Pallares, S. | bioRxiv Science

bioRxiv2024

Intraspecific variation of thermal tolerance in freshwater insects along elevational gradients: the case of a widespread diving beetle

Species distributed along wide elevational gradients are likely to experience local adaptation and exhibit high plasticity of thermal tolerance traits, as these gradients are characterised by steep environmental changes over short geographic distances (i.e., strong selection differentials). However, the prevalence of adaptive clinal intraspecific variation in thermal tolerance with elevation remains unclear, and this aspect has been poorly studied in freshwater insects. We explored variation in upper (heat coma temperature) and lower (supercooling point) thermal limits and acclimation capacity among Iberian populations of the widespread aquatic beetle Agabus bipustulatus (fam. Dytiscidae) across a 2,000-m elevational gradient, from lowland to alpine areas. As minimum, maximum and mean temperatures decline with elevation, we predicted that higher elevation populations will show lower heat tolerances and higher cold tolerances. We also explored whether acclimation capacity is positively related with climatic variability across different elevations. We found significant variation in upper and lower thermal limits among populations of A. bipustulatus, but no evidence of local adaptation to different thermal conditions along the altitudinal gradient, as relationships between thermal limits and elevation or climatic variables were in general not significant. Plasticity of upper and lower thermal limits was overall and consistently low in all populations. These results suggest conservatism of the thermal niche, which might be the result of gene flow counteracting the effects of divergent selection, or adaptations in other traits that buffer the exposure of populations to climate extremes. The limited adaptive potential and plasticity of thermal tolerance found here for A. bipustulatus imply that even generalist species, distributed along wide environmental gradients, may have little resilience to global warming.

ecology↗

bioRxiv2019

Heat tolerance and acclimation capacity in unrelated subterranean arthropods living under common and stable thermal conditions

Cave-dwelling ectotherms, which have evolved for millions of years under stable thermal conditions, could be expected to have adjusted their physiological limits to the narrow range of temperatures they experience and be highly vulnerable to global warming. However, the few existing studies on thermal physiology in subterranean invertebrates point that they have lost some of the thermoregulatory mechanisms common in surface species, but there is a lack of evolutionary adjustment to ambient temperature. The question remains whether this surprising homogeneous thermal niche is common for the whole subterranean biodiversity or just a peculiarity of the species tested. In order to test for the generality of such previous findings, we studied basal heat tolerance and thermal plasticity in four species of distant arthropod groups (Coleoptera, Diplopoda and Collembola) with different evolutionary histories but under similar selection pressures, as they have been exposed to the same constant environmental conditions for a long time (inhabiting the same cave). Adult specimens were exposed at different temperatures for one week to determine upper lethal temperatures. Then, surviving individuals from each treatment were exposed to a fixed stressful temperature to determine acclimation capacity. Upper lethal temperatures were similar to those reported for other subterranean species (between 20-25{degrees}C), except from that of the diplopod (28{degrees}C), widely exceeding the cave temperature (13-14{degrees}C). Acclimation responses differed from a positive acclimation response in two of the species to no acclimation capacity or detrimental acclimation effects in the others. Our results show that thermal niche features are not as homogeneous as expected across the subterranean fauna and unrelated to ambient conditions. We show that some species, likely those that colonized subterranean environments more recently, still retain thermoregulation capacity to face temperature changes. Thus, subterranean species, even those living under similar climatic conditions, might be very differently affected by global warming.

ecology↗