Phenotypic and genetic constraints on the reversible acclimation of thermal performance curves in the zebrafish, Danio rerio.

In response to seasonal variation in environmental temperature, many ectotherms are able to adjust the thermal dependence of their physiological processes via a plastic change in phenotype known as acclimation. Thermal acclimation can induce a reversible plasticity in the shape and position of thermally-sensitive physiological traits among environments. Although often assumed to be beneficial, acclimation is rarely perfect and selection experiments have demonstrated that thermal acclimation can induce both adaptive and non-adaptive plastic changes in phenotype between environments. A lack of consideration for the costs involved with large phenotypic changes may assist in explaining why an adaptive benefit to acclimation is not generally found. In this thesis, I examine three possible constraints on the acclimation of reversible thermal performance curves (TPCs) in the zebrafish, Danio rerio. These constraints are: the effect of cue reliability on the induction of seasonal acclimation responses, the distribution of genetic variance in reversible TPCs for selection in different environments and the influence of a thermodynamic effect on the evolution of TPCs. I tested the effect of correlated (seasonally matching) and conflicting (seasonally opposite) temperature and photoperiod cues on the acclimation of thermal sensitivity for two whole- animal performance traits. I found that thermal performance curves for burst swimming and feeding rate were significantly affected by acclimation to 18 or 30 °C, but that photoperiod had no effect on the shape or position of thermal performance curves in either trait. This study is the first that has found no effect of covariation between temperature and photoperiod cues on locomotor performance and supports the intuitive idea that animals living at low latitude are not heavily reliant on photoperiod as a seasonal cue for environmental variability. Second, I examined the quantitative genetic variation in thermal performance curves of male Danio rerio thermally acclimated to 16 and 32 °C. I found a significant genetic difference in the distribution of genetic variance between zebrafish acclimated to different temperatures. The results show that the genetic architecture of a reversible plastic trait changes between seasonal thermal environments within a single population of zebrafish. I found no evidence of a genetic trade-off between curve width and height in either acclimation temperature. Instead, I found positive genetic co-variance in performance at all  temperatures for 32 °C-acclimated males, with the greatest genetic variance found in environments with the greatest thermal stress. The results suggest that the height of TPC is not constrained in warm acclimated D. rerio but future selection for cold-adaptation in 16 °C-acclimated treatment may reduce performance in extreme warm temperatures tested in this study. Last, I examined whether the acclimation of TPCs in zebrafish are constrained by a thermodynamic effect known as ‘hotter is better’. Under hotter is better, warmer populations are predicted to have greater fitness or performance than cold populations due to the rate-limiting effect of temperature in cold conditions. I found that a genetic correlation indicating hotter is better was present in 16 °C-acclimated male zebrafish, but this correlation was reversed in 32 °C-acclimated fish. These results indicate that a thermodynamic constraint on reversible changes in thermal sensitivity may limit the evolution of plasticity of the thermal optimum in contrast to the predictions of current optimality models of reversible phenotypic plasticity. Together these results demonstrate that both phenotypic and genetic constraints on the production of reversible seasonal phenotypes may limit the acclimation of TPCs in some environments. Significantly, I found that the genetic architecture of reversible plastic thermal performance curves can vary within the same population of organisms exposed to different thermal environments. The evolution of TPCs in both warm- and cold- acclimated zebrafish is unconstrained by a thermal specialist-generalist trade-off but the evolution of TPCs in cold-acclimated D. rerio may be constrained by hotter is better. These results are the first to examine quantitative genetic variance in a reversible plastic response and one of the first to examine the evolution of thermal sensitivity in a vertebrate animal. I discuss the findings of these studies in light of predicted constraints on the evolution of thermal sensitivity and the potential significance of these results to the thermal adaptation of physiological traits under future climate change