Investigating the pathways of pathogen defence senescence in Drosophila melanogaster

Animals commonly become more susceptible to infection as they age. This thesis focused on determining the extent to which deterioration of individual immune mechanisms in Drosophila melanogaster contribute to overall pathogen defence senescence. I investigated how barrier defences and systemic pathogen defences senesce in flies infected with the fungal pathogen Beauveria bassiana, testing if these senescent processes are sexually dimorphic. For flies infected by dusting fungal spores onto the cuticle, both sexes became substantially more susceptible to infection while they aged. However, when fungal spores were injected through the cuticle directly into the body, only females demonstrated an age-dependent decline in pathogen defence. This suggests that pathogen defence senescence in females is largely due to impaired systemic defences, whereas for males barrier defence deterioration is mostly responsible. Sex-specific selection on males and females may have led to senescence of pathogen defence occurring by different routes in the two sexes. Exposure to some pathogens can be reduced through hygiene behaviours; I subsequently investigated if ageing-impairment of hygienic grooming behaviour underlies D. melanogaster pathogen defence senescence. Using a dye-dusting assay, I found no evidence that cuticle cleaning ability declined with age. However, although flies of all ages cleaned rapidly, considerably more dye ii particles initially attached to the cuticle of older flies, an ageing effect that was stronger in females than males. This may indicate that as individuals age, they must invest more resources in grooming behaviour to maintain body hygiene. I next tested whether age-associated changes in the cuticular hydrocarbon layer explains the increased adherence of material to the cuticle of older flies. Solvent extraction techniques were used to remove cuticular hydrocarbons and subsequently exchange them between young and old flies. Old flies washed with solvent extract from young flies took on the dye powder adherence characteristics of young flies, and vice versa; an effect that was more pronounced in females than in males. These data strongly suggest that the age-dependent increase in particle adherence to the D. melanogaster cuticle is driven by cuticular hydrocarbon changes. Most previous insect pathogen defence senescence studies focussed on changes in the cellular immune response and expression of humoral immune genes. My research demonstrates that senescence of additional non-immunological processes is also important.