Skin sloughing in susceptible and resistant amphibians regulates infection with a fungal pathogen

Cutaneous shedding in amphibians causes shifts in bacterial microbiomes

Considerable research has focused on microbes on amphibian skin, as they act as the first line of defense against invading pathogens. This effort has generated substantial data on patterns across species, space, time, and ontogeny, alongside a growing list of beneficial antifungal symbionts. Though there is evidence of stability in amphibian skin microbial communities, there is also an indication that regular skin shedding reduces cultivable bacteria, with regrowth and recolonization in the period between sheds. This suggests that skin communities are in constant flux, and we lack an understanding of how the membership and structure of those communities are affected by shedding events. In this study, we conducted experiments on cane toads (Rhinella marina) to investigate the influence of shedding on skin microbiomes. We first used quantitative PCR to verify a positive correlation between bacterial loads and time in the days after shedding. We then resampled individuals over time to describe changes in community composition in the 38 h after shedding using amplicon sequencing. Similar to trends of bacterial loads, we found increases in alpha diversity over time after shedding, suggesting that shedding reduces bacterial diversity as it knocks down bacterial loads. During the 38-h period, community structure became similar to pre-shed communities in some individuals, but there was no consistent pattern in structural changes among individuals. In light of the amphibian chytridiomycosis pandemic, understanding how physiological events such as skin shedding affect beneficial bacteria and communities on amphibians would provide important insight into amphibian ecology.

Chytridiomycosis causes high amphibian mortality prior to the completion of metamorphosis

Amphibian populations are undergoing extensive declines globally. The fungal disease chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), is a primary contributor to these declines. The amphibian metamorphic stages (Gosner stages 42-46) are particularly vulnerable to a range of stressors, including Bd. Despite this, studies that explicitly examine host response to chytridiomycosis throughout the metamorphic stages are lacking. We aimed to determine how Bd exposure during the larval stages impacts metamorphic development and infection progression in the endangered Fleay's barred frog (Mixophyes fleayi). We exposed M. fleayi to Bd during pro-metamorphosis (Gosner stages 35-38) and monitored infection dynamics throughout metamorphosis. We took weekly morphological measurements (weight, total body length, snout-vent-length and Gosner stage) and quantified Bd load using qPCR. While we observed minimal impact of Bd infection on animal growth and development, Bd load varied throughout ontogeny, with an infection load plateau during the tadpole stages (Gosner stages 35-41) and temporary infection clearance at Gosner stage 42. Bd load increased exponentially between Gosner stages 42 and 45, with most exposed animals becoming moribund at Gosner stage 45, prior to the completion of metamorphosis. There was variability in infection outcome of exposed individuals, with a subgroup of animals (n = 5/29) apparently clearing their infection while the majority (n = 21/29) became moribund with high infection burdens. This study demonstrates the role that metamorphic restructuring plays in shaping Bd infection dynamics and raises the concern that substantial Bd-associated mortality could be overlooked in the field due to the often cryptic nature of these latter metamorphic stages. We recommend future studies that directly examine the host immune response to Bd infection throughout metamorphosis, incorporating histological and molecular methods to elucidate the mechanisms responsible for the observed trends.

Constant-temperature predictions underestimate growth of a fungal amphibian pathogen under individual host thermal profiles

Ectotherm body temperatures fluctuate with environmental variability and host behavior, which may influence host-pathogen interactions. Fungal pathogens are a major threat to ectotherms and may be highly responsive to the fluctuating thermal profiles of individual hosts, especially cool-loving fungi exposed to high host temperatures. However, most studies estimate pathogen thermal performance based on averages of host or surrogate environmental temperatures, potentially missing effects of short-term host temperature shifts such as daily or hourly heat spikes. We recorded individual thermal profiles of Australian rainforest frogs using temperature-sensitive radio-transmitters. We then reproduced a subset of individual thermal profiles in growth chambers containing cultures of the near-global amphibian pathogen Batrachochytrium dendrobatidis (Bd) to investigate how realistic host temperature profiles affect Bd growth. We focused on thermal profiles that exceed the thermal optimum of Bd because the effects of realistic heat spikes on Bd growth are unresolved. Our laboratory incubation experiment revealed that Bd growth varied in response to relatively small differences in heat spike characteristics of individual frog thermal profiles, such as a single degree or a few hours, highlighting the importance of individual host behaviors in predicting population-level disease dynamics. The fungus also grew better than predicted under the most extreme and unpredictable frog temperature profile, recovering from two days of extreme (nearly 32 °C) heat spikes without negative effects on overall growth, suggesting we are underestimating the growth potential of the pathogen in nature. Combined with the previous finding that Bd reduces host heat tolerance, our study suggests that this pathogen may carry a competitive edge over hosts in the face of anthropogenic climate change.

