Thermal Performance Curves of Multiple Isolates of Batrachochytrium dendrobatidis, a Lethal Pathogen of Amphibians

The evolution of thermal performance curves in fungi farmed by attine ant mutualists in above-ground or below-ground microclimates

Fungi are abundant and ecologically important at a global scale, but little is known about whether their thermal adaptations are shaped by biochemical constraints (i.e., the hotter is better model) or evolutionary tradeoffs (i.e., the specialist-generalist model). We tested these hypotheses by generating thermal performance curves of fungal cultivars farmed by six species of Panamanian fungus-farming "attine" ants. These fungi represent evolutionary transitions in farming strategies, as four cultivars are farmed by ants below ground at stable temperatures near 25 °C and two cultivars are farmed above ground at variable temperatures. We generated thermal performance curves using a common garden experiment confining fungal isolates to different temperatures and then used a Bayesian hierarchical modelling approach to compare competing temperature sensitivity models. Some thermal performance traits differed consistently across farming strategies, with above-ground cultivars having: (1) higher tolerance to low temperatures (CTLmin) and (2) higher maximum growth rate at the optimal temperature (rmax). However, two core assumptions shared by the hotter is better model or specialist-generalist model were not supported as above-ground cultivars did not show systematic increases in either their optimal temperature (Topt) or thermal tolerance breadth. These results harness ant farming systems as long-term natural experiments to decouple the effects of environmental thermal variation and innate physiological temperature sensitivity on fungal thermal evolution. The results have clear implications for predicting climate warming-induced breaking points in animal-microbe mutualisms.

Testing for thermal acclimation in zoospores of an amphibian pathogen

Thermal acclimation effects on locomotory performance have been widely documented for macroscopic organisms, but such responses remain largely unexplored in microorganisms. Metabolic theory of ecology (MTE) predicts faster responses in smaller organisms, with potential consequences for host-parasite interactions in variable temperature environments. We investigated thermal acclimation effects on zoospores of the amphibian fungal pathogen Batrachochytrium dendrobatidis (Bd), quantifying (1) thermal performance for maximum zoospore velocity and (2) high temperatures needed to immobilize 50% (CT50max) or 100% (CT100max) of zoospores. We obtained measurements within 18 min following a temperature shift. We found significant curvilinear acclimation effects on maximum zoospore velocity and CT50max, although the latter pattern might have been driven by confoundment with zoospore density. We also observed a significant positive effect of the trial start temperature on CT50max, consistent with a rapid acclimation response to the start temperature on a time scale of ~1-6 min (i.e. too rapid for our experimental acclimation treatments to detect), implying that zoospores either have constitutive heat tolerance (i.e. no acclimation) or fully acclimate CTmax to new temperatures within ~10 min. To explore the plausibility of such a rapid response, we analyzed published CTmax acclimation times for macroscopic eukaryotes, resulting in a predicted interquartile range of 3.11-25.98 min when mass-scaled to the size of a Bd zoospore. Taken together, these results suggest that Bd zoospores do exhibit thermal acclimation response on the rapid time scale predicted by MTE, possibly giving Bd an advantage over slower-acclimating hosts in variable-temperature environments.

Landscape-scale drivers of spatial dynamics and genetic diversity in an emerging wildlife pathogen

Aquatic pathogens often cannot tolerate drying, and thus their spread, and diversity across a landscape may depend on interactions between hydrological conditions and the movement of infected hosts. The aquatic fungus Batrachochytrium dendrobatidis (Bd) is a nearly ubiquitous pathogen of amphibians and particular lineages have been associated with host declines. By coupling amphibian surveys with molecular pathogen detection and genotyping techniques, we characterized the spatial dynamics and genetic diversity of Bd on a landscape containing both permanent and ephemeral ponds. In doing so, we aimed to clarify how pathogen loads and prevalences vary across seasons and among habitat types, and which host species move the pathogen from place to place. At the start of spring breeding, Bd prevalence was lower on amphibians sampled from ephemeral ponds. For the remainder of the amphibian active season, prevalence was similar across both ephemeral and permanent ponds, with variation in prevalence being well-explained by a hump-shaped relationship with host body temperature. The first amphibians to arrive at these ephemeral ponds infected were species that breed in ephemeral ponds and likely emerged infected from terrestrial hibernacula. However, species from permanent ponds, most of which hibernate aquatically, later visited the ephemeral ponds and these animals had a greater prevalence and load of Bd, suggesting that migrants among ponds and pond types also move Bd across the landscape. The Bd we sampled was genetically diverse within ponds but showed little genetic structure among ponds, host species, or seasons. Taken together, our findings suggest that Bd can be diverse even at small scales and moves readily across a landscape with help from a wide variety of hosts.

High phenotypic diversity correlated with genomic variation across the European Batrachochytrium salamandrivorans epizootic

Recognizing the influence of pathogen diversity on infection dynamics is crucial for mitigating emerging infectious diseases. Characterising such diversity is often complex, for instance when multiple pathogen variants exist that interact differently with the environment and host. Here, we explore genotypic and phenotypic variation of Batrachochytrium salamandrivorans (Bsal), an emerging fungal pathogen that is driving declines among an increasing number of European amphibian species. For thirteen isolates, spanning most of the known temporal and geographical Bsal range in Europe, we mapped phenotypic diversity through numerous measurements that describe varying reproductive rates in vitro across a range of temperatures. Bsal isolates are revealed to have different thermal optima and tolerances, with phenotypic variation correlating with genomic diversity. Using a mechanistic niche model of the fire salamander (Salamandra salamandra) as an example, we illustrate how host steady-state body temperature and Bsal thermal range variation may influence pathogen growth through space and time across Europe. Our combined findings show how the identity of emergent pathogen variants may strongly influence when and which host populations are most at risk.

