A formal assessment of gene flow and selection in the fire ant Solenopsis invicta

Recent studies of the introduced fire ant Solenopsis invicta suggest that introduced polygyne (with multiple queens per nest) populations are strongly influenced by male-mediated gene flow from neighboring monogyne (single queen per nest) populations and selection acting on a single locus, general protein-9 (Gp-9). This investigation formally tests this hypothesis and determines if these processes can account for the genotypic structure of polygyne S. invicta. To increase the statistical power of this test, we considered the genotypes of polygyne queens and workers at both Gp-9 and the closely linked, selectively neutral locus Pgm-3. We then constructed and analyzed a novel mathematical model to delimit the effects of monogyne male gene flow and selection on the joint genotypes at the Pgm-3/Gp-9 superlocus. Using this framework, a hierarchical maximum-likelihood method was developed to estimate the best-fitting gene flow and selection parameters based on the fit of our model to data from both the current study and an earlier one of the same population. In each case, selection on polygyne queens and workers alone, with no monogyne male gene flow, provides the most parsimonious explanation for the observed genotype frequencies. The apparent discrepancy between this result and the empirical evidence for monogype male gene flow indicates that undocumented factors, such as other forms of selection in polygyne males or workers, are operating in introduced polygyne S. invicta.