Abstract
Model organisms can be useful for studying climate change impacts, but it is unclear whether domestication to laboratory conditions has altered their thermal tolerance and therefore how representative of wild populations they are. Zebrafish in the wild live in fluctuating thermal environments that potentially reach harmful temperatures. In the laboratory, zebrafish have gone through four decades of domestication and adaptation to stable optimal temperatures with few thermal extremes. If maintaining thermal tolerance is costly or if genetic traits promoting laboratory fitness at optimal temperature differ from genetic traits for high thermal tolerance, the thermal tolerance of laboratory zebrafish could be hypothesized to be lower than that of wild zebrafish. Furthermore, very little is known about the thermal environment of wild zebrafish and how close to their thermal limits they live. Here, we compared the acute upper thermal tolerance (critical thermal maxima; CTmax) of wild zebrafish measured on-site in West Bengal, India, to zebrafish at three laboratory acclimation/domestication levels: wild-caught, F1 generation wild-caught and domesticated laboratory AB-WT line. We found that in the wild, CTmax increased with increasing site temperature. Yet at the warmest site, zebrafish lived very close to their thermal limit, suggesting that they may currently encounter lethal temperatures. In the laboratory, acclimation temperature appeared to have a stronger effect on CTmax than it did in the wild. The fish in the wild also had a 0.85-1.01°C lower CTmax compared to all laboratory populations. This difference between laboratory-held and wild populations shows that environmental conditions can affect zebrafish's thermal tolerance. However, there was no difference in CTmax between the laboratory-held populations regardless of the domestication duration. This suggests that thermal tolerance is maintained during domestication and highlights that experiments using domesticated laboratory-reared model species can be appropriate for addressing certain questions on thermal tolerance and global warming impacts.
Highlights
Climate change is predicted to cause a continued increase in global water temperatures and increase the frequency and severity of extreme heat waves (Meehl and Tebaldi, 2004; IPCC, 2013; Seneviratne et al, 2014)
Weight was included in the model as a covariate. Since this analysis was performed on fish acclimated to a temperature of 28◦C, and for the India population only included fish tested at one site while the remaining populations originated from fish collected at several sites, we investigated whether any differences in critical thermal maxima (CTmax) could be due to differing genetic diversity by calculating the coefficient of variation (CV) for each population
Individual variation in CTmax did not differ between the populations
Summary
Climate change is predicted to cause a continued increase in global water temperatures and increase the frequency and severity of extreme heat waves (Meehl and Tebaldi, 2004; IPCC, 2013; Seneviratne et al, 2014). Thermal stress triggers a multitude of physiological acclimation responses within the organism in order to improve overall function at non-optimal temperatures (Dietz and Somero, 1992; Angilletta, 2009) This phenotypic plasticity can be advantageous to organisms living in heterogeneous environments and could be so under a climate change scenario. The thermal acclimation process can be both energetically costly (Angilletta, 2009) and come at a cost to fitness (Krebs and Loeschcke, 1994) and maintaining the capacity to acclimate when living in homogeneous environments, such as in a laboratory, might be disadvantageous and selected against This means that using laboratory model species for experiments on thermal tolerance might underestimate the acclimation potential of animals in nature. Highly domesticated species are frequently used to answer questions on thermal tolerance and global warming impacts (Gilchrist et al, 1997; Schaefer and Ryan, 2006; Paaijmans et al, 2013)
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