Abstract
BackgroundEctothermic organisms are thought to be severely affected by global warming since their physiological performance is directly dependent on temperature. Latitudinal and temporal variations in mean temperatures force ectotherms to adapt to these complex environmental conditions. Studies investigating current patterns of thermal adaptation among populations of different latitudes allow a prediction of the potential impact of prospective increases in environmental temperatures on their fitness.Methodology/Principal FindingsIn this study, temperature reaction norms were ascertained among 18 genetically defined, natural clones of the microbial eukaryote Paramecium caudatum. These different clones have been isolated from 12 freshwater habitats along a latitudinal transect in Europe and from 3 tropical habitats (Indonesia). The sensitivity to increasing temperatures was estimated through the analysis of clone specific thermal tolerances and by relating those to current and predicted temperature data of their natural habitats.All investigated European clones seem to be thermal generalists with a broad thermal tolerance and similar optimum temperatures. The weak or missing co-variation of thermal tolerance with latitude does not imply local adaptation to thermal gradients; it rather suggests adaptive phenotypic plasticity among the whole European subpopulation. The tested Indonesian clones appear to be locally adapted to the less variable, tropical temperature regime and show higher tolerance limits, but lower tolerance breadths.Conclusions/SignificanceDue to the lack of local temperature adaptation within the European subpopulation, P. caudatum genotypes at the most southern edge of their geographic range seem to suffer from the predicted increase in magnitude and frequency of summer heat waves caused by climate change.
Highlights
Temperature is one of the most important environmental factors determining a variety of ecosystem elements, e.g. species ecophysiology, abundance and distribution, as well as species diversity and population dynamics [1,2,3,4]
Intraspecific Variation in Thermal Performance Using the Lactin-2 model [49] to describe thermal performance curves (TPCs) of individual Paramecium caudatum clones resulted in typical left-skewed TPCs (Figure 3). These clonal specific TPCs allowed the calculation of ecophysiological key characteristics, which were qualitatively distinguishable between the different P. caudatum clones
All European clones showed their highest growth rates at the same experimental temperature of 28uC, while the calculated optimum temperatures (Topt) of the fitted TPCs ranged from 27.9460.23uC (ESH-2) to 29.4960.15uC (GRL-1)
Summary
Temperature is one of the most important environmental factors determining a variety of ecosystem elements, e.g. species ecophysiology, abundance and distribution, as well as species diversity and population dynamics [1,2,3,4]. To make predictions of organisms’ and population responses to global warming, studies on genetic and phenotypic diversity over a species’ geographic range are important. Such investigations can unveil patterns of evolutionary temperature adaptation to the current thermal heterogeneity on Earth by determining which ectotherms have a high acclimatisation capacity and which only occur at specific temperatures. Ectothermic organisms are thought to be severely affected by global warming since their physiological performance is directly dependent on temperature. Studies investigating current patterns of thermal adaptation among populations of different latitudes allow a prediction of the potential impact of prospective increases in environmental temperatures on their fitness
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