Abstract
The ability of organisms to perform at different temperatures could be described by a continuous nonlinear reaction norm (i.e., thermal performance curve, TPC), in which the phenotypic trait value varies as a function of temperature. Almost any shift in the parameters of this performance curve could highlight the direct effect of temperature on organism fitness, providing a powerful framework for testing thermal adaptation hypotheses. Inter-and intraspecific differences in this performance curve are also reflected in thermal tolerances limits (e.g., critical and lethal limits), influencing the biogeographic patterns of species’ distribution. Within this context, here we investigated the intraspecific variation in thermal sensitivities and thermal tolerances in three populations of the invasive snail Cornu aspersum across a geographical gradient, characterized by different climatic conditions. Thus, we examined population differentiation in the TPCs, thermal-coma recovery times, expression of heat-shock proteins and standard metabolic rate (i.e., energetic costs of physiological differentiation). We tested two competing hypotheses regarding thermal adaptation (the “hotter is better” and the generalist-specialist trade-offs). Our results show that the differences in thermal sensitivity among populations of C. aspersum follow a latitudinal pattern, which is likely the result of a combination of thermodynamic constraints (“hotter is better”) and thermal adaptations to their local environments (generalist-specialist trade-offs). This finding is also consistent with some thermal tolerance indices such as the Heat-Shock Protein Response and the recovery time from chill-coma. However, mixed responses in the evaluated traits suggest that thermal adaptation in this species is not complete, as we were not able to detect any differences in neither energetic costs of physiological differentiation among populations, nor in the heat-coma recovery.
Highlights
Temperature is an important environmental factor that affects species distribution [1,2], influencing at the same time all life functions of organisms through changes in the rates of physiological and biochemical processes [3,4,5]
The thermal sensitivity of most biological rate processes at the whole-organism level operates within the ranges of critical temperature extremes, with the performance of a biological trait gradually increasing with temperature from a critical minimum (CTmin) to an optimum (Topt) before dropping precipitously as temperature approaches a critical maximum (CTmax) [6,7]
It is well known that the ability of an organism to perform at different temperatures can be described by a continuous nonlinear reaction pattern in which the phenotypic trait value varies as a function of temperature [8,9,10]
Summary
Temperature is an important environmental factor that affects species distribution [1,2], influencing at the same time all life functions of organisms through changes in the rates of physiological and biochemical processes [3,4,5]. Adaptive evolution or phenotypic plasticity can modify a TPC by means of vertical shifts in the shape that produces changes in average performance or fitness, by horizontal shifts that produce changes in the Topt, or by width shifts that produce changes in the niche width [9,15] In keeping with this line of thinking, it is believed that these kinds of changes have an effect on fitness or performance trade-offs [9,12] that impose a cost to thermal specialization and result in either specialists that are well adapted to local conditions but poorly adapted to alternative environments, or generalists that are broadly adapted to a range of environments but not well adapted to any particular one (i.e., generalist-specialist trade-off) [9,16,17]. This is interesting in invasive species, since they exhibit great physiological tolerance to thermal variation [18,19], as well as rapid phenotypic responses and adaptation to novel environments [20,21]
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