On the frontiers of speciation: Male-based delimitation of Mesoamerican ant species in the genus Nylanderia (Hymenoptera: Formicidae)

One of the most widely accepted biological paradigms across sexually reproducing organisms is that genitalia evolve rapidly in comparison to other traits and are therefore taxonomically informative. However, the evidence cited in support of this is limited to taxonomic literature in which the shapes of genitalic structures are assumed with a priori bias to be species-specific. This phenomenon is assumed to be near-universal but is rarely tested within and between populations of closely related species. The ant genus Nylanderia is large, near-globally distributed, and includes over 125 described species with most diversity in the Neotropics still undescribed: In Mesoamerica, approximately 20 unnamed species still await description. Nylanderia workers are often morphologically cryptic and require an oft-unavailable association with males to delimit species. At least four Neotropical species ‘globetrot’ outside their native ranges via human-mediated dispersal, one of which is the invasive tawny crazy ant (Nylanderia fulva). In this study, we compared male genitalia sampled from 14 different Nylanderia species across Mexico, Mesoamerica, and parts of South America, with emphasis on population-level sampling to evaluate intra- versus interspecific morphological boundaries in the shapes of five different male structures: parameres, volsella, penial sclerites, subgenital plates, and wings. To characterize shape, we applied geometric morphometric analysis of two data sets: (1) two-dimensional images of slide mounted genitalia; and (2) three-dimensional models from nano-CT scans of genital capsules. The results are essential to (1) understand how male morphology informs species boundaries, and (2) to distinguish otherwise oft-indistinguishable endemic species from invasive species.