The effect of thermal microenvironment in upper thermal tolerance plasticity in tropical tadpoles. Implications for vulnerability to climate warming

Plasticity of Gene Expression and Thermal Tolerance: Implications for Climate Change Vulnerability in a Tropical Forest Lizard

AbstractTropical ectotherms are thought to be especially vulnerable to climate change because they have evolved in temporally stable thermal environments and therefore have decreased tolerance for thermal variability. Thus, they are expected to have narrow thermal tolerance ranges, live close to their upper thermal tolerance limits, and have decreased thermal acclimation capacity. Although models often predict that tropical forest ectotherms are especially vulnerable to rapid environmental shifts, these models rarely include the potential for plasticity of relevant traits. We measured phenotypic plasticity of thermal tolerance and thermal preference as well as multitissue transcriptome plasticity in response to warmer temperatures in a species that previous work has suggested is highly vulnerable to climate warming, the Panamanian slender anole lizard (Anolis apletophallus). We found that many genes, including heat shock proteins, were differentially expressed across tissues in response to short-term warming. Under long-term warming, the voluntary thermal maxima of lizards also increased, although thermal preference exhibited only limited plasticity. Using these data, we modeled changes in the activity time of slender anoles through the end of the century under climate change and found that plasticity should delay declines in activity time by at least two decades. Our results suggest that slender anoles, and possibly other tropical ectotherms, can alter the expression of genes and phenotypes when responding to shifting environmental temperatures and that plasticity should be considered when predicting the future of organisms under a changing climate.

Thermal limits along tropical elevational gradients: Poison frog tadpoles show plasticity but maintain divergence across elevation

Temperature is arguably one of the most critical environmental factors impacting organisms at molecular, organismal, and ecological levels. Temperature variation across elevation may cause divergent selection in physiological critical thermal limits (CTMAX and CTMIN). Generally, high elevation populations are predicted to withstand lower environmental temperatures than low elevation populations. Organisms can also exhibit phenotypic plasticity when temperature varies, although theory and empirical evidence suggest that tropical ectotherms have relatively limited ability to acclimate. To study the effect of temperature variation along elevational transects on thermal limits, we measured CTMAX and CTMIN of 934 tadpoles of a poison frog species, Epipedobates anthonyi, along two elevational gradients (200-1700 m asl) in southwestern Ecuador to investigate their thermal tolerance across elevation. We also tested if tadpoles could plastically shift their critical thermal limits in response to exposure to different temperatures representing the range of temperatures they experience in nature (20 °C, 24 °C, and 28 °C). Overall, we found that CTMAX did not change across elevation. In contrast, CTMIN was lower at higher elevations, suggesting that elevational variation in temperature influences this thermal trait. Moreover, all populations shifted their CTMAX and CTMIN according to treatment temperatures, demonstrating an acclimation response. Overall, trends in CTMIN among high, mid, and low elevation populations were maintained despite plastic responses to treatment temperature. These results demonstrate that, for tadpoles of E. anthonyi across tropical elevational gradients, temperature acts as a selective force for CTMIN, even when populations show acclimation abilities in both, CTMAX and CTMIN. Our findings advance our understanding on how environmental variation affects organisms' evolutionary trajectories and their abilities to persist in a changing climate in a tropical biodiversity hotspot.

Changes in thermal sensitivity of Rhinella arenarum tadpoles (Anura: Bufonidae) exposed to sublethal concentrations of different pesticide fractions (Lorsban® 75WG)

The intensive use of agrochemicals and the rapid increase of global temperatures have modified the thermal conditions of aquatic environments, thus increasing amphibians' vulnerability to global warming and positioning them at great risk. Commercial formulations of chlorpyrifos (COM) are the pesticides most widely used in agricultural activities, with a high toxic potential on amphibians. However, little is known about the separate effects of the active ingredient (CPF) and adjuvants (AD). We studied the thermal sensitivity at different concentrations and pesticide fractions in Rhinella arenarum tadpoles, on thermal tolerance limits (CTmax = Critical thermal maximum and CTmin = Critical thermal minimum), swimming speed (Ss), Optimum temperature (Top), and Thermal breadth 50 (B50). Our results demonstrate that the pesticide active ingredient, the adjuvants, and the commercial formulation of chlorpyrifos differentially impair the thermal sensitivity of R. arenarum tadpoles. The pesticide fractions affected the heat and the cold tolerance (CTmax and CTmin), depending on the concentrations they were exposed to. The locomotor performance (Ss, Top, and B50) of tadpoles also varied among fractions, treatments, and environmental temperatures. In the context of climate change, the outcomes presented are particularly relevant, as mean temperatures are increasing at unprecedented rates, which suggests that tadpoles inhabiting warming and polluted ponds are currently experiencing deleterious conditions. Considering that larval stages of amphibians are the most susceptible to changing environmental conditions and the alarming predictions about environmental temperatures in the future, it is likely that the synergism between high temperatures and pesticide exposure raise the threat of population deletions in the coming years.

