| Summary: | Social environments can influence health and fitness in a variety of organisms. Since both social contact and isolation can be stressful, elucidating the mechanisms underlying the patterns of variation is essential to understanding how social environment contributes to overall phenotypes. Using a Drosophila melanogaster fruit fly model, I investigated how social contact influences ageing, immunity and the microbiome, in a sex-specific manner. Throughout, I used same sex pairs or groups to avoid the costs of mating and reproduction. Firstly I examined the effect of social environment on actuarial ageing and functional senescence in unwounded and wounded individuals, as well as stress responses. I found that pairing acted to decrease lifespan and stress resistance for both sexes, but when combined with wounding this effect was more severe for males. Climbing ability decreased with age, but this was more severe for paired females than those kept isolated. I next challenged the immune system directly by using a bacterial injection, as well as measuring gene expression and phagocytosis. In contrast to prior work, I found that older paired flies lived longer post-infection than flies that lived alone. Furthermore, gene expression and flow cytometry data suggested that some immune response pathways are more socially-responsive than others. I next determined the effect of social contact on the bacterial community associated with D. melanogaster. I found that the male microbiome is affected disproportionately compared to females, indicating that changes in species richness and evenness are not solely the result of horizontal transfer. I also found that age of cohabitants can have a marked effect on the microbiome composition and found that this can have important physiological implications - paired males are less able to cope with oral infections. Taken together, these findings suggest that the effects of social environments differ both between sexes and between traits.
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