The Pheromonal Basis of Reproductive Constraint in Eusocial Insects

The reproductive division of labour is a key characteristic of eusociality. In Apis mellifera honeybees, this is maintained in large part through the secretion of pheromones, particularly by the queen, which prevent the sexual development of workers. The aim of this thesis was primarily to investigate the mechanism of action of the principal reproductively constraining honeybee queen pheromone: Queen Mandibular Pheromone (QMP). The secondary aim was to investigate the role of less well-studied pheromones, such as brood pheromone (BP) and E-β-ocimene (EBO), on reproductive constraint.
Although many disparate elements of QMP-mediated physiological effects have been characterised, there has yet to be identified any unifying mechanism which explains how the physical interaction of a worker honeybee with QMP is able to bring about reproductive repression of worker ovaries. Similarly, QMP can act incredibly widely across the insect orders, repressing reproduction in the bumblebee Bombus terrestris and the fruit fly Drosophila melanogaster, and it is unknown whether QMP is acting via the same mechanisms in these other insect species as it is able to act in honeybees. Additionally, although the larval pheromones BP and EBO have been shown to bring about reproductive repression in workers, the mechanism by which these do so is completely unknown.
In chapter 3 I investigate different elements of the effect of QMP on honeybees at a physiological level, demonstrating that honeybees lack plasticity in their adult reproductive constraint mediated by QMP, and that QMP likely induces starvation in honeybee workers in a similar manner to fruit flies. In chapter 4 I attempt to establish an in cavea B. terrestris model for investigating the mechanism of action of QMP and pentacosane in this species. In chapter 5 I demonstrate a lack of reproducibility for the published work showing that BP and EBO bring about reproductive repression in honeybees, and show that these pheromones do not induce reproductive constraint in D. melanogaster. In chapter 6 I investigate the plasticity of QMP effect in D. melanogaster, and identify that diapause is likely the mechanism through which QMP is able to bring about repression of reproduction in this species, but that the repression is likely mediated through different tissues than cold-induced diapause.