Genetic and morphological variation over space and time in the invasive fire ant Solenopsis invicta

Abstract

Social insects are among the most successful and damaging of invasive taxa. We studied spatial and temporal variation in two traits, colony genetic structure and worker mass, associated with social insect success in the introduced fire ant Solenopsis invicta. Our aim was to determine if changes in social structure occurred over time and if variation in worker size was related to worker genotype. We sampled 1139 workers from five multiple-queen S. invicta nests on six dates over a one-year period. The genotypes of workers were determined at ten microsatellite loci and at the selected locus general protein-9 (Gp-9). We found little evidence for genetic differentiation of workers sampled from distinct nests or from different dates at the microsatellite loci. However, worker Gp-9 genotype frequencies varied among nests and over time. In addition, worker mass was affected by nest-of-origin, sampling date, ploidy level, and Gp-9 genotype. Our results suggest that large numbers of queens contribute to the production of workers in introduced S. invicta nests throughout the year. Colony boundaries are semi-permeable, although the among-nest variation in Gp-9 genotype frequencies and worker mass does suggest that boundaries are present. In addition, selection operating on Gp-9 genotype depends on nest environment. Finally, worker mass is affected by both endogenous and exogenous factors in S. invicta. Overall, our data suggests that the key traits of colony social structure and worker size reflect the effects of variable selection in invasive social insects.