Changes in the symbiotic yeasts of Drosophila melanogaster in course of adaptation to substrates with extra NaCl content

The model organism Drosophila melanogaster is a convenient object for studying the mechanisms of adaptation to adverse environments. In course of an evolutionary experiment conducted at the Department of Biological Evolution of Moscow State University, different Drosophila strains adapt to food substrates with different NaCl content (0, 2, 4, and 7%). Previously, we have shown that some strains of symbiotic yeast can contribute to the adaptation of Drosophila to high-salt substrates. However, the relationship between NaCl concentration and the abundance and composition of yeasts in Drosophila has not yet been studied in detail. Here, we explore the quantitative and qualitative composition of yeast component of the Drosophila microbiome in 11 laboratory lines of D. melanogaster and five food substrates inhabited by them. We find out that as the concentration of salt in the substrate increases, the yeast microbiome of Drosophila undergoes significant changes. The total abundance of yeasts changes nonlinearly: it is the lowest at 0% NaCl, the highest at 2-4%, intermediate at 7%. At a low salt concentration, the species Pichia occidentalis strongly predominates; at intermediate concentrations, the species diversity of yeasts increases while maintaining the same dominant; at 7%, a new dominant Starmerella bacillaris appears and the composition of minor components changes considerably. The results are consistent with the hypothesis that S. bacillaris may help the flies to withstand high NaCl concentrations at the early stages of adaptation, but later these useful symbionts can be lost as the genetic adaptations of insects (or other components of their microbiome) to salty food gradually develop.