Population genetic structure of a native, urban invader ant Tapinoma sessile

Biological invasions are becoming more prevalent throughout the world due to the rise of global trade and the expansion of urban areas. Ants are among the most prolific invaders, with many exhibiting a multi-queen colony structure, dispersal through budding and a lack of inter-nest aggression. Although these characteristics are generally associated with the invasions of exotic ant species, they may also facilitate the spread of native ants into novel habitats (i.e., urban areas). Native to North American forests, the odorous house ant Tapinoma sessile has become highly abundant in urban environments throughout the United States. Forest-dwelling colonies are typically small, monodomous, and monogyne, whereas urban colonies tend to be several orders of magnitude larger, polydomous, and polygyne. Here, we explore and compare the population genetic and breeding structure of T. sessile within and between urban and natural environments in several locations across its distribution range. We found the social structure of a colony to be a plastic trait in both habitats, although extreme polygyny was confined to urban habitats. Additionally, we identified strong differentiation between almost every nest, habitat and locality, but found no intra-habitat isolation-by-distance in any locality. Finally, no polydomous colonies were found in any natural habitat (i.e., nests lacking genetic differentiation and behavioral antagonism), suggesting T. sessile can only achieve unicoloniality within urbanized areas. Overall, our results highlight strong differentiation between urban and natural populations of the ant, reinforcing the powerful role cities have in shaping the evolutionary outcome of species.