Like the mule, hybrids between species or lineages are often considered an evolutionary dead end. However, recent studies suggest that hybrids can be important in shaping evolution. While most hybrid gene mixtures are unsuccessful, some are beneficial and result in new strategies to cope with changing ecological and social environments. In Pogonomyrmex seed-harvester ants, hybridization has produced a unique social and reproductive structure, in which queens must mate with males of two different genetic lineages to produce both workers and daughter queens. Daughters of the queen and males of the same lineage develop into queens. Hybrid daughters between the queen and males of the alternate lineage become sterile workers. This system of genetic caste determination violates the widely held assumption that whether a female becomes a worker or queen has to be based on differences in nutrition or other environmental factors, and not genetics, because gene combinations producing sterility should be selected against. This system occurs within two closely related harvester ant species, P. rugosus and P. barbatus, leading to the hypothesis that it originally evolved as a result of incomplete hybridization between the two species that lead to the development of two interdependent genetic lineages. However, preliminary evidence suggests that this is not the case and that it may have evolved initially in one species (P. barbatus) then spread to the other. To test these hypotheses, the extent and consequences of hybridization within and between these species of seed-harvester ants will be examined using mitochondrial and nuclear genetic analyses. This work will contribute to our understanding of the role that hybridization plays in evolution, especially the evolution of social systems. This research will also provide opportunities for a graduate student and several undergraduates to learn molecular techniques and analyses with broad applications in evolutionary biology.
