Biology subjects

Tarkington, J. A.

Publications and source records attributed to Tarkington, J. A..

3 recordsLinked to original sources

bioRxiv2022

Comparing evolvability and pleiotropy across environments

The environment can play an important role in determining evolutionary outcomes (Reboud and Bell 1997; Stanton et al. 2000; Gresham et al. 2008; Cooper and Lenski 2010; Becks and Agrawal 2012; Bailey et al. 2015). Populations may increase in fitness more after evolution in one environment than in another (Hegreness et al. 2008). This may be because the distance between the ancestral genotype and the top of the locally accessible fitness peak may be greater in one environment or the other. Additionally, the rate at which mutations occur and populations move up the peaks could differ. For this reason, comparing evolvability across environments presents an interesting problem. Here, we show an environment impacts evolvability in two ways, directly by impacting the rate of adaptative evolution, and indirectly by limiting the range of fitness outcomes that are possible. We show that correlated responses are often highly idiosyncratic, due to variation in the range of possible fitness outcomes in the environment and differences in pleiotropic effects across environments but can also be predicted from the ancestral growth rate regardless of the environment in which populations evolve. Interestingly, we also show a negative correlation between increase in an environment X following evolution in environment Y and the increase in environment Y following evolution in environment X. These results highlight the necessity to measure fitness in both environments when comparing the evolvability or repeatability of evolution across environments.

evolutionary biology↗

bioRxiv2021

Sex, amitosis, and evolvability in the ciliate Tetrahymena thermophila

Understanding the mechanisms that generate genetic variation, and thus contribute to the process of adaptation, is a major goal of evolutionary biology. Mutation and genetic exchange have been well studied as mechanisms to generate genetic variation. However, there are additional processes that may also generate substantial genetic variation in some populations and the extent to which these variation generating mechanisms are themselves shaped by natural selection is still an open question. Tetrahymena thermophila is a ciliate with an unusual mechanism of nuclear division, called amitosis, which can generate genetic variation among the asexual descendants of a newly produced sexual progeny. We hypothesize that amitosis thus increases the evolvability of newly produced sexual progeny relative to species that undergo mitosis. To test this hypothesis, we used experimental evolution and simulations to compare the rate of adaptation in T. thermophila populations founded by a single sexual progeny to parental populations that had not had sex in many generations. The populations founded by a sexual progeny adapted more quickly than parental populations in both laboratory populations and simulated populations. This suggests that the additional genetic variation generated by amitosis of a heterozygote can increase the rate of adaptation following sex and may help explain the evolutionary success of the unusual genetic architecture of Tetrahymena and ciliates more generally.

evolutionary biology↗

bioRxiv2020

Temperature affects the repeatability of evolution in the microbial eukaryote Tetrahymena thermophila

Evolutionary biologists have long sought to understand what factors affect the repeatability of adaptive outcomes. To better understand the role of temperature in determining the repeatability of adaptive trajectories, we evolved populations of different genotypes of the ciliate Tetrahymena thermophila at low and high temperatures and followed changes in growth rate over 4,000 generations. As expected, growth rate increased with a decelerating rate for all populations; however, there were differences in the patterns of evolution at the two temperatures. The growth rates of the different genotypes converged as evolution proceeded at both temperatures, but this convergence was quicker at the higher temperature. Likewise, we found greater repeatability of evolution, in terms of change in growth rate, among replicates of the same genotype at the higher temperature. Finally, we found no evidence of trade-offs in fitness between temperatures, but did observe asymmetry in the correlated responses, whereby evolution in a high temperature increases growth rate at the lower temperature significantly more than the reverse. These results demonstrate the importance of temperature in determining the repeatability of evolutionary trajectories.

evolutionary biology↗