Anticipatory plasticity: how early life environmental cues affect development and behaviour

In this thesis, I examine how social conditions experienced during early life stages can have effects into adulthood. I use the fruit fly Drosophila melanogaster as a model species to explore this; using larval social conditions that produce an anticipatory response to reproductive competition in adult males, namely high density and the presence of adult males. I demonstrate that the type of social conditions experienced by the larvae can have distinct effects on aspects of cognition, stress, immunity and the microbiome. First, I demonstrate that manipulating larval density can influence adult learning ability and relative expression of a synaptic growth gene. I show that adult lifespan is negatively affected by the presence of adult males during larval stages, and that development time is longer for those reared at high larval density, but a number of adult female reproductive traits are not influenced by these conditions. I find type of larval social conditions and sex-specific responses to specific stressors in young adults. In particular, females from adult presence larval conditions show increased cold and desiccation stress tolerance, but decreased post-infection lifespan after bacterial injection with Bacillus thuringiensis, suggesting a trade-off may be occurring. I also examine the effect of these conditions on microbiome composition, finding distinct effects of pupal versus adult stages, and an increased microbial diversity in adult presence pupae. Overall, my results suggest that there are a number of factors in adult fruit flies that can be affected by larval social environments. These responses are often dependent on the type of social conditions experienced, the adult trait examined, and the sex of the fly. This study highlights early life conditions can have important and long-lasting consequences.