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

Abstract

In the Rhytidoponera impressa group, populations of Type A (queenright, monogynous) colonies produce numerous colony-founding queens and have markedly female-biassed sex ratios of investment, the mean proportional investment in females (PF) being 0.824 for thirteen populations of confusa, chalybaea, and purpurea (0.839 for ten confusa populations). (All proportional means are retransformed from the angular transformation.) By contrast, Type B (worker-reproductive, usually polygynous) colonies reproduce by colony fission and show less female-biassed investment ratios, with mean PF values (12 populations, 2 species) ranging from 0.358 to 0.721, for estimates of colony fission costs ranging from 10% to 50% of the worker force. The population PF for Type A colonies shows significant positive correlations with the relative frequency (r=0.63, df= 11, P < 0.025) and density (r=0.70, df=11, P<0.01) of sympatric Type B colonies, indicating that Type A colonies compensate for male bias in the community sex ratio generated by Type B colonies. In one population (of purpurea) consisting entirely of Type A colonies, the estimated population PF was 0.736. In the absence of evidence for inbreeding or local mate competition, this suggests worker control of the investment ratio. There is a substantial amount of unexplained intrapopulation variation in colony sex ratios. Colony-level PF values for Type A colonies range from 0.00 to 1.00, and are not significantly correlated with colony size, number of alates, worker size, or weights of alates. Introduction