Signatures of functional bacteriome structure in a tropical direct-developing amphibian species

Background

Host microbiomes may differ under the same environmental conditions and these differences may influence susceptibility to infection. Amphibians are ideal for comparing microbiomes in the context of disease defense because hundreds of species face infection with the skin-invading microbe Batrachochytrium dendrobatidis (Bd), and species richness of host communities, including their skin bacteria (bacteriome), may be exceptionally high. We conducted a landscape-scale Bd survey of six co-occurring amphibian species in Brazil’s Atlantic Forest. To test the bacteriome as a driver of differential Bd prevalence, we compared bacteriome composition and co-occurrence network structure among the six focal host species.

Results

Intensive sampling yielded divergent Bd prevalence in two ecologically similar terrestrial-breeding species, a group with historically low Bd resistance. Specifically, we detected the highest Bd prevalence in Ischnocnema henselii but no Bd detections in Haddadus binotatus. Haddadus binotatus carried the highest bacteriome alpha and common core diversity, and a modular network partitioned by negative co-occurrences, characteristics associated with community stability and competitive interactions that could inhibit Bd colonization.

Conclusions

Our findings suggest that community structure of the bacteriome might drive Bd resistance in H. binotatus, which could guide microbiome manipulation as a conservation strategy to protect diverse radiations of direct-developing species from Bd-induced population collapses.

Microbial Diversity of the Chinese Tiger Frog (Hoplobatrachus rugulosus) on Healthy versus Ulcerated Skin

Simple Summary

As amphibians’ skin is highly sensitive to the environment, skin defects such as ulceration may pose a particular threat to them. Our study has found a stark difference in the microbial communities between healthy and ulcerated Hoplobatrachus rugulosus skin. The proportion and type of bacteria differed between the two groups, and we suggest that ulceration on the skin may lead to changes in skin microbial communities. The functional pathways of skin microbes may be influenced by ulceration on the skin surface of H. rugulosus. We also found that Vogesella is more abundant in healthy H. rugulosus, which may be a potential probiotic candidate for the reduction or removal of pathogens.

Abstract

The Chinese tiger frog (Hoplobatrachus rugulosus) is extensively farmed in southern China. Due to cramped living conditions, skin diseases are prevalent among unhealthy tiger frogs which thereby affects their welfare. In this study, the differences in microbiota present on healthy versus ulcerated H. rugulosus skin were examined using 16S rRNA sequences. Proteobacteria were the dominant phylum on H. rugulosus skin, but their abundance was greater on the healthy skin than on the ulcerated skin. Rhodocyclaceae and Comamonadaceae were the most dominant families on the healthy skin, whereas Moraxellaceae was the most dominant family on the ulcerated skin. The abundance of these three families was different between the groups. Acidovorax was the most dominant genus on the healthy skin, whereas Acinetobacter was the most dominant genus on the ulcerated skin, and its abundance was greater on the ulcerated skin than on the healthy skin. Moreover, the genes related to the Kyoto Encyclopedia of Genes and Genomes pathways of levels 2–3, especially those genes that are involved in cell motility, flagellar assembly, and bacterial chemotaxis in the skin microbiota, were found to be greater on the healthy skin than on the ulcerated skin, indicating that the function of skin microbiota was affected by ulceration. Overall, the composition, abundance, and function of skin microbial communities differed between the healthy and ulcerated H. rugulosus skin. Our results may assist in developing measures to combat diseases in H. rugulosus.

