Abstract
Supergenes are large clusters of linked genes that control complex phenotypes. In several ant clades, supergenes determine whether one or multiple queens reproduce in mature colonies, but how supergenes affect colony social structure has been inferred indirectly. We show experimentally that a supergene in Formica ant workers alters the social structure (single- or multi-queen) of developing colonies. We crossed queens and males carrying alternative social supergene genotypes, and let the queens establish single-queen colonies, which we paired in the laboratory. The presence of a paternally-inherited P haplotype in workers was sufficient to make the colonies more likely to fuse and become multi-queened, regardless of the genotype of their mother. The dominant effect of the P haplotype on colony social structure likely contributes to the spread of multi-queen colonies. This controlled experiment provides direct evidence that the P haplotype in workers steers social organization towards multiple queens during early colony ontogeny.