Adaptation of Drosophila melanogaster to unfavorable feed substrate is accompanied by expansion of trophic niche

Adaptation to stress factors is frequently accompanied by negative side effects that are manifested in a decreased adaptation to an environment in which this stress factor is absent. This can cause ecological specialization of forms adapted to the stress conditions to the point of their separation into a new species (ecological speciation). On the other hand, the existence of eurybiont species with a wide spectrum of ecological tolerance indicates that adaptation to marginal conditions can presumably occur without negative side effects or even be accompanied by positive effects (which leads to expansion of the ecological niche). There are few experimental confirmations of the reality of such an evolutionary scenario. During an evolutionary experiment on the Drosophila melanogaster, experimental fly lines successfully adapted to life on stressful feed substrates with an increased NaCl content for 20 generations. This was manifested in an increase in the total amount of descendants produced by parents from adapted lines on stressful substrate for a fixed time as compared with the control (unadapted) line, as well as in a reduction of the larval development delay caused by high NaCl concentrations. Not only did the adaptation to a standard (favorable) feed medium in lines adapted to salt not decrease, but it even increased. Thus, adaptation to unfavorable medium actually resulted in trophic niche expansion. Along with similar results previously obtained during a study of the D. melanogaster adaptation to a depleted feed medium based on starch, these data indicate that an adaptation to marginal conditions that is accompanied by positive side effects and does not lead to constriction but to expansion of the trophic niche can be a common phenomenon in such eurybiont species as D. melanogaster, which probably largely explains their eurybiontity. The small number of experimentally confirmed examples of such a scenario can be partly explained be the small number of attempted searches for them. Adaptive changes of symbiotic microbiota, which are efficiently transmitted in the Drosophila from parents to descendants when the descendants eat the same substrate as the parents, can be one possible mechanism of “wide-profile” adaptation output during the development of the stressful substrate. Thus, a growth of the number of symbiotic Lactobacilli in the Drosophila intestine can promote acceleration of larvae growth, extension of imago life, and an increased efficiency of the used feed substrate. Further studies are required to detect the specific mechanisms responsible for the changes in adaptability found during the evolutionary experiment.