Evolution of wingless reproductives in ants and the new phylogeny
Christian Peeters, Laboratoire d'Ecologie, Université Paris 6/CNRS, 7 quai Saint Bernard, Paris, 75005, France
Winged queens are inferred to be ancestral in ants, and they occur in a majority of species. Yet in a significant number of taxa belonging to almost all the subfamilies, winged queens have been completely replaced by wingless reproductives. These are either morphologically distinct from the worker caste (i.e. ‘ergatoid queens’ or ‘intermorphs’, combining queen and worker traits in external morphology and reproductive organs), or workers that mate and reproduce (i.e. ‘gamergates’, mostly restricted to poneromorph species). Only winged queens can start new colonies independently, and their absence in a species always implies that there has been a permanent switch to colony fission. Fission occurs together with independent foundation in only few species, and winged queens are then retained. Obligate fission is a derived strategy in which the reproductive(s) depend totally on workers; there is no solitary stage. Since all ants have wingless workers, fission is synonymous with short-range dispersal on foot and thus producing winged queens no longer brings benefits for fission. Accordingly, colony-level reproductive investment changes, and the caste system can be modified. The recent multi-gene phylogeny (Moreau et al. 2006; Ward, Brady, Fisher & Schultz in prep.) is used as a framework to discuss the repeated evolutionary replacement of winged queens by wingless reproductives. Two distinct patterns in colonial investment are revealed. In some species, dedicated wingless reproductives are produced in very small numbers, and they are highly dimorphic in army ants. In other species, substantial numbers of reproductively competent individuals are produced, but most remain sterile and function as laborers together with the workers. This is akin to the gamergate strategy since individuals that fail to reproduce can benefit the colony. What is common to all species with wingless reproductives are adaptations for the success of colony fission, about which there are little empirical data.