Genetic diversity and temperature increases associated with global climate change are known to independently influence population growth and extinction risk. Whether increasing temperature may influence the effect of genetic diversity on population growth, however, is not known. We address this issue in the model protist system Tetrahymena thermophila. We test the hypothesis that at temperatures closer to the species’ thermal optimum (i.e., the temperature at which population growth is maximal, or Topt), genetic diversity should have a weaker effect on population growth compared to temperatures away from the thermal optimum. To do so, we grew populations of T. thermophila with varying levels of genetic diversity at increasingly warmer temperatures and quantified their intrinsic population growth rate, r. We found that genetic diversity increases population growth at cooler temperatures, but that as temperature increases, this effect weakens. We also show that a combination of changes in the amount of expressed genetic diversity (G), plastic changes in population growth across temperatures (E), and strong GxE interactions, underlie this temperature effect. Our results uncover important but largely overlooked temperature effects that have implications for the management of small populations with depauperate genetic stocks in an increasingly warming world. --

We sourced five clonal lines (B2086.2, A*III, CU438.1, A*V, and CU427.4) of the protist Tetrahymena thermophila from the Cornell University Tetrahymena Stock Center from across three putatively different genetic backgrounds (A*III and A*V have genetic background A, B2086.2 has background B, while CU438.1 and CU427.4, have background C). The lines were reared in Carolina Biological protist medium® (Burlington, NC) in 200mL autoclaved borosilicate jars, and a 16-8 day/night cycle at 22ºC within Percival growth chambers (Perry, IA).

To determine whether temperature alters the effects of genetic diversity on population growth, we manipulated the temperature and initial genetic diversity of microcosm populations. To manipulate genetic diversity, we started populations with a varying number of clonal lines (1, 2, 3, 4 or 5 lines). Monoclonal cultures were initialized with 50 individual protists. For all other treatments, the initial abundance of each clone depended on the total number of clones present, to control for possible effects of initial density: 2-clone populations started with 25 individuals/clone, 3-clone populations started with ~16 individuals/clone, 4-clone populations started with ~12 individuals/clone, and 5-clone populations started with 10 individuals/clone. Each monoclonal population, and each combination of four and five clones, was replicated 4 times. Each combination of two and three clones was replicated twice, for a total of 84 experimental populations per temperature. All experimental microcosms were reared in 3mL of growth media in 35 mm petri dishes.

We crossed the five genetic diversity treatments with four temperature treatments (19, 22, 25, 28 ºC) along the rising portion of the thermal performance curve (TPC) of the organism (Fig 1c), for a total 336 experimental microcosms. The TPC was itself was estimated as the intrinsic growth rate, r, of a well-mixed population (i.e., comprising the same number of initial individuals per clone, all starting at 3 ind/mL in 3mL petri dishes), at seven different temperatures (13, 19, 22, 27, 30, 32, 35 and 37 ºC). Experimental microcosms were grown in Percival growth chambers with all other environmental variables mimicking rearing conditions.

After a 24-hr incubation period, we estimated final population size by sub-sampling each microcosm and counting individual cells under a stereomicroscope (Leica, M205 C). Assuming exponential growth, the intrinsic growth rate (r, which has units of t^-1) of each microcosm population was calculated as [log(Nf)-log(Ni)]/time, with time=1 day, N_f being the final abundance, and N_i the initial density (=50 ind for all experimental microcosms).