Population genetics and colony structure of the argentine ant (Linepithemahumile) in its native and introduced ranges

Introduced species often possess low levels of genetic diversity relative t o source populations us a consequence of the small population sizes associa ted with founder events. Additionally. native and introduced populations of the same species can possess divergent genetic structuring at both large a nd small geographic scales. Thus, genetic systems that have evolved in the context of high diversity may function quite differently in genetically hom ogeneous introduced populations. Here we conduct a genetic analysis of nati ve and introduced populations of the Argentine ant (Linepithema humile) in which we show that the population-level changes that have occurred during i ntroduction have produced marked changes in the social structure of this sp ecies. Native populations of the Argentine ant are characterized by a patte rn of genetic isolation by distance, whereas this pattern is absent in intr oduced populations. These differences appear to arise both from the effects of recent range expansion in the introduced range as well as from differen ces in gene flow within each range. Relatedness within nests and colonies i s lower in the introduced range than in the native range as a consequence o f the widespread genetic similarity that typifies introduced populations, I n contrast, nestmates and colony-mates in the native range are more closely related, and local genetic differentiation is evident. Our results shed li ght on the problem posed for kin selection theory by the low levels of rela tedness that are characteristic of many unicolonial species and suggest tha t the loss of genetic variation may be a common mechanism for the transitio n to a unicolonial colony structure.