Evolution under pH stress and high population densities leads to increased density-dependent fitness in the protistTetrahymena thermophila

Abstract

Abiotic stress is a major force of selection that organisms are constantly facing. While the evolutionary effects of various stressors have been broadly studied, it is only more recently that the relevance of interactions between evolution and underlying ecological conditions, that is, eco-evolutionary feedbacks, have been highlighted. Here, we experimentally investigated the interaction between adaptation to pH-stress, and ecological conditions, specifically demography. Using the protist species Tetrahymena thermophila, we studied how four different genotypes evolved in response to stressfully low pH conditions and high population densities. We found that genotypes underwent evolutionary changes, some shifting up and others shifting down their intrinsic rates of increase (r 0 ). Overall, evolution at low pH led to the convergence of r 0 and intraspecific competitive ability (α) across the four genotypes. Given the strong correlation between r 0 and α, we argue that this convergence was a consequence of selection for increased density-dependent fitness at low pH under the experienced high density conditions. Increased density-dependent fitness was either attained through increase in r 0 , or decrease of α, depending on the genetic background. In conclusion, we show that demography can interact with abiotic stress, to impact the direction of evolution under abiotic stress.FM, FA and EAF designed the experiment. FM, AA and SM performed the experimental work. Statistical analyses were done by FM and EAF. FM, FA, AW and EAF interpreted the results. FM, FA and EAF wrote the first version of the manuscript and all authors commented the final version.