males for species identity based on a species-specific trait then choose among conspecific males using different sets of traits. This suggests sexual isolation and sexual selection are not the same processes. To answer this, one must understand the role of traits in mating decisions within and between closely related species.Because closely related species often show non-overlapping differences in many relevant sexual traits combined with limited variation in each species, one would ideally want to study their hybrids to reveal a continuum of variation in traits between species. With this, we can determine which trait combination of traits females prefer both within species and beyond their conspecific variation. Then, we can build sexual selection fitness landscapes for each set of females. To test this approach in linking sexual selection with sexual isolation, we conducted no-choice single mating experiments with two species, Drosophila athabasca (West Northern; WN) and Drosophila mahican (eastern A; EA)(see Yukilevich et al. 2016) and their F1 and F2 hybrids. We paired a female from each species with either type of male (conspecific, F1, F2 or heterospecific) and determined mating success after five minutes of observation. Simultaneously we recorded 1) the courtship song, measured 2) morphology, and 3) pheromone profile (cuticular hydrocarbons) of each male. This enabled us to create significant variation in male traits and map their mating success with either WN or EA females. From this information, we can see which traits females prefer in conspecific males and in hybrids. This reveals the shape and direction of sexual selection on traits within and between species and helps identify traits used in sexual selection and isolation.