Latitudinal variation in the sociality of allodapine bees (Hymenoptera : Apidae): sex ratios, relatedness and reproductive differentiation

Variation in sociality of two Australian allodapine bees was investigated o ver a latitudinal range of 10 degrees, spanning subtropical to cool tempera te forests in eastern Australia. Exoneura robusta is univoltine and singly brooded in southern populations but is able to produce two broods per seaso n in northern populations, leading to opportunities for sib rearing by olde r brood members. In contrast, sympatric colonies of E. angophorae show no s uch variation with latitude, and all populations exhibit some colonies wher e opportunities for sib rearing arise. Patterns of ovarian differentiation, relatedness and sex allocation were examined over a one-year period for th ese two sympatric species in four populations. Within each species, the tim ing of ovarian development and the degree of reproductive skew were similar at all sites, but the onset of egg-laying and brood development was earlie r in E. angophorae than in E. robusta. Relatedness and sex allocation patte rns in northern populations of these species were not markedly different fr om southern populations. Selection for female-biased sex allocation and hig h reproductive skew in Exoneura probably stems from strong benefits from co operative nesting, and our results suggest that opportunities for sib reari ng (and eusociality) in these species may depend on latitudinally mediated brood development rates, and are unrelated to levels of sex bias and relate dness.