Rates of environmental change influence the capacity for phenotypic and molecular adjustments in tropical ectotherms

Abstract

For ectotherms, environmental temperature (Ta) is the most important abiotic factor that affects their body temperature and a variety of organismal (e.g., fitness, physiology), ecological (distribution, interactions), and evolutionary (speciation) processes. In tropical regions, variation in daily and seasonal Ta is small compared to temperate regions. As a result, tropical ectotherms have evolved to maximize performance and fitness over a narrow range of Ta, living near their thermal physiological limits and optimal temperatures. Because of this, tropical ectotherms are particularly vulnerable to climate change since even small perturbations in Ta can lead to important deleterious effects. However, in some extreme tropical zones, natural selection has favoured the evolution of adaptive physiological responses that allow ectotherms to regulate thermal stress by entering into metabolic arrest. As part of this adaptive response, both cellular and physiological processes are actively suppressed. It is unclear however, whether this adaptive response is regulated by the capacity of tropical ectotherms to sense the magnitude and rate of Tachange. Tropical rocky shores offer a natural laboratory to explore this question. These are perhaps one of the most stressful and fluctuating habitatson Earth, characterised by a vertical thermal variability gradient with different rates of temperature change. This study focuses on the physiological and molecular mechanisms of thermoregulation (phenotypic plasticity) using the mussel Mytiliseptavirgatusas biological system. Here, I explored the extend to which varying warming rates affect physiological performance and gene expression during rapid heating and acclimation, and how this relationship may differ between populations from different thermal habitats. Using heat ramping experiments, I examined four physiological variables of interest: heart rate, osmolality, weight loss percentage and gene expression profiles. Faster rates significantly decreased weight loss and average thermal equilibrium but did not show changes in heart rate and osmolality. Higher protein-folding genes were upregulated at the expense of pro-survival pathways. Higher-shore populations displayed similar patterns at the fast rate with higher protein folding gene expression, faster signaling activation and transduction as well as stronger repair and immune ability against heat stress. As physiological indicators may not have the resolution to reflect minute changes in molecular machinery in response to thermal stress, an integrative approach that incorporates molecular techniques in physiological studies can provide important insights into the effect of warming rates and climate change on tropical ectotherms