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Page 14 of 18 Yakovleva et al. Microbiome Res Rep 2024;3:19 https://dx.doi.org/10.20517/mrr.2023.56

Figure 5. Composition of the yeast microbiome in the fly lines adapted to the substrate with different salinity and in the corresponding
substrates (the data from the paper of Dmitrieva et al. [44] ). 30 flies or 3 mL of the substrate, ten replicates. Designation of the fly lines is
the same as in Figure 1.

correlation analysis [Tables 3 and 4] revealed the possible antagonism between the bacterial and yeast
components of the fly microbiome. In samples with a relatively high abundance and/or diversity of yeasts,
the bacteria tended to be less diverse, and vice versa. The bacterial component of the microbiome appears to
be more sensitive to the salt concentration than the yeast component.

As the salinity of the substrate increases, the abundance of bacteria tends to become higher in the flies than
in the substrate [Figure 4, blue bars]. Apparently, the saltier the substrate, the smaller the contribution of
bacteria entering the intestine with the substrate to their total number in the fly. The number of bacteria per
1 mg of substrate was three orders of magnitude less than the number of bacteria per fly in the sample with
7% NaCl. A fruit fly weighs just about 1 mg, and the weight of the intestinal contents is presumably much
less; therefore, the ratio between bacterial abundance in the fly gut and the substrate is even higher. This
may be due to the water the fly drinks, thus decreasing the salinity of the intestinal content compared to the
substrate’s salinity. This probably makes it possible for less salt-tolerant bacteria to grow in the gut. In
contrast, the relative abundance of yeast in the flies, compared to that in the substrate, did not change
significantly with the increase in salt concentration [Figure 4, orange bars]. This is probably because yeasts
are generally more salt-tolerant than bacteria, and flies may selectively consume yeast biomass from the
substrate.

Our results suggest that as the salinity of the substrate increases, yeasts tend to play a more significant role
in D. melanogaster adaptation to the environment, while the importance of the bacteria decreases. This