Caste plasticity and task allocation in a primitively eusocial insect

Abstract

Phenotypic plasticity is the ability of a given genotype to express different phenotypes in response to different environmental conditions. Plasticity is a key trait when an individual’s role in the social environment, and hence its optimum phenotype, fluctuates unpredictably. Extensive evidence exists for the capacity of individuals to be sensitive to their environments and exhibit phenotypic plasticity as a response. The selective advantage this can bring has been explored in a range of taxa. Plasticity is especially important in primitively eusocial insects where small colony sizes and little morphological caste differentiation mean that individuals can readily adopt and change specialised behavioural or physiological roles within their lifetimes. In this thesis, I examine the extent of within lifetime plasticity and task specialisation using a remarkable nest aggregation of the ground nesting primitively eusocial sweat bee, Lasioglossum malachurum. In Chapter 2, I perform a queen removal experiment and present evidence of workers taking on a reproductive role (worker queen). I find that they produce no fewer offspring than an original queen (foundress queen), but their social control of the group is likely constrained since workers from worker queen nests forage less than those in foundress queen nests. In order to quantify differences between these nest types, in Chapter 3, I use RNA-seq to explore the gene expression of worker queens, foundress queens and their workers examined in Chapter 2. I find that worker queens show extensive plasticity at the gene expression level but this does not perfectly match foundress queens in key caste differentiating genes. Furthermore, workers in worker queen nests show a generalised response to queen removal even after a new reproductive is established suggesting that all individuals respond to queen removal even if observable behavioural traits, such as foraging, continue to be carried out. Finally in Chapter 4, I present evidence of task specialisation within the worker caste of L. malachurum for foraging behaviour but not for guarding behaviour. I also discuss why there might be inactive workers within the nest, including the possibility that they prioritise direct fitness over helping behaviour. Together, this work provides novel evidence for the extent of phenotypic plasticity in primitively eusocial species, and highlights the importance of the ecology of L. malachurum for understanding the level of complexity in its behaviour.