Evolutionary Trends in Adaptation of Drosophila melanogaster Fruit Flies and Their Symbiotic Microbiome to Growth Media with Different NaCl Concentrations

Many experimental evolution studies have shown the ability of model animal species, such as Drosophila melanogaster, to adapt quickly to various adverse conditions such as unfavorable growth medium. It is usually assumed by default that such adaptation is due to changes in the genes of the macroorganisms under study. However, it is also known that the microbiome can influence various biological processes in macroorganisms. Here we performed an evolutionary experiment in which some D. melanogaster lines were reared on a growth medium with two regimes of high NaCl concentration (4% and 7%), while the others were reared on the standard (favorable) medium. We evaluated the reproductive efficiency of experimental lines on the same two different unfavorable growth media five years after the experiment started. Our tests confirmed that the lines reared on the salty substrate became more tolerant to high NaCl concentration, with lines reared on 7% being overall better than lines reared on 4%, especially when tested on 7% medium. Moreover, we found that pre-inoculation of the high salt medium with homogenized salt-tolerant flies tended to improve reproductive efficiency on this medium (compared to pre-inoculation with homogenized control flies). In addition, we found out that individual history of the laboratory line is also an important factor that affects the results of evolutionary experiment. Finally, we show that general adaptive traits (overall number of mature offspring and their rate of development) can change linearly (number of mature offspring) and nonlinearly (rate of development) when fruit flies progressively adapt to growing NaCl concentration (0 → 4% → 7%).