Attributes Of Royalty In The Primitively Eusocial Wasp Ropalidia marginata : Pheromone, Ovaries And Behavior

Abstract

This thesis has looked at the proximate mechanisms by which eusociality is maintained in colonies of the primitively eusocial wasp Ropalidia marginata. Unlike other typical primitively eusocial species, the R, marginata queen is remarkably docile and non-interactive and hence cannot possibly use aggression to maintain her status. Recent evidence hints at pheromonal queen signalling through the Dufour’s gland. Hence, queen-worker difference in Dufour’s gland composition has been studied in details. Queens and workers differ with respect to overall composition of Dufour’s glands, categories of compounds, and individual compounds as well. The Dufour’s gland compounds may be having a bouquet effect in queen signalling, with individual compounds being less important than the overall composition. The queen pheromone also appears to be an honest signal of fertility, as compounds that differ consistently between queens and workers are correlated with ovarian development of queens, and solitary foundresses and potential queens, who are intermediate between queens and workers in ovarian development, are intermediate in their Dufour’s gland profile as well. When the queen is removed from a colony, one of the workers (potential queen, PQ) shows high aggression, and if the queen is not returned, goes on to become the next queen of the colony. The aggression of PQ comes down as a function of time since queen removal, and correlated with this, the ovaries of PQ increase. Dufour’s gland profile of PQ is similar to workers immediately after queen removal, but comes closer to queens with passage of time. This hints at an interesting transition in maintenance of eusociality from “queen control” by aggression to “queen signal” by pheromone during the queen establishment phase. It has generally been assumed that one set of chemicals can carry multiple information, namely queen signal and colony signal. Initial statistical analysis of chemical composition data showed that perhaps both caste and colony signals can be conveyed by the Dufour’s gland compounds, but detailed analysis cast some doubt on this, as the Dufour’s gland compounds could not be separated into non-overlapping subsets with respect to importance in caste and colony discrimination. A bioassay showed that the wasps do not make colony discrimination from Dufour’s gland compounds. This suggests that the ability to statistically differentiate groups of organisms from their chemical profiles does not guarantee similar discrimination by the organisms themselves, emphasising the need for bioassays to resolve such issues.