Seasonal variation in the prevalence of a fungal pathogen and unexpected clearance from infection in a susceptible frog species

The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) causes the disease chytridiomycosis, which is a primary driver for amphibian population declines and extinctions worldwide. For highly susceptible species, such as the green and golden bell frog Litoria aurea, large numbers of Bd-related mortalities are thought to occur during the colder season (winter), when low temperatures favour the growth of the pathogen. However, extant L. aurea populations are persisting with Bd. We measured Bd prevalence and infection levels of wild L. aurea using capture-mark-recapture and radio-tracking methods. Using this information, we sought to determine host and environmental correlates of Bd prevalence and infection load. Mean ± SE infection load was higher in frogs sampled in autumn (431.5 ± 310.4 genomic equivalents; GE) and winter (1147.5 ± 735.8 GE), compared to spring (21.8 ± 19.3 GE) and summer (0.9 ± 0.8 GE). Furthermore, prevalence of Bd infection in L. aurea was highest in winter (43.6%; 95% CI 33.1-54.7%) and lowest in summer (11.2%; 95% CI 6.8-17.9%). Both prevalence and infection load decreased with increasing temperature. Seven frogs cleared their fungal infection during the coolest months when Bd prevalence was highest; however, these clearances were not permanent, as 5 frogs became infected again. Understanding the factors that allow amphibians to clear their Bd infections when temperatures are optimal for Bd growth presents the potential for manipulating such factors and provides an important step in future research.

Host gene expression in wildlife disease: making sense of species-level responses

Emerging infectious diseases are significant threats to wildlife conservation, yet the impacts of pathogen exposure and infection can vary widely among host species. As such, conservation biologists and disease ecologists have increasingly aimed to understand species-specific host susceptibility using molecular methods. In particular, comparative gene expression assays have been used to contrast the transcriptomic responses of disease-resistant and disease-susceptible hosts to pathogen exposure. This work usually assumes that the gene expression responses of disease-resistant species will reveal the activation of molecular pathways contributing to host defence. However, results often show that disease-resistant hosts undergo little gene expression change following pathogen challenge. Here, we discuss the mechanistic implications of these "null" findings and offer methodological suggestions for future molecular studies of wildlife disease. First, we highlight that muted transcriptomic responses with minimal immune system recruitment may indeed be protective for nonsusceptible hosts if they limit immunopathology and promote pathogen tolerance in systems where susceptible hosts suffer from genetic dysregulation. Second, we argue that overly narrow investigation of responses to pathogen exposure may overlook important, constitutively active molecular pathways that underlie species-specific defences. Finally, we outline alternative study designs and approaches that complement interspecific transcriptomic comparisons, including intraspecific gene expression studies and genomic methods to detect signatures of selection. Collectively, these insights will help ecologists extract maximal information from conservation-relevant transcriptomic data sets, leading to a deeper understanding of host defences and, ultimately, the implementation of successful conservation interventions.

Applied ecoimmunology: using immunological tools to improve conservation efforts in a changing world

Ecoimmunology is a rapidly developing field that explores how the environment shapes immune function, which in turn influences host-parasite relationships and disease outcomes. Host immune defence is a key fitness determinant because it underlies the capacity of animals to resist or tolerate potential infections. Importantly, immune function can be suppressed, depressed, reconfigured or stimulated by exposure to rapidly changing environmental drivers like temperature, pollutants and food availability. Thus, hosts may experience trade-offs resulting from altered investment in immune function under environmental stressors. As such, approaches in ecoimmunology can provide powerful tools to assist in the conservation of wildlife. Here, we provide case studies that explore the diverse ways that ecoimmunology can inform and advance conservation efforts, from understanding how Galapagos finches will fare with introduced parasites, to using methods from human oncology to design vaccines against a transmissible cancer in Tasmanian devils. In addition, we discuss the future of ecoimmunology and present 10 questions that can help guide this emerging field to better inform conservation decisions and biodiversity protection. From better linking changes in immune function to disease outcomes under different environmental conditions, to understanding how individual variation contributes to disease dynamics in wild populations, there is immense potential for ecoimmunology to inform the conservation of imperilled hosts in the face of new and re-emerging pathogens, in addition to improving the detection and management of emerging potential zoonoses.

Xela DS2 and Xela VS2: Two novel skin epithelial-like cell lines from adult African clawed frog (Xenopus laevis) and their response to an extracellular viral dsRNA analogue

