© 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd Blackwell Publishing LtdContrasting population genetic structure for workers and queens in the putatively unicolonial ant Formica exsecta

The theory of inclusive fitness provides a powerful explanation for reproductive altruism in social insects, whereby workers gain inclusive fitness benefit by rearing the brood of related queens. Some ant species, however, have unicolonial population structures where multiple nests, each containing numerous queens, are interconnected and individuals move freely between nests. In such cases, nestmate relatedness values may often be indis-tinguishable from zero, which is problematic for inclusive fitness-based explanations of reproductive altruism. We conducted a detailed population genetic study in the polygynous ant Formica exsecta, which has been suggested to form unicolonial populations in its native habitat. Analyses based on adult workers indeed confirmed a genetic structuring consistent with a unicolonial population structure. However, at the population level the genetic struc-turing inferred from worker pupae was not consistent with a unicolonial population struc-ture, but rather suggested a multicolonial population structure of extended family-based nests. These contrasting patterns suggest limited queen dispersal and free adult worker dis-persal. That workers indeed disperse as adults was confirmed by mark–recapture measure