Genetic basis for queen-worker dimorphism in a social insect.

Eusocial insects are characterized by reproductive division of labor, cooperative brood care, and the presence of a sterile worker caste. It is generally accepted that caste determination, including the differentiation of females into sterile workers and reproductive queens, is determined by environmental factors. In contrast, we find that in the red harvester ant, Pogonomyrmex barbatus, an individual's genotype at a particular microsatellite locus predicts its caste. We propose that this microsatellite locus is in tight linkage disequilibrium with at least one locus that plays an important role in caste determination. We call this the caste locus. We hypothesize that the system of caste determination we observe segregates the population into two distinct genetic lineages, each of which has distinct alleles at the microsatellite locus and also has distinct alleles, we propose, at caste. Workers are the offspring of parents from different lineages, and are thus heterozygous at caste, whereas queens are the offspring of parents from the same lineage, and are, therefore, homozygous at caste. This mode of caste determination has important consequences for the evolution of multiple mating by females and for control of the sex ratio and reproductive allocation in social insect colonies.