PS 7-88 - Genetic and evolutionary constraints in thermal performance curves

Abstract: Rising temperatures are known to affect microbial population growth, which plays a central role in the global carbon cycle. The Temperature Performance Curve (TPC) of microbial organisms’ intrinsic growth rate (r) follows a unimodal curve, where growth rate peaks at a certain optimal temperature but is lower at both colder and warmer temperatures. TPCs determine microbial responses to temperature, but how these TPCs will adapt to future climates remains an open question in thermal ecology. Here, we study possible genetic and evolutionary constraints on TPC shape by addressing: 1) whether there is genetic variation in microbial TPCs for selection to act on (i.e. heritability), 2) what aspects of TPC shape are more likely to be under selection, and, 3) which aspects of TPC shape are more likely to respond to said selection—and how—across temperatures?



To do so, we quantified the TPC of intrinsic growth rate (r-TPCs) of 22 distinct genotypes of the model protist system Tetrahymena thermophila to estimate genetic, environmental and GxE variation needed to calculate heritability in r-TPC shape. We fitted a Sharpe-Schoolfield model to extract parameter values that govern variation in TPC shape (CTmin, Ea, rpeak, Topt), then estimated selection acting on each parameter and their uni- and multivariate evolvabilities using tools from quantitative genetics.



All r-TPCs showed strong unimodal temperature effects on intrinsic growth rate (r) and large amounts of variation across genotypes. We also show the existence of temperature-dependent selection acting on different aspects of the r-TPC. Specifically, we find that environmental variation amounted to 71.7% of all observed r-TPC variation, genetic variation explained 6.1%, Gene-by-Environment interactions explained 11.7%, and there was 10.5% of residual variation. Despite large amounts of E, r-TPCs were highly heritable (Heritability >0.7), thus allowing selection to drive adaptive evolution in new climates. We also uncovered temperature-dependent selection acting on different r-TPC shape parameters, revealing possible parameters that could evolve under selection in new climates –and how. Our study highlights the importance of evolution in shaping r-TPCs and how temperature-dependent selection may result in adaptation to novel thermal environments in microbial populations.