The genetic and mechanistic basis of worker sterility in the honey bee

Abstract

Worker sterility is a defining feature of social insects. However, the evolution of sterility is a conundrum because workers ‘altruistically’ forgo personal reproduction. To understand how worker sterility has evolved, it is important to identify both its genetic and mechanistic basis. In this thesis I utilise an ‘evo-devo’ framework to propose that the mechanistic basis of worker sterility can be conceptualised as ‘reproductive control points’ – specific mechanisms that reduce the reproductive capacity of workers. I provide empirical evidence for two control points in honey bee (Apis mellifera) workers. The first control point is when the queen’s pheromone triggers the abortion of adult honey bee workers’ oocytes at mid-oogenesis. I show that when workers are exposed to the queen’s pheromone, their germ cells degenerate midway through development. I also find that the candidate gene, Anarchy, is important for the mid-oogenesis control point where it causes increased programmed cell death activity in the ovaries of workers exposed to a queen. The second control point is the loss of ovarioles in adult honey bee workers. I show that the number of ovarioles declines as honey bee workers age, due to programmed cell death. The mechanism underlying all the reproductive control points, and therefore worker sterility, is likely to be programmed cell death. My thesis is therefore an important contribution to a mechanistic understanding of worker sterility, and provides insights into how this trait emerged from a solitary ancestor. In addition, this interdisciplinary thesis reflects upon my empirical research by examining how key molecular biology techniques are developed. I conclude that the techniques are developed in a characteristic, staged process with easily-identified common elements. The techniques of molecular biology lead to immense scientific progress and my examination of their development provides a compelling case for the importance of basic research.