Evidence that microgynes of Myrmica rubra ants are social parasites that attack old host colonies

Ant microgynes are miniaturised queen forms found together with normal queens (macrogynes) in species occurring across the ant phylogeny. Their role is not yet fully understood: in some cases, they seem to be nonparasitic alternative reproductive morphs, in others incipient social parasites, and thus potential models for studying the evolution of social parasitism. Whether they are regarded as parasitic or not has traditionally been based on genetic differentiation from syntopic macrogynes and/or the queen/worker ratio of their offspring rather than measuring fitness traits. In this study, the authors confirmed previously reported genetic differentiation between microgynes and macrogynes of Myrmica rubra in a population studied for the first time. Further, virulence and infectivity of M.rubra microgynes were measured in a controlled laboratory experiment. Nests headed only by macrogynes (controls), only by microgynes, and naturally and artificially mixed nests were kept under identical conditions. The authors found reduction in worker numbers of both naturally and artificially mixed macrogyne/microgyne nests compared with controls, and strong reduction but also surprising variation in fitness of nests headed only by microgynes. Microgyne nests produced workers, males and microgynes. Microgynes did not themselves reproduce in artificially mixed nests, but reproduced most in naturally mixed nests that had lost their macrogyne queen. This, together with higher mortality of field-collected macrogyne queens from naturally infested colonies and greater estimated relative age of macrogyne queens in naturally infected nests, suggests that they preferentially exploit older host colonies. the authors concluded that M.rubra microgynes are intraspecific social parasites specialised on exploiting old host colonies.

Authors: Schär S, Nash DR. ;Full Source: Journal of Evolutionary Biology. 2014 Sep 16. doi: 10.1111/jeb.12482. [Epub ahead of print] ;