Dietary nutrient balance shapes phenotypic traits of Drosophila melanogaster in interaction with gut microbiota

Nutrient composition of dietary supply is affecting insects’ phenotypes by modulating their physiological processes. However, diet could also affect gut microbiota composition and abundance, with indirect consequences for the host. In this study, we reared Drosophila melanogaster on five different diets; three with balanced sugar:yeast ratio, but with increasing caloric content (2:2, 8:8, 16:16, in weight %), and two with imbalanced sugar:yeast ratio, either with low sugar and high yeast content (2:16) or vice-versa (16:2). In each of these diets, we compared flies with conventional vs. artificially altered gut microbiota with antibiotics that reduced the bacterial load. The antibiotic treatment also had the surprising effect of increasing the amount of live yeast associated with the flies. We characterized flies from these ten treatments (5 diets x 2 microbiota) in terms of development, body mass, food preference, body reserves, metabolic rate and a range of stress tolerance traits (heat, cold, starvation and desiccation tolerance). Diets, and to a lesser extent antibiotics treatment, affected development rate, weight, and cold tolerance of adult flies. Other traits such as energy reserves, metabolic rate, food preference, or starvation tolerance were affected by diet alone. When detected, the effect of antibiotic treatment was stronger in yeast-poor diets, suggesting that gut bacterial community might help to counterbalance nutritional deficiencies. These results show that changes in dietary factors lead to a global re-organization of fly’s physiology and development while the manipulation of gut microorganisms had minor effects that were mainly seen in case of protein restriction.