Long-Term Evolution of the Complex Eukaryote Tetrahymena thermophila

Evolutionary biologists have long sought to understand what factors affect the rate and repeatability of adaptive outcomes. To better understand the role of temperature in determining adaptive trajectories, we evolved populations of different genotypes, including populations that just had sex and populations that hadn’t, of the ciliate Tetrahymena thermophila at two temperatures and followed changes in growth rate over 1000s of generations. As expected, growth rate increased with a decelerating rate for all populations; however, there were differences in the patterns of evolution at warmer and colder temperatures. Initial differences in growth rates between the genotypes decreased as evolution proceeded at both temperatures, but this convergence was quicker at the higher temperature. Likewise, we found greater repeatability of evolution among replicate populations of the same genotype at the higher temperature. We also found no evidence of trade-offs in fitness between temperatures, but did observe asymmetry in the correlated responses, whereby evolution in a warmer temperature increases growth rate at the lower temperature significantly more than the reverse. We also find the populations founded from a single newly produced sexual progeny are more evolvable than their parents who have not had sex in many generations. These results demonstrate the importance of genotype and the environment in determining evolutionary trajectories.