Larval density affects phenotype and surrounding bacterial community without altering gut microbiota in Drosophila melanogaster. Does larval density affects bacteria diversity?

Larval crowding represents a complex stressful situation arising from inter-individual competition for time- and space-limited resources. The foraging of a large number of individuals may alter chemical and bacterial composition of food, and in turn affect individual’s traits. Here we used Drosophila melanogaster to explore these assumptions. First, we used a wide larval density gradient to evidence the impact of crowding on phenotypical traits. We confirmed that high densities increased development time and pupation height, and decreased viability and body mass. Next, we measured concentrations of common metabolic wastes (ammonia, uric acid) and characterized bacterial communities, both in food and in larvae, for three contrasted larval densities (low, medium and high). Ammonia concentration increased in food from medium and high larval densities, but remained low in larvae regardless of the larval density. Uric acid did not accumulate in food but was detected in larvae. Surprisingly, bacterial composition remained stable in guts of larvae whatever their rearing density, although it drastically changed in the food. Overall, these results indicate that crowding deeply affects individuals, but also their abiotic and biotic surroundings. Environmental bacterial community likely adapts to altered nutritional situations resulting from crowding, putatively acting as scavengers of larval metabolic wastes.