The skin epithelial layer acts as an important immunological barrier against pathogens and is capable of recognizing and responding to pathogen-associated molecular patterns (PAMPs) in human and mouse models. Although presumed, it is unknown whether amphibian skin epithelial cells exhibit the ability to respond to PAMPs such as viral double-stranded RNA (dsRNA). To address this, two cell lines from the dorsal skin (Xela DS2) and ventral skin (Xela VS2) of the African clawed frog (Xenopus laevis) were established. Xela DS2 and Xela VS2 cells have an epithelial-like morphology, express genes associated with epithelial cells, and lack senescence-associated beta-galactosidase activity. Cells grow optimally in 70% Leibovitz's L-15 medium supplemented with 15% fetal bovine serum at 26 °C. Upon treatment with poly(I:C), a synthetic analogue of viral dsRNA and known type I interferon inducer, Xela DS2 and Xela VS2 exhibit marked upregulation of key antiviral and pro-inflammatory transcripts suggesting frog epithelial cells participate in the recognition of extracellular viral dsRNA and production of local inflammatory signals; similar to human and mouse models. Currently, these are the only known Xenopus laevis skin epithelial-like cell lines and will be important for future research in amphibian epithelial cell biology, initial host-pathogen interactions, and rapid screening of the effects of environmental stressors, including contaminants, on frog skin epithelial cells.

Immunological Aspects of Chytridiomycosis

Amphibians are currently the most threatened vertebrate class, with the disease chytridiomycosis being a major contributor to their global declines. Chytridiomycosis is a frequently fatal skin disease caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). The severity and extent of the impact of the infection caused by these pathogens across modern Amphibia are unprecedented in the history of vertebrate infectious diseases. The immune system of amphibians is thought to be largely similar to that of other jawed vertebrates, such as mammals. However, amphibian hosts are both ectothermic and water-dependent, which are characteristics favouring fungal proliferation. Although amphibians possess robust constitutive host defences, Bd/Bsal replicate within host cells once these defences have been breached. Intracellular fungal localisation may contribute to evasion of the induced innate immune response. Increasing evidence suggests that once the innate defences are surpassed, fungal virulence factors suppress the targeted adaptive immune responses whilst promoting an ineffectual inflammatory cascade, resulting in immunopathology and systemic metabolic disruption. Thus, although infections are contained within the integument, crucial homeostatic processes become compromised, leading to mortality. In this paper, we present an integrated synthesis of amphibian post-metamorphic immunological responses and the corresponding outcomes of infection with Bd, focusing on recent developments within the field and highlighting future directions.

The amphibian complement system and chytridiomycosis

Understanding host immune function and ecoimmunology is increasingly important at a time when emerging infectious diseases (EIDs) threaten wildlife. One EID that has emerged and spread widely in recent years is chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which is implicated unprecedented amphibian declines around the world. The impacts of Bd have been severe for many amphibian species, but some populations have exhibited signs of persistence, and even recovery, in some regions. Many mechanisms may underpin this pattern and amphibian immune responses are likely one key component. Although we have made great strides in understanding amphibian immunity, the complement system remains poorly understood. The complement system is a nonspecific, innate immune defense that is known to enhance other immune responses. Complement activation can occur by three different biochemical pathways and result in protective mechanisms, such as inflammation, opsonization, and pathogen lysis, thereby providing protection to the host. We currently lack an understanding of complement pathway activation for chytridiomycosis, but several studies have suggested that it may be a key part of an early and robust immune response that confers host resistance. Here, we review the available research on the complement system in general as well as amphibian complement responses to Bd infection. Additionally, we propose future research directions that will increase our understanding of the amphibian complement system and other immune responses to Bd. Finally, we suggest how a deeper understanding of amphibian immunity could enhance the conservation and management of amphibian species that are threatened by chytridiomycosis.

Experimental methodologies can affect pathogenicity of Batrachochytrium salamandrivorans infections

Controlled experiments are one approach to understanding the pathogenicity of etiologic agents to susceptible hosts. The recently discovered fungal pathogen, Batrachochytrium salamandrivorans (Bsal), has resulted in a surge of experimental investigations because of its potential to impact global salamander biodiversity. However, variation in experimental methodologies could thwart knowledge advancement by introducing confounding factors that make comparisons difficult among studies. Thus, our objective was to evaluate if variation in experimental methods changed inferences made on the pathogenicity of Bsal. We tested whether passage duration of Bsal culture, exposure method of the host to Bsal (water bath vs. skin inoculation), Bsal culturing method (liquid vs. plated), host husbandry conditions (aquatic vs. terrestrial), and skin swabbing frequency influenced diseased-induced mortality in a susceptible host species, the eastern newt (Notophthalmus viridescens). We found that disease-induced mortality was faster for eastern newts when exposed to a low passage isolate, when newts were housed in terrestrial environments, and if exposure to zoospores occurred via water bath. We did not detect differences in disease-induced mortality between culturing methods or swabbing frequencies. Our results illustrate the need to standardize methods among Bsal experiments. We provide suggestions for future Bsal experiments in the context of hypothesis testing and discuss the ecological implications of our results.