Ranavirus infection does not reduce heat tolerance in a larval amphibian

Extreme heat events and emerging infectious diseases negatively impact wildlife populations, but the interacting effects of infection and host heat tolerance remain understudied. The few studies covering this subject have demonstrated that pathogens lower the heat tolerance of their hosts, which places infected hosts at a greater risk experiencing lethal heat stress. Here, we studied how ranavirus infection influenced heat tolerance in larval wood frogs (Lithobates sylvaticus). In line with similar studies, we predicted the elevated costs of ranavirus infection would lower heat tolerance, measured as critical thermal maximum (CT_max), compared to uninfected controls. Ranavirus infection did not reduce CT_max and there was a positive relationship between CT_max and viral loads. Our results demonstrate that ranavirus-infected wood frog larvae had no loss in heat tolerance compared to uninfected larvae, even at viral loads associated with high mortality rates, which contradicts the common pattern for other pathogenic infections in ectotherms. Larval anurans may prioritize maintenance of their CT_max when infected with ranavirus to promote selection of warmer temperatures during behavioral fever that can improve pathogen clearance. Our study represents the first to examine the effect of ranavirus infection on host heat tolerance, and because no decline in CT_max was observed, this suggests that infected hosts would not be under greater risk of heat stress.

The time course of acclimation of critical thermal maxima is modulated by the magnitude of temperature change and thermal daily fluctuations

Plasticity in the critical thermal maximum (CT_max) helps ectotherms survive in variable thermal conditions. Yet, little is known about the environmental mechanisms modulating its time course. We used the larvae of three neotropical anurans (Boana platanera, Engystomops pustulosus and Rhinella horribilis) to test whether the magnitude of temperature changes and the existence of fluctuations in the thermal environment affected both the amount of change in CT_max and its acclimation rate (i.e., its time course). For that, we transferred tadpoles from a pre-treatment temperature (23 °C, constant) to two different water temperatures: mean (28 °C) and hot (33 °C), crossed with constant and daily fluctuating thermal regimes, and recorded CT_max values, daily during six days. We modeled changes in CT_max as an asymptotic function of time, temperature, and the daily thermal fluctuation. The fitted function provided the asymptotic CT_max value (CT_max∞) and CT_max acclimation rate (k). Tadpoles achieved their CT_max∞ between one and three days. Transferring tadpoles to the hot treatment generated higher CT_max∞ at earlier times, inducing faster acclimation rates in tadpoles. In contrast, thermal fluctuations equally led to higher CT_max∞ values but tadpoles required longer times to achieve CT_max∞ (i.e., slower acclimation rates). These thermal treatments interacted differently with the studied species. In general, the thermal generalist Rhinella horribilis showed the most plastic acclimation rates whereas the ephemeral-pond breeder Engystomops pustulosus, more exposed to heat peaks during larval development, showed less plastic (i.e., canalized) acclimation rates. Further comparative studies of the time course of CT_max acclimation should help to disentangle the complex interplay between the thermal environment and species ecology, to understand how tadpoles acclimate to heat stress.

