Maintaining low intragroup relatedness: evolutionary stability of nonkin social groups

Communal sociality, involving cooperation among unrelated group members, occurs in all of the six families of bees. However, most communal species also occupy nests that persist over many generations. Why, then, do females not remain in their natal nests with their relatives? We examined how sex-biased dispersal contributes to maintaining low intragroup relatedness by documenting dispersal using microsatellite data in the communal halictine bee, Lasioglossum (Chilalictus) hemichalceum. The composition and genotypes of brood from nests excavated late in the reproductive season (late-season) were compared with those of adults from nests excavated from the same aggregation in early winter (post-season). Females left their natal nests if nests were large and if their brothers were present, demonstrating female-biased dispersal to avoid inbreeding in a social insect for the first time. Thus, the occurrence of within-nest mating in communal species, combined with strong selection against inbreeding in the Hymenoptera, appears to select for female-biased dispersal. This in turn results in a population-wide decrease in intracolony relatedness, thereby maintaining cooperation among nonkin in communal Hymenoptera.