Housing density affects the survivorship post infection of Drosophila melanogaster males in a pathogen dependent...

The impact of social interaction on physiology and behaviour has been widely studied in social insects, but less so in non-eusocial insects. Group housing can signal higher infection risk and reproductive competition, potentially leading to context-dependent trade-offs between reproductive versus immune investment. In this study, we housed Drosophila melanogaster males singly or in groups of two or sixteen for two days and assessed their post infection survival. We found that singly held males exhibited higher mortality post-infection with Pseudomonas entomophila compared to males housed in groups of sixteen. Males held singly or as pairs did not significantly differ in their mortality. However, there was no difference in mortality between the three groups when the pathogen was Bacillus thuringiensis. Singly held males showed elevated levels of cat1, sod and puc and lower levels of rel, indicating elevated stress levels and lower immunity in them. Environmental factors other than the housing density did not account for the difference in survivorship between singly held and group-held flies post-infection with P. entomophila. We observed no trade-off between reproductive investment and post-infection survivorship. Thus, our results show a complex interplay between social isolation, stress related mechanisms and immune function of a non-eusocial insect.