Heat hardening of a larval amphibian is dependent on acclimation period and temperature

Plasticity in heat tolerance provides ectotherms the ability to reduce overheating risk during thermal extremes. However, the tolerance-plasticity trade-off hypothesis states that individuals acclimated to warmer environments have a reduced plastic response, including hardening, limiting their ability to further adjust their thermal tolerance. Heat hardening describes the short-term increase in heat tolerance following a heat shock that remains understudied in larval amphibians. We sought to examine the potential trade-off between basal heat tolerance and hardening plasticity of a larval amphibian, Lithobates sylvaticus, in response to differing acclimation temperatures and periods. Lab-reared larvae were exposed to one of two acclimation temperatures (15°C and 25°C) for either 3 or 7 days, at which time heat tolerance was measured as critical thermal maximum (CT_max ). A hardening treatment (sub-critical temperature exposure) was applied 2 h before the CT_max assay for comparison to control groups. We found that heat-hardening effects were most pronounced in 15°C acclimated larvae, particularly after 7 days of acclimation. By contrast, larvae acclimated to 25°C exhibited only minor hardening responses, while basal heat tolerance was significantly increased as shown by elevated CT_max temperatures. These results are in line with the tolerance-plasticity trade-off hypothesis. Specifically, while exposure to elevated temperatures induces acclimation in basal heat tolerance, shifts towards upper thermal tolerance limits constrain the capacity for ectotherms to further respond to acute thermal stress.

Effects of fine-scale habitat quality on activity, dormancy, habitat use, and survival after reproduction in Rana dybowskii (Chordata, Amphibia)

Amphibians are facing population declines and extinctions, and protecting and supplementing refuges can help species survive. However, the microhabitat requirements of most species are unknown, and artificial shelters or burrows have not been well tested for amphibians. Some amphibians exhibit complex behaviour during the transition from post-reproductive dormancy to activity. However, little is known about the ecology, post-reproductive dormancy, and terrestrial activity of amphibians. Here, habitat site selection in experimental enclosures and the effects of shelters (stones, soil) and shade (with and without shade netting) on the activity, exposed body percentage, burrow depth, body-soil contact percentage, and survival of Rana dybowskii were investigated during post-reproductive dormancy and post-dormant activity. The results showed that R. dybowskii live individually under leaves, soil, stones or tree roots. Furthermore, although the dormant sites of frogs are significantly different, the distribution of male and female frogs in these sites is similar. Shading and shelter significantly affected the exposed body percentage, burrow depth and body-soil contact percentage of frogs compared with soil. In the stone group, soil and stone form the frog's refuge/burrow, whereas in the soil group, the refuge/burrow is composed entirely of soil. Even though the soil group has a deeper burrow and a larger area of soil contact with the body, it still has a higher exposure rate than the stone group. Frog activity frequency was affected by shelter and shade; the interaction of shelter and time and the interaction of shading and time were significant. The soil group had a higher activity frequency than the stone group, and the no-shade group had a higher activity frequency than the shade group. Shelter and shading differences do not significantly affect frog survival; however, the death rate during post-reproductive dormancy is lower than that during the active period.

There and back again: A meta-analytical approach on the influence of acclimation and altitude in the upper thermal tolerance of amphibians and reptiles

Realistic predictions about the impacts of climate change onbiodiversity requires gathering ecophysiological data and the critical thermal maxima (CTMax) is the most frequently used index to assess the thermal vulnerability of species. In the present study, we performed a systematic review to understand how acclimation and altitude affect CTMax estimates for amphibian and non-avian reptile species. We retrieved CTMax data for anurans, salamanders, lizards, snakes, and turtles/terrapins. Data allowed to perform a multilevel random effects meta-analysis to answer how acclimation temperature affect CTMax of Anura, Caudata, and Squamata and also meta-regressions to assess the influence of altitude on CTMax of frogs and lizards. Acclimation temperature influenced CTMax estimates of tadpoles, adult anurans, salamanders, and lizards, but not of froglets. In general, the increase in acclimation temperature led to higher CTMax values. Altitudinal bioclimatic gradient had an inverse effect for estimating the CTMax of lizards and anuran amphibians. For lizards, CTMax was positively influenced by the mean temperature of the wettest quarter. For anurans, the relationship is inverse; we recover a trend of decreasing CTMax when max temperature of warmest month and precipitation seasonality increase. There is an urgent need for studies to investigate the thermal tolerance of subsampled groups or even for which we do not have any information such as Gymnophiona, Serpentes, Amphisbaena, and Testudines. Broader phylogenetic coverage is mandatory for more accurate analyses of macroecological and evolutionary patterns for thermal tolerance indices as CTMax.