, Volume 12, Issue 4, pp 285-299

Genetic relatedness and colony organization in a species complex of ponerine ants

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In Australian ants of the Rhytidoponera impressa group there are two distinct types of colonies which occur sympatrically in most species: monogynous, queenright colonies (Type A), and monogynous or (usually) polygynous worker-reproductive colonies (Type B) in which 1–15 (mean 4.0) mated workers occur in lieu of a queen.

Both colonies produce winged males, and Type A colonies also rear numerous colony-founding alate queens. Type B colonies seldom produce queens and usually reproduce by colony fission (budding off daughter colonies with one or more mated workers). In Rhytidoponera confusa and chalybaea there is no evidence of reproductive isolation between the two colony types: allele frequencies at several polymorphic enzyme loci are essentially identical in sympatric Type A and Type B colonies, despite marked genetic differentiation among geographically distinct populations and between species.

Allozyme markers confirm that the queen in Type A colonies mates only once and is the mother of all colony members, including males. Regression estimates of relatedness among workers (w) and males (m) in queenright colonies give b w.w≈0.70 and b m.w≈0.20. Corresponding estimates for Type B colonies (b w.w≈0.30, b m.w≈0.16) reveal lower levels of relatedness. Cohabiting mated workers are often closely related, and share reproductive output approximately equally. Uninseminated laying workers are found in about one-quarter of Type B colonies.

Workers in Type A colonies behave more aggressively during colony disturbance than non-reproductive workers in Type B colonies. Colony size, worker size, and male weights are significantly greater in Type A than Type B colonies. Despite evidence that worker altruism and ergonomic efficiency are better developed in Type A colonies, intra- and inter-specific comparisons within the genus Rhytidoponera indicate selective factors favoring worker-reproductive colonies over queenright colonies in xeric environments and in patchy mesic habitats.