Abstract
Microbiota and symbiotic interactions are priority topics that are often explored using the Drosophila model organism. However, existing knowledge of natural relationships, i.e. in situ, between Drosophila flies and microbial symbionts is fragmented. Understanding coevolution between a host and its microbial symbionts requires a detailed understanding of the co-effects between symbiotic partners. In this PhD, I empirically studied the interactions between Drosophila flies (the model organism D. melanogaster and the pest species D. suzukii) and extracellular microbial symbionts (bacteria and yeasts) using wild strains under near-natural conditions. I investigated three main questions: (i) how do Drosophila flies acquire and transmit their microbial symbionts along their life cycle; (ii) how do these microorganisms affect host development; (iii) how do these microorganisms interact? My work revealed that yeasts and bacteria are not simply sources of nutrition (i.e. resource acquisition) for Drosophila but also influence how fly larva allocate resources between different life history traits (i.e. developmental plasticity). Second, the conducted study on microbial acquisition and transmission phenomena under near-natural conditions showed that symbionts are partially acquired from the environment, conserved through different life stages, and transmitted between generations and through mating. Thirdly, I found substantial interactions between microbial symbionts that affect their multiplication and transmission between host generations. These results reveal natural interactions of some complexity between Drosophila flies and their microbial symbionts. This demonstrates not only that these interactions are durable but also composed of nested effects that are simultaneous and invisible in obligatorily simplified laboratory systems. In addition, this work brings new elements likely to improve population control of the pest D. suzukii.