[Adaptation of Drosophila melanogaster to stressful nutritional conditions leads to the expansion of the trophic niche].

Adaptation to stress factors is often accompanied by negative side effects that are manifested in lower fitness in the absence of the stress factor. This can lead to ecological specialization of the populations adapted to stressful environment and, ultimately, to ecological speciation. However, the existence of eurytopic species with a wide spectrum of ecological tolerance implies that adaptation to marginal conditions apparently can proceed without negative side effects or even involve positive effects, leading to niche expansion. Experimental evidence in favour of this evolutionary scenario is scarce. In the course of the evolutionary experiment that lasted for 20 generations, the laboratory populations of Drosophila melanogaster successfully adapted to stressful media with high NaCl concentration. The adaptation is manifested through the higher number of offspring produced during a fixed time interval by a pair of parents from the adapted lineages on the stressful medium compared to the control (unadapted) lineage, and in the less pronounced delay in larval development caused by high NaCl concentration. The adaptation to stressful medium did not entail fitness costs on the standard (favorable) medium; moreover, it resulted in more effective reproduction in favorable conditions (expansion of the trophic niche). These results, together with those obtained earlier during the study of adaptation of D. melanogaster to nutrient-poor starch based medium, imply that adaptation to marginal conditions accompanied by positive (rather than negative) side effects, leading to the expansion of the trophic niche, may be a frequent phenomenon in eurytopic species like D. melanogaster, probably explaining, to some extent, their ecological tolerance. Scarcity of experimentally confirmed examples of such evolutionary scenario is probably due to low number of attempts to find them. One possible mechanism of 'multi-purpose adaptations' obtained during the acclimation to environmental stress is the adaptive changes of symbiotic microbiota which, in Drosophila, is efficiently transferred between generations if offspring eat the medium on which their parents had lived. For instance, high quantities of symbiotic lactobacilli in the gut can enhance larval growth, life span of adults, and the efficiency of substrate utilization. Further studies are needed to reveal the mechanisms responsible for the changes in fitness observed in the course of the experiment.