Gene functions of the Ambystoma altamirani skin microbiome vary across space and time but potential antifungal genes are widespread and prevalent

Amphibian skin microbiomes can play a critical role in host survival against emerging diseases by protecting their host against pathogens. While a plethora of biotic and abiotic factors have been shown to influence the taxonomic diversity of amphibian skin microbiomes it remains unclear whether functional genomic diversity varies in response to temporal and environmental factors. Here we applied a metagenomic approach to evaluate whether seasonality, distinct elevations/sites, and pathogen presence influenced the functional genomic diversity of the A. altamirani skin microbiome. We obtained a gene catalogue of 92 107 nonredundant annotated genes and a set of 50 unique metagenome assembled genomes (MAGs). Our analysis showed that genes linked to general and potential antifungal traits significantly differed across seasons and sampling locations at different elevations. Moreover, we found that the functional genomic diversity of A. altamirani skin microbiome differed between B. dendrobatidis infected and not infected axolotls only during winter, suggesting an interaction between seasonality and pathogen infection. In addition, we identified the presence of genes and biosynthetic gene clusters (BGCs) linked to potential antifungal functions such as biofilm formation, quorum sensing, secretion systems, secondary metabolite biosynthesis, and chitin degradation. Interestingly genes linked to these potential antifungal traits were mainly identified in Burkholderiales and Chitinophagales MAGs. Overall, our results identified functional traits linked to potential antifungal functions in the A. altamirani skin microbiome regardless of variation in the functional diversity across seasons, elevations/sites, and pathogen presence. Our findings suggest that potential antifungal traits found in Burkholderiales and Chitinophagales taxa could be related to the capacity of A. altamirani to survive in the presence of Bd, although further experimental analyses are required to test this hypothesis.

Glutathione is required for growth and cadmium tolerance in the amphibian chytrid fungus, Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd) is a lethal amphibian pathogen, partly due to its ability to evade the immune system of susceptible frog species. In many pathogenic fungi, the antioxidant glutathione is a virulence factor that helps neutralise oxidative stressors generated from host immune cells, as well as other environmental stressors such as heavy metals. The role of glutathione in stress tolerance in Bd has not been investigated. Here, we examine the changes in the glutathione pool after stress exposure and quantify the effect of glutathione depletion on cell growth and stress tolerance. Depletion of glutathione repressed growth and release of zoospores, suggesting that glutathione is essential for life cycle completion in Bd. Supplementation with <2 mM exogenous glutathione accelerated zoospore development, but concentrations >2 mM were strongly inhibitory to Bd cells. While hydrogen peroxide exposure lowered the total cellular glutathione levels by 42 %, glutathione depletion did not increase the sensitivity to hydrogen peroxide. Exposure to cadmium increased total cellular glutathione levels by 93 %. Glutathione-depleted cells were more sensitive to cadmium, and this effect was attenuated by glutathione supplementation, suggesting that glutathione plays an important role in cadmium tolerance. The effects of heat and salt were exacerbated by the addition of exogenous glutathione. The impact of glutathione levels on Bd stress sensitivity may help explain differences in host susceptibility to chytridiomycosis and may provide opportunities for synergistic therapeutics.

Temperature shifts associated with bat arousals during hibernation inhibit the growth of Pseudogymnoascus destructans

Temperature is a critically important factor in many infectious disease systems, because it can regulate responses in both the host and the pathogen. White-nose syndrome (WNS) in bats is a severe infectious disease caused by the temperature-sensitive fungus, Pseudogymnoascus destructans (Pd). One feature of WNS is an increase in the frequency of arousal bouts (i.e. when bat body temperatures are elevated) in Pd-infected bats during hibernation. While several studies have proposed that increased frequency of arousals may play a role in the pathophysiology of WNS, it is unknown if the temperature fluctuations might mediate Pd growth. We hypothesized that exposure to a high frequency of elevated temperatures would reduce Pd growth due to thermal constraints on the pathogen. We simulated the thermal conditions for arousal bouts of uninfected and infected bats during hibernation (fluctuating from 8 to 25°C at two different rates) and quantified Pd growth in vitro. We found that increased exposure to high temperatures significantly reduced Pd growth. Because temperature is one of the most critical abiotic factors mediating host-pathogen interactions, resolving how Pd responds to fluctuating temperatures will provide insights for understanding WNS in bats and other fungal diseases.

The Potential for Physiological Performance Curves to Shape Environmental Effects on Social Behavior

As individual animals are exposed to varying environmental conditions, phenotypic plasticity will occur in a vast array of physiological traits. For example, shifts in factors such as temperature and oxygen availability can affect the energy demand, cardiovascular system, and neuromuscular function of animals that in turn impact individual behavior. Here, we argue that nonlinear changes in the physiological traits and performance of animals across environmental gradients—known as physiological performance curves—may have wide-ranging effects on the behavior of individual social group members and the functioning of animal social groups as a whole. Previous work has demonstrated how variation between individuals can have profound implications for socially living animals, as well as how environmental conditions affect social behavior. However, the importance of variation between individuals in how they respond to changing environmental conditions has so far been largely overlooked in the context of animal social behavior. First, we consider the broad effects that individual variation in performance curves may have on the behavior of socially living animals, including: (1) changes in the rank order of performance capacity among group mates across environments; (2) environment-dependent changes in the amount of among- and within-individual variation, and (3) differences among group members in terms of the environmental optima, the critical environmental limits, and the peak capacity and breadth of performance. We then consider the ecological implications of these effects for a range of socially mediated phenomena, including within-group conflict, within- and among group assortment, collective movement, social foraging, predator-prey interactions and disease and parasite transfer. We end by outlining the type of empirical work required to test the implications for physiological performance curves in social behavior.