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.

. Abstract
. Plasticity is a key trait when an individual’s role in the social environment, and hence its optimum phenotype, fluctuates unpredictably. Plasticity is especially important in primitively eusocial insects where small colony sizes and little morphological caste differentiation mean that individuals may find themselves switching from non-reproductive to reproductive roles. To understand the scope of this plasticity, workers of the primitively eusocial sweat bee Lasioglossum malachurum were experimentally promoted to the reproductive role (worker-queens) and their performance compared with foundress-queens. We focussed on how their developmental trajectory as workers influenced three key traits: group productivity, monopolisation of reproduction, and social control of foraging nest-mates. No significant difference was found between the number of offspring produced by worker-queens and foundress-queens. Genotyping of larvae showed that worker-queens monopolised reproduction in their nests to the same extent as foundress queens. However, non-reproductives foraged less and produced a smaller total offspring biomass when the reproductive was a promoted worker: offspring of worker-queens were all males, which are the cheaper sex to produce. Greater investment in each offspring as the number of foragers increased suggests a limit to both worker-queen and foundress-queen offspring production when a greater quantity of pollen arrives at the nest. The data presented here suggest a remarkable level of plasticity and represent one of the first quantitative studies of worker reproductive plasticity in a non-model primitively eusocial species.
.
. Significance statement
. The ability of workers to take on a reproductive role and produce offspring is expected to relate strongly to the size of their colony. Workers in species with smaller colony sizes should have greater reproductive potential to insure against the death of the queen. We quantified the reproductive plasticity of workers in small colonies of sweat bees by removing the queen and allowing the workers to control the reproductive output of the nest. A single worker then took on the reproductive role and hence prevented her fellow workers from producing offspring of their own. These worker-queens produced as many offspring as control queens, demonstrating remarkable worker plasticity in a primitively eusocial species.
.