Dispersal in the inquiline social parasite ant Plagiolepis xene

Inquiline ants are highly specialised social parasites. They usually do not produce their own worker caste but instead use the worker force of the host ant colony to ensure the rearing of their sexual progeny. Several barriers are expected to severely limit their migration, and the mechanism allowing them to disperse remains largely enigmatic. Here, we tested two hypotheses to account for the low level of infestation of inquiline parasites, in populations of the parasite ant Plagiolepis xene and its host P. pygmaea: (1) the establishment of a new P. xene colony is such a rare event that a single colonisation should be expected per population, and (2) once a P. xene colony is established in one location, it has very little chance to succeed in infecting a neighbour genetically unrelated colony. We sampled nests from both species along four separate transects, and genotyped host and parasite individuals at eight polymorphic microsatellite loci. Our genetic data contradict both hypotheses: multiple colonisation events were recorded in all four transects sampled and, in at least one case, P. xene has successfully migrated from one host colony of P. pygmaea to a spatially close unrelated nest. This shows that the dispersion capacity of the social parasite is sufficiently effective to ensure its long-term survival.