Mate Discrimination of Colocasiomyia xenalocasiae and C. alocasiae (Diptera: Drosophilidae) as a Possible Factor Contributing to their Co-Existence on the Same Host Plant

Mate discrimination contributes to the co-existence of related species by reducing the risk of interspecific copulation. In pollination mutualistic systems where pollinators utilize host plants as mating places, sharing of host plants with other related species could increase non-adaptive interspecific copulation. Although such host-sharing species are expected to have strong mate discrimination systems, little is known about whether and how they discriminate species for mating. Here, we investigate mate discrimination of two fly species, Colocasiomyia xenalocasiae and C. alocasiae (Diptera: Drosophilidae), which share host plants; they are essentially anthophilous, depending exclusively on specific aroid host plants throughout their entire life cycles. Our field observations showed that the males of C. alocasiae and C. xenalocasiae preferentially paired with conspecific, but not heterospecific, females. This indicates that they discriminate species for mating in the natural habitat. Such mate discrimination was also observed under laboratory conditions. To investigate how these flies discriminate species, we defined distinct behavioral elements in courtship sequence in both species, and compared sexual interactions in each element between conspecific and heterospecific pairs. We found that males discriminate female whilst tapping, whereas females discriminate male before or during males’ attempted mounting. This suggests that mate discrimination systems in both males and females reduce the incidence of heterospecific mounting; mounting is a necessary step in the sequence of courtship for successful copulation. The mate discrimination system found in this study potentially allows for the co-existence of C. xenalocasiae and C. alocasiae on the same host plant by effectively suppressing interspecific copulation.