Evolutionary transition from single to multiple mating in fungus-growing ants

Queens of leaf cutter ants exhibit the highest known levels of multiple mat ing (up to 10 mates per queen) among ants. Multiple mating may have been se lected to increase genetic diversity among nestmate workers, which is hypot hesized to be critical in social systems with large, long-lived colonies un der severe pressure of pathogens. Advanced fungus-growing (leafcutter) ants have large numbers (10(4)-10(6) workers) and long-lived colonies, whereas basal genera in the attine tribe have small (< 200 workers) colonies with p robably substantially shorter lifespans. Basal attines are therefore expect ed to have lower queen mating frequencies, similar to those found in most o ther ants. We tested this prediction by analysing queen mating frequency an d colony kin structure in three basal attine species: Myrmicocrypta ednaell a, Apterostigma collare and Cyphomyrmex longiscapus. Microsatellite marker analyses revealed that queens in all three species were single mated, and t hat worker-to-worker relatedness in these basal attine species is very clos e to 0.75, the value expected under exclusively single mating. Fungus growi ng per se has therefore not selected for multiple queen mating. Instead, th e advanced and highly productive social structure of the higher attine ants , which is fully dependent on the rearing of an ancient clonal fungus, may have necessitated high genetic diversity among nestmate workers. This is no t the case in the lower attines, which rear fungi that were more recently d erived from free-living fungal populations.