Experimental evolution of Chlamydomonas reinhardtii in fluctuating versus constant temperature stress

Human-driven climate change has amplified the magnitude and reduced the predictability of natural environmental fluctuations. Temperature, a major physiological stressor, is rising in both its mean and the frequency of extremes. Understanding how organisms adapt to shifts in both thermal mean and variability is therefore critical for predicting their performance under future climates. Here, we investigated the standing variation in thermal optima of ten Chlamydomonas reinhardtii strains from a broad geographic sample. We then experimentally evolved one strain for ∼900 generations under three distinct temperature stress conditions: cold, hot and fluctuating. We found that those grown in constant environmental conditions became specialized to their environment of 20°C or 37°C. Contrary to our prediction that adaptation to fluctuating temperatures would compromise fitness at each extreme, those evolved in a fluctuating regime performed comparably to the specialists, suggesting no long-term cost of this generalist phenotype. We also tested the prediction that replicate lines evolved in a fluctuating regime may diverge from one another more than temperature specialists, but found no difference in the amount of among-line variance between growth regimes. Overall, despite our predictions that a fluctuating regime would hinder adaptation, we found a high degree of adaptability even when evolved in a variable environment.