Causes and consequences of genetic caste-bias in the eusocial Hymenoptera

The eusocial Hymenoptera is one of the most successful groups to ever colonise the planet. They are defined by reproductive division of labour, whereby reproduction in a colony is dominated by one or a few queens, whilst all other individuals are more-or-less sterile. There has been much interest in understanding the mechanistic basis of caste determination, and particular interest in the evidence for genotypic variation in the propensity of individuals to become queens. However, the mechanisms underlying such genotypic variation in caste fate are as yet unknown. In this thesis, I examine potential causes and consequences of genetic caste-bias using the honey bee, Apis mellifera, and Acromyrmex leaf-cutting ants. In A. mellifera, individuals with queen-biased genotypes were shown to have a higher growth rate than those from genotypes with no caste-bias or a worker-bias. The variable response, in terms of growth rate, of genotypes to treatment with a juvenile hormone analogue suggested that variation in caste propensity could potentially arise from differential responses to environmentally induced physiological cues. In addition, an immune challenge by the fungal parasite Ascosphaera apis was found to affect the growth rate of A. mellifera larvae and caste determination at the colony level, suggesting that resistance to parasites may also be important in determining caste fate. In many species, workers have some control over caste determination, and I show that Ac. octospinosus workers were able to discriminate between nestmate and non-nestmate brood, even after larvae had been in contact with the fungus garden. That workers were able to discriminate between larvae within the nest suggests that they may be able to differentiate closely related kin during caste determination, despite homogenisation of fine scale cues. Finally, a positive relationship between patriline queen-worker skew and fluctuating asymmetry, an indicator of developmental stress, was found in Ac. echinatior, suggesting that subtle costs to caste-biasing strategies could explain the rarity of queen-biased genotypes. Overall this work shows that genotypic differences in the physiological response to environmental cues may underlie genetic caste-bias in the social insects. However, understanding the interaction between physiology, environment and genotype is essential if we are to understand the ultimate determinants of individual caste fate.