Phenotypic variance, plasticity and heritability estimates of critical thermal limits depend on methodological context

Abstract

Summary 1.Biologists have long been concerned with measuring thermal performance curves and limits because of their significance to fitness. Basic experimental design may have a marked effect on the outcome of such measurements, and this is true especially of the experimental rates of temperature change used during assessments of critical thermal limits to activity. To date, the focus of work has almost exclusively been on the effects of rate variation on mean values of the critical limits. 2. If the rate of temperature change used in an experimental trial affects not only the trait mean but also its variance, estimates of heritable variation would also be profoundly affected. Moreover, if the outcomes of acclimation are likewise affected by methodological approach, assessment of beneficial acclimation and other hypotheses might also be compromised. 3. In this article, we determined whether this is the case for critical thermal limits using a population of the model species Drosophila melanogaster and the invasive ant species Linepithema humile . 4. We found that effects of the different rates of temperature change are variable among traits and species. However, in general, different rates of temperature change resulted in different phenotypic variances and different estimates of heritability, presuming that genetic variance remains constant. We also found that different rates resulted in different conclusions regarding the responses of the species to acclimation, especially in the case of L. humile . 5. Although it seems premature to dismiss past generalities concerning interspecific and acclimationrelated variation in critical thermal limits, we recommend that conditions during trials be appropriately selected, carefully reported and rigorously controlled.

Evolutionary stasis and lability in thermal physiology in a group of tropical lizards

“…Estimation of CT min and CT max is potentially confounded by the rate of temperature change, body size and starting conditions [24,26,27]. We performed linear regressions with mean population CT min and CT max as the dependent variables against the population means for rate of temperature change, initial experimental temperature and body mass (see the electronic supplementary material, table S2).…”

Impact of medium-term exposure to CO2 enriched seawater on the physiological functions of the velvet swimming crab Necora puber

“…Tolerance to heat was not affected, contrary to what was found for the edible crab Cancer pagurus (Metzger et al 2007) and the spider crab Hyas araneus (Walther et al 2009). The thermal tolerance values for Necora puber presented here, after accounting for methodological and acclimatisation differences (see Garland & Adolph 1991, Terblanche et al 2007, Chown et al 2009, are comparable to those of Hopkin et al (2006). In addition, even when methodological differences -we used passive endpoints (Lutterschmidt & Hutchison 1997), while Metzger et al (2007) used 'pejus' and 'critical' temperatures (Frederich & Pörtner 2000), as well as our slower ramping rate (see Terblanche et al 2007) -are accounted for, our results appear to indicate a greater resilience to combined OA and warming in N. puber than in the intertidal C. pagurus, and the subtidal H. araneus (Walther et al 2009).…”

Cellular damage as induced by high temperature is dependent on rate of temperature change – investigating consequences of ramping rates on molecular and organismal phenotypes in Drosophila melanogaster Meigen 1830

“…0.1 vs 0.06°Cmin -1 ) (Mora and Maya, 2006;Terblanche et al, 2007;Chown et al, 2009;Overgaard et al, 2012). Rezende et al (Rezende et al, 2011) commented on this observation and stated that it is '...a puzzling result because slower heating rates should allow individuals to acclimatize to new temperatures and because slow heating pre-exposes individuals to non-lethal high temperatures ('hardening'), which increases heat shock resistance (see e.g.…”