Social and genetic structure of a supercolonial weaver ant, Polyrhachis robsoni, with dimorphic queens

Abstract

We studied a population of the Australian weaver ant Polyrachis robsoni with regard to variation in the morphology of its winged queens using six newlydeveloped microsatellite markers. Morphometrically the queens fell clearly into two groups, macrogynes and microgynes, with the latter an isometric reduction of the former. Aggression tests showed that hostility between ants from different nests was minimal. Nests frequently contained numbers of both queen types, with microgynes about twice as numerous as macrogynes. Nestmate workers, microgynes, and macrogynes, were significantly related to others within their caste, with macrogynes more highly related than the other castes. Relatedness values between these groups of nestmates were also significant. Pairwise relatedness values were consistent with both queen morphs producing workers. At the population level, microgynes from different nests were also significantly related and there was a weak inverse relationship between pairwise relatedness value between individuals and distance between nests. We conclude that this species is supercolonial and that the two queen morphs are part of the same population.

The dimorphic queens of Polyrhachis australis (Hymenoptera: Formicidae)

“…Rüppell & Heinze 1999 and references therein). This finding is also consistent with reports for P. robsoni (Heinze & Hölldobler 1993;van Zweden et al 2007;B Hölldobler pers. comm.…”

“…Although the relative number of macrogynes increased as the number of queens in a nest decreased, this did not result in a majority of macrogynes in monogynous nests or in polygynous nests with small numbers of queens (consistent with the findings of van Zweden et al 2007 in the case of P. robsoni). This was probably due to the effects of dependent nest foundation: microgynes walk to found new nests, and remain the sole queen there (or the founding queens, if more than one walk abroad together) unless and until others join.…”

“…They are not. Intermediate sizes persist, as reported by Downes (2015) for a smaller sample size, whereas in the neighbouring congener P. robsoni there are no individuals intermediate in size between microgynes and macrogynes, and the former are consistently in excess (Bellas & Hölldobler 1985;van Zweden et al 2007). It may be teleological to suggest that the alternative reproductive strategy implied by this bimodality is not yet fully expressed in P. australis, but that is not an unreasonable expectation given the status of the size morphs in other species, and it is an expectation that has been repeatedly voiced by other authors for other ants (e.g.…”

“…Every new case is therefore of interest both in its own right and in relation to those species closest to it phylogenetically. Bellas and Hölldobler (1985) reported size bimodality among queens of the formicine weaver ant Polyrhachis ?doddi, (now P. robsoni), a finding later confirmed and extended by Heinze and Hölldobler (1993) and van Zweden et al (2007). This species, judged by Rüppell and Heinze (1999) as being effectively a missing link between species whose queens are wing-dimorphic and size-dimorphic, and hence one with excellent potential for future studies investigating causal links between the two, ranges along the northernmost tropical coast of Queensland as far south as Townsville where its distribution appears to overlap marginally (S Robson pers.…”

“…It may be teleological to suggest that the alternative reproductive strategy implied by this bimodality is not yet fully expressed in P. australis, but that is not an unreasonable expectation given the status of the size morphs in other species, and it is an expectation that has been repeatedly voiced by other authors for other ants (e.g. Rüppell & Heinze 1999;Howard 2006, van Zweden et al 2007Steiner et al 2010). In this context, it would be useful to examine queens of P. mondoi, a species that according to Kohout (2006) is phylogenetically even closer to P. australis than is P. robsoni.…”