Metabolic and immunological responses of Drosophila melanogaster to dietary restriction and bacterial infection differ substantially between genotypes in a population

To respond to changing environmental conditions, a population may either shift towards better-adapted genotypes or adapt on an individual level. The present work aimed to quantify the relevance of these two processes by comparing the responses of defined Drosophila populations to different stressors. To do this, we infected two homogeneous populations (isofemale lines), which differ significantly in fitness, and a synthetic heterogeneous population with a specific pathogen and/or exposed them to food restriction. Pectobacterium carotovorum was used to infect Drosophila larvae either fed standard or protein-restricted diets. In particular, the two homogeneous groups, which diverged in their fitness, showed considerable differences in all parameters assessed (survivorship, protein and lipid contents, phenol-oxidase activity, and antibacterial rate). Under fully nutritious conditions, larvae of the homogeneous population with low fitness exhibited lower survivorship and protein levels, as well as higher phenol-oxidase activity and antibacterial rate compared with the fitter population. A protein-restricted diet and bacterial infection provoked a decrease in survivorship, and antibacterial rate in most populations. Bacterial infection elicited an opposite response in protein and lipid content in both isofemale lines tested. Interestingly, the heterogeneous population showed a complex response pattern. The response of the heterogeneous population followed the fit genotype in terms of survival and antibacterial activity but followed the unfit genotype in terms of phenol-oxidase activity. In conclusion, our results show that defined genotypes exhibit highly divergent responses to varying stressors that are difficult to predict. Furthermore, the responses of heterogeneous populations do not follow a fixed pattern showing a very high degree of plasticity and differences between different genotypes.

Methods
Two isofemale (homogenous) lines of Drosophila melanogaster were selected for use in the present study based on emergence percentage (survivorship) and referred to as  A and B. Line A was the population that produced the lowest emergence rate while line B was the population that produced the highest emergence rate of the eight isofemale lines tested. A heterogeneous population (experimental) was created by mixing the eight isofemale lines altogether. To assess the effects of protein-restricted diet and/or infection by  Pectobacterium carotovorum on the response of the populations A and B (genotypes) and the heterogeneous population (Exp), first instar Drosophila larvae were allowed to develop under four environmental conditions. These conditions were uninfected+standard diet, uninfected+restricted diet, infected+standard diet, and infected+restricted diet. Parameters such as survivorship, lipid and protein contents, phenol-oxidase activity, and antibacterial rate were assessed following exposure. Moreover, the extent of plasticity among isofemale lines (genotypes) and the heterogeneous population, was calculated using the relative distance plasticity index (RDPI). Survivorship was observed as the number of emerged adults out of ten larvae. The lipid and protein contents, phenol-oxidase activity and antibacterial rate were determined spectrophotometrically in the whole body homogenate of D. melanogaster larvae.