Evolutionary trends in adaptation of Drosophila melanogaster fruit flies and their symbiotic microbiome to growth mediums 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. 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 (favourable) medium. We evaluated the reproductive efficiency of experimental lines on the same two different unfavourable growth mediums 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. Besides, we found out that individual laboratory line history is also an important factor that affect evolutionary experiment results. Finally, we show that general adaptive traits (overall number of mature offsprings and their development speed) can change linearly (number of mature offsprings) and non-linearly (development speed) when fruit flies progressively adapt to growing NaCl concentration (0 → 4% → 7%).