Abstract
The extraordinary lifelong partner commitment in social insects is expected to increase choosiness in both sexes and therefore to be associated with particularly low hybridization frequencies. Yet, more and more studies reveal that in many ant taxa hybrids are surprisingly common, with up to half of all female sexuals receiving sperm from allospecific males in extreme cases. In a few ant species, hybridization has led to the evolution of reproductively isolated new lineages with a bizarre system of genetic caste differentiation: colonies produce hybrid workers and pure-lineage female sexuals. This requires that colonies either contain multiple queens or that queens mate multiple times. In most other cases, hybridization appears to be an evolutionary dead end and fertile hybrid queens are rarely found. In such cases, haplodiploid sex determination appears to decrease the costs of mating with an allospecific male. As long as hybrid workers are viable, a cross-mated queen can partially rescue its fitness by producing males from unfertilized eggs. Mating with an allospecific partner may thus be an option for queens when conspecific mates are not available. The morphological similarity of most ant males, perhaps resulting from the lack of sexual conflict, may similarly contribute to the commoness of hybridization.