Genetic determination of caste in harvester ants

Two recent papers by Volny and Gordon and Julian et al. show that caste in harvester ants Pogonomyrmex barbatus and P. rugosus can be determined exclusively by genotype. This is in contrast to all previous examples of caste-determination in social insects, where environmental factors have been shown to be the most important determinants. Genetic caste determinism is hypothesized to have arisen through a hybridization event, and might be a possible mechanism for the evolution of eusociality.