Résumé
Microbial symbionts can either provide resources to their hosts or mediate host plasticity through physiological signaling. Symbionts that improve resource availability would be beneficial in all environments, which favors the evolution of stable mutualism. However, when symbiont affects plasticity, fitness consequences would depend on the environmental context, with consequences on coevolution. In order to separate symbiont effects on host resources and plasticity, we propose a new multivariate framework based on the analysis of trade-offs between fitness components. We applied the framework to a series of experiments on the symbiosis between bacteria, yeasts and three fruit-fly species, Drosophila melanogaster, D. simulans and D. suzukii. In particular, we focused on the trade-off between speed of larval development and adult size that is largely documented in holometabolous insects. We found evidence for symbiont effects on both host resources availability and developmental plasticity. Taking into account host sex, genotype and species revealed a major effect of host species on whether yeast affects plasticity or not. Besides, evidence suggests Drosophila's bacterial symbionts may influence plasticity more than yeast does. Further developments of the framework will enable the simultaneous analysis of a greater number of fitness components and should encompass other types of variation (e.g. genetic). Until our framework's evolutionary predictions are tested, its results can be used to improve the mass-production of insect (e.g. for the sterile insect technique) so as to fine-tune insect vigor and phenotype to contextual needs