Sustained immune activation is associated with susceptibility to the amphibian chytrid fungus

The disease chytridiomycosis caused by the fungus Bd has devastated amphibian populations worldwide. Functional genomic contributions to host susceptibility remain enigmatic and vary between species and populations. We conducted experimental Bd infections in Rana yavapaiensis, a species with intraspecific variation in chytridiomycosis susceptibility, to assess the skin and spleen transcriptomic response to infection over time. We predicted that increased immune gene expression would be associated with a positive disease outcome, but we instead found that surviving frogs had significantly reduced immune gene expression compared to susceptible frogs and to uninfected controls. MHC class IIβ gene expression was also significantly higher in susceptible frogs compared to surviving frogs. Furthermore, susceptible frogs expressed a significantly larger number of distinct class IIβ alleles, demonstrating a negative correlation between class IIβ expression, functional diversity, and survival. Expression of the MHC class IIβ locus previously associated with Bd disease outcomes was a significant predictor of Bd infection intensity at early infection stages but not at late infection stages, suggesting initial MHC-linked immune processes are important for ultimate disease outcomes. We infer through disease association and phylogenetic analysis that certain MHC variants are linked to the immune expression that was negatively associated with survival, and we hypothesize that frogs that did not express these alleles could better survive infections. Our study finds that MHC expression at early and late infection stages predicts Bd infection intensity, and suggests that generating a sustained immune response against Bd may be counterproductive for surviving chytridiomycosis in this partially susceptible species.

Incapacitating effects of fungal coinfection in a novel pathogen system

As globalization lowers geographic barriers to movement, coinfection with novel and enzootic pathogens is increasingly likely. Novel and enzootic pathogens can interact synergistically or antagonistically, leading to increased or decreased disease severity. Here we examine host immune responses to coinfection with two closely related fungal pathogens: Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal). Both pathogens have had detrimental effects on amphibian populations, with Bd now largely enzootic, while Bsal is currently spreading and causing epizootics. Recent experimental work revealed that newts coinfected with Bd and Bsal had significantly higher mortality than those infected with either pathogen alone. Here we characterize host immunogenomic responses to chytrid coinfection relative to single infection. Across several classes of immune genes including pattern recognition receptors, cytokines, and MHC, coinfected host gene expression was weakly upregulated or comparable to that seen in single Bd infection, but significantly decreased when compared to Bsal infection. Combined with strong complement pathway downregulation and keratin upregulation, these results indicate that coinfection with Bd and Bsal compromises immune responses active against Bsal alone. As Bsal continues to invade naïve habitats where Bd is enzootic, coinfection will be increasingly common. If other Bd-susceptible species in the region have similar responses, interactions between the two pathogens could cause severe population and community-level declines.

Phylogenetic investigation of skin sloughing rates in frogs: relationships with skin characteristics and disease-driven declines

Amphibian skin is highly variable in structure and function across anurans, and plays an important role in physiological homeostasis and immune defence. For example, skin sloughing has been shown to reduce pathogen loads on the skin, such as the lethal fungus Batrachochytrium dendrobatidis ( Bd), but interspecific variation in sloughing frequency is largely unknown. Using phylogenetic linear mixed models, we assessed the relationship between skin turnover rate, skin morphology, ecological traits and overall evidence of Bd-driven declines. We examined skin sloughing rates in 21 frog species from three continents, as well as structural skin characteristics measured from preserved specimens. We found that sloughing rate varies significantly with phylogenetic group, but was not associated with evidence of Bd-driven declines, or other skin characteristics examined. This is the first comparison of sloughing rate across a wide range of amphibian species, and creates the first database of amphibian sloughing behaviour. Given the strong phylogenetic signal observed in sloughing rate, approximate sloughing rates of related species may be predicted based on phylogenetic position. While not related to available evidence of declines, understanding variation in sloughing rate may help explain differences in the severity of infection in genera with relatively slow skin turnover rates (e.g. Atelopus).

