Multiple queens and biased sex ratios in the independent-founding Ropalidia Guerin-Meneville, 1831 (Hymenoptera: Vespidae) wasps

摘要

The social wasp genus Ropalidia includes both 'swarm-founding' species with large, multi-queen colonies, and 'independent-founding' species that have smaller colonies and fewer queens. Swarm-founding evolved from independent-founding, and a key element of this transition was the evolution of queen cycling, which elevates relatedness despite high average queen numbers. Queen cycling is a form of split-sex ratios; colonies with few queens specialise on new queen production, while colonies with many queens specialise on male production. This is consistent with worker preferences resulting from relatedness asymmetry (RA) to male and female brood but can also result from other factors. We characterised the demographic and genetic structure of colonies from three species of independent-founding Ropalidia (R. excavata, R. revolutionalis and R. gregaria) to better understand the evolutionary origins of split-sex ratios and queen cycling in their swarm-founding relatives. We found that colonies of all threespecies varied in genetic structure, driving variation in RA. In colonies where the workers' RA was high (i.e. workers were more related to female than male brood), the proportion of males in the brood was lower, consistent with the relatedness-based preferences of the workers. We also found that the proportion of males decreased in colonies with more mated and reproductively developed females. Our model explained 23% of the deviance in sex ratio across all three species and suggests that the workers manipulate the sex ratio in response to RA but that other factors are clearly important also. These results further suggest that split-sex ratios and queen cycling in the swarm-founding wasps are rooted in worker control of the colony sex ratio, and thatsuch worker control may have occurred in their independent-founding ancestors, enabling the subsequent evolution of large, multi-queen societies.