The evolution of two periphallic sclerites of the male terminalia in Scaptomyza and related genera (Diptera: Drosophilidae)

Insect terminalia is highly variable, especially in species with internal fertilization. Since the male terminalia evolves at higher rates than other morphological traits, it has an important role in the early stages of speciation. The most analyzed male terminalia sclerite is the aedeagus, which is the intromittent copulatory organ and its morphological variation is often correlated with reproductive isolation. However, other terminalia sclerites involved with the mating behavior, including periphallic sclerites as the surstyli, cercal ventral lobes, and epandrial lobes, also merit attention. The genus Scaptomyza is a suitable system for studying the evolution of these periphallic sclerites, since it contains species with well developed cercal and epandrial lobes, which often bear strong setae, in resemblance to the surstyli. Interestingly, it is possible to observe similar terminalia modifications in other Drosophilidae genera, including in the Drosophila melanogaster species group. However, the evolutionary patterns of variation of these sclerites remain poorly understood. The goal of this study is to analyze concordant patterns of differentiation of cercal and posterior epandrial lobes across the family Drosophilidae in a phylogenetic framework. For this, we analyzed male terminalia figures of species included in a phylogeny of 155 Drosophilidae genomes (Suvorov et al., 2020), reconstructed the ancestral states of cercal and posterior epandrial lobes, and generated scanning-electron-microscopy images of four Drosophilidae species with different terminalia modifications that represent the analyzed patterns of differentiation.