Characteristics of monolayer formation in vitro by the chytrid Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis is a globally distributed generalist pathogen that has driven many amphibian populations to extinction. The life cycle of B. dendrobatidis has two main cell types, motile zoospores, and sessile reproductive sporangia. When grown in a nutrient-rich liquid medium, B. dendrobatidis forms aggregates of sporangia that transition into monolayers on surfaces and at the air-liquid interface. Pathogenic microorganisms use biofilms as mechanisms of group interactions to survive under harsh conditions in the absence of a suitable host. We used fluorescent and electron microscopy, crystal violet, transcriptomic, and gas chromatographic analyses to understand the characteristics of B. dendrobatidis monolayers. The cell-free monolayer fraction showed the presence of extracellular ribose, mannose, xylose, galactose, and glucose. Transcriptome analysis showed that 27%, 26%, and 4% of the genes were differentially expressed between sporangia/zoospores, monolayer/zoospores, and sporangia/monolayer pairs respectively. In pond water studies, zoospores developed into sporangia and formed floating aggregates at the air-water interface and attached film on the bottom of growth flasks. We propose that B. dendrobatidis can form surface-attached monolayers in nutrient-rich environments and aggregates of sporangia in nutrient-poor aquatic systems. These monolayers and aggregates may facilitate dispersal and survival of the fungus in the absence of a host. We provide evidence for using a combination of plant-based chemicals, allicin, gingerol, and curcumin as potential anti-chytrid drugs to mitigate chytridiomycosis.

Mechanistic basis for loss of water balance in green tree frogs infected with a fungal pathogen

Chytridiomycosis, a lethal skin disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), disrupts skin function of amphibians, interfering with ionic and osmotic regulation. To regulate fungal loads, amphibians increase their rate of skin sloughing. However, sloughing also causes a temporary loss of ionic and osmotic homeostasis due to disruption of the skin, a key osmoregulatory organ. The combined effects of increased sloughing frequency and chytridiomycosis contribute to the high rates of mortality from Bd infections. However, the mechanisms responsible for the loss of cutaneous osmotic regulation remain unknown. We measured the changes in whole animal water uptake rates, in vitro transcutaneous water fluxes across the ventral skin, and the mRNA expression of epithelial water transport proteins (aquaporins, AQPs) and junctional proteins in Bd-infected and uninfected Litoria caerulea skin. We hypothesize that infected frogs would show reduction/inhibition in cutaneous water transporters responsible for regulating water balance, and sloughing would exacerbate cutaneous water fluxes. We found that infected, nonsloughing frogs had an impaired rate of water uptake and showed increased rates of in vitro water efflux across the ventral skin. In uninfected frogs, the expression of AQPs and junction genes increased significantly with sloughing, which may assist in regulating cutaneous water movements and barrier function in the newly exposed skin. In contrast, infected frogs did not show this postsloughing increase in AQP gene expression. The combination of increased sloughing frequency, impaired water uptake rates, and increased rates of water loss likely contributes to the loss of osmotic homeostasis in frogs infected with Bd.

Diverse genotypes of the amphibian-killing fungus produce distinct phenotypes through plastic responses to temperature

Phenotypes are the target of selection and affect the ability of organisms to persist in variable environments. Phenotypes can be influenced directly by genes and/or by phenotypic plasticity. The amphibian-killing fungus Batrachochytrium dendrobatidis (Bd) has a global distribution, unusually broad host range, and high genetic diversity. Phenotypic plasticity may be an important process that allows this pathogen to infect hundreds of species in diverse environments. We quantified phenotypic variation of nine Bd genotypes from two Bd lineages (Global Pandemic Lineage [GPL] and Brazil) and a hybrid (GPL-Brazil) grown at three temperatures (12, 18 and 24°C). We measured five functional traits including two morphological traits (zoospore and zoosporangium sizes) and three life history traits (carrying capacity, time to fastest growth and exponential growth rate) in a phylogenetic framework. Temperature caused highly plastic responses within each genotype, with all Bd genotypes showing phenotypic plasticity in at least three traits. Among genotypes, Bd generally showed the same direction of plastic response to temperature: larger zoosporangia, higher carrying capacity, longer time to fastest growth and slower exponential growth at lower temperatures. The exception was zoospore size, which was highly variable. Our findings indicate that Bd genotypes have evolved novel phenotypes through plastic responses to temperature over very short timescales. High phenotypic variability likely extends to other traits and may facilitate the large host range and rapid spread of Bd.

