Thelytokous Versus Arrhenotokous Worker Reproduction in the Cape Honeybee and Other Eusocial Hymenoptera

Abstract

The workers of eusocial Hymenoptera are normally sterile, but under certain conditions this is not true. The most frequent of these is when the colony has lost its queen and no eggs exist from which a new queen can be reared (BOURKE 1988). If such a colony cannot produce reproductive individuals it would be an evolutionary dead end. Although workers cannot mate they can lay unfertilised haploid eggs that develop into male reproductives (CROZIER 1977). In many species of eusocial hymenoptera this is exactly what happens (reviewed by BOURKE 1988). The production of haploid males through unfertilised gametes is termed arrhenotokous parthenogenesis or just arrhenotoky. In contrast, workers of the Cape honeybee Apis mellqera capensis ( VERMA and RUTTNER 1983), as well as those of four ant species (Cataglyphus cursor ( CAGNIANT 1979), Pristomyrmex pungens (ITOW et al. 1984), Cerapachys biroi (TSUJI and YAMAUCHI 1995) and Platytherea punctata ( HEINZE and HOLLDOBLER 1995)can lay diploid eggs which develop into females, and can hence be reared to new queens. This type of parthenogenesis is called thelytoky. RUTTNER (1988), working with A . m. capensis, showed that the type of parthenogenetic reproduction is possibly determined at a single locus with the allele for thelytoky being recessive to arrhenotoky. Following on this result, MORITZ (1986) modelled the dynamics of these two alleles. He concluded that the “model showed that there are conditions, arising from the Cape environment, as opposed to other areas, that favour the natural development of thelytokous laying workers in Honeybees” (my italics). In this first attempt three important assumptions were made: that males are diploid, that the population is in Hardy-Weinberg equilibrium, and that bees are not structured into colonies. In this note, I extend MORITZ’S (1986) model by relaxing these three assumptions and by investigating the importance of the queen’s mating frequency and the question whether egg number or colony resources limit the number of reproductives a worker colony can raise. My study suggests that thelytoky is the preferred reproductive mode of the Cape honeybee. However, this bias is due to its life history and not because of a high risk of queenloss. It also shows that the queen’s mating frequency and the question whether worker reproduction is limited by colony resources or the sheer number of eggs, are of importance in determining which reproductive strategy would be stable.