Founder effects and the evolution of asymmetrical sexual isolation in a rapidly-speciating clade

Sexual isolation resulting from differences in mate choice behaviors is a hallmark of rapidly-speciating lineages. When present, asymmetrical sexual isolation may provide insights into the mechanisms responsible for the evolutionary change in mate signaling traits. In particular, Kaneshiro's hypothesis suggests that divergence in sexual characters between populations may arise in allopatry when 'derived' founding populations are subject to severe population bottlenecks, accompanied by a relaxation of sexual selection relative to 'ancestral' source populations. In the present study, we tested predictions of asymmetrical sexual isolation between two allopatric species of Hawaiian Laupala crickets, representing 'ancestral' (L. makaio) and 'derived' (L. nigra) taxa. While crickets in this genus are notable for rapid divergence of male courtship songs, these species share similar song types, thus suggesting that patterns of sexual isolation are likely due to other mating cues. Analysis of behavioral responses in conspecific and heterospecific 'no-choice' mating trials revealed pronounced asymmetrical isolation in the direction predicted by Kaneshiro's hypothesis, wherein we observed a significant reduction in mating success for crosses involving 'derived' males paired with 'ancestral' females, compared to the reciprocal heterospecific and both conspecific pairings. Further dissection of courtship behaviors suggested this difference did not reflect male mate choice, but rather, marked reduced spermatophore acceptance rates by 'ancestral' females paired with 'derived' males. The results are discussed with respect to founder effect models of speciation and the potential role of chemosensory signals in mate choice in these species.