SIMPLE GENETIC-BASIS FOR IMPORTANT SOCIAL TRAITS IN THE FIRE ANT SOLENOPSIS-INVICTA

Variation in queen phenotype and reproductive role in the fire ant Sol enopsis invicta has been shown to have a simple genetic basis in a sin gle introduced population in the United States. The evidence consists of an association between this variation and queen genotype at Pgm-3, a phosphoglucomutase-encoding gene. In the present study, we surveyed Pgm-3 allele and genotype frequencies in diverse populations from the native and introduced ranges of this ant to learn whether this simple genetic basis for reproductive traits is a general feature of the spec ies or a genetic anomaly in introduced ants stemming from a recent bot tleneck or the invasion of novel habitats. No egg-laying queens living in polygyne (multiple-queen) nests possessed the homozygous genotype Pgm-3(a/a) in any of the study populations, yet nonreproductive female s from such nests (workers as well as queens that had not yet initiate d oogenesis) possessed this genotype at moderate frequencies. Remarkab ly, Pgm-3(a/a) was the most common genotype among all classes of femal es, including egg-laying queens, in monogyne (single-queen) nests from all populations studied. Genotype proportions at Pgm-3 in polygyne po pulations typically departed strongly from the proportions expected un der Hardy-Weinberg equilibrium, whereas those in monogyne populations did not. These patterns establish that a single mendelian gene influen ces queen reproductive role in S. invicta and that this gene uniformly is under strong directional selection in the polygyne social form onl y. Moreover, the perfect association of Pgm-3 genotype and reproductiv e role in all populations, combined with the known function of phospho glucomutase in insect metabolism, suggest that this gene may directly influence queen phenotypes rather than merely serving as a marker for a linked gene that causes the effects.