Epidermal epidemic: unravelling the pathogenesis of chytridiomycosis

Chytridiomycosis, a lethal fungal skin disease of amphibians, fatally disrupts ionic and osmotic homeostasis. Infected amphibians increase their skin shedding rate (sloughing) to slow pathogen growth, but the sloughing process also increases skin permeability. Healthy amphibians increase active ion uptake during sloughing by increasing ion transporter abundance to offset the increased skin permeability. How chytridiomycosis affects the skin function during and between sloughing events remains unknown. Here, we show that non-sloughing frogs with chytridiomycosis have impaired cutaneous sodium uptake, in part because they have fewer sodium transporters in their skin. Interestingly, sloughing was associated with a transient increase in sodium transporter activity and abundance, suggesting that the newly exposed skin layer is initially fully functional until the recolonization of the skin by the fungus again impedes cutaneous function. However, the temporary restoration of skin function during sloughing does not restore ionic homeostasis, and the underlying loss of ion uptake capacity is ultimately detrimental for amphibians with chytridiomycosis.

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

Amphibian skin is a mucosal surface in direct and continuous contact with a microbially diverse and laden aquatic and/or terrestrial environment. As such, frog skin is an important innate immune organ and first line of defence against pathogens in the environment. Critical to the innate immune functions of frog skin are the maintenance of physical, chemical, cellular, and microbiological barriers and the complex network of interactions that occur across all the barriers. Despite the global decline in amphibian populations, largely as a result of emerging infectious diseases, we understand little regarding the cellular and molecular mechanisms that underlie the innate immune function of amphibian skin and defence against pathogens. In this review, we discuss the structure, cell composition and cellular junctions that contribute to the skin physical barrier, the antimicrobial peptide arsenal that, in part, comprises the chemical barrier, the pattern recognition receptors involved in recognizing pathogens and initiating innate immune responses in the skin, and the contribution of commensal microbes on the skin to pathogen defence. We briefly discuss the influence of environmental abiotic factors (natural and anthropogenic) and pathogens on the immunocompetency of frog skin defences. Although some aspects of frog innate immunity, such as antimicrobial peptides are well-studied; other components and how they contribute to the skin innate immune barrier, are lacking. Elucidating the complex network of interactions occurring at the interface of the frog's external and internal environments will yield insight into the crucial role amphibian skin plays in host defence and the environmental factors leading to compromised barrier integrity, disease, and host mortality.

Review of the Amphibian Immune Response to Chytridiomycosis, and Future Directions

The fungal skin disease, chytridiomycosis (caused by Batrachochytrium dendrobatidis and B. salamandrivorans), has caused amphibian declines and extinctions globally since its emergence. Characterizing the host immune response to chytridiomycosis has been a focus of study with the aim of disease mitigation. However, many aspects of the innate and adaptive arms of this response are still poorly understood, likely due to the wide range of species' responses to infection. In this paper we provide an overview of expected immunological responses (with inference based on amphibian and mammalian immunology), together with a synthesis of current knowledge about these responses for the amphibian-chytridiomycosis system. We structure our review around four key immune stages: (1) the naïve immunocompetent state, (2) immune defenses that are always present (constitutive defenses), (3) mechanisms for recognition of a pathogen threat and innate immune defenses, and (4) adaptive immune responses. We also evaluate the current hot topics of immunosuppression and immunopathology in chytridiomycosis, and discuss their respective roles in pathogenesis. Our synthesis reveals that susceptibility to chytridiomycosis is likely to be multifactorial. Susceptible amphibians appear to have ineffective constitutive and innate defenses, and a late-stage response characterized by immunopathology and Bd-induced suppression of lymphocyte responses. Overall, we identify substantial gaps in current knowledge, particularly concerning the entire innate immune response (mechanisms of initial pathogen detection and possible immunoevasion by Bd, degree of activation and efficacy of the innate immune response, the unexpected absence of innate leukocyte infiltration, and the cause and role of late-stage immunopathology in pathogenesis). There are also gaps concerning most of the adaptive immune system (the relative importance of B and T cell responses for pathogen clearance, the capacity and extent of immunological memory, and specific mechanisms of pathogen-induced immunosuppression). Improving our capacity for amphibian immunological research will require selection of an appropriate Bd-susceptible model species, the development of taxon-specific affinity reagents and cell lines for functional assays, and the application of a suite of conventional and emerging immunological methods. Despite current knowledge gaps, immunological research remains a promising avenue for amphibian conservation management.

Effects of Emerging Infectious Diseases on Amphibians: A Review of Experimental Studies

Numerous factors are contributing to the loss of biodiversity. These include complex effects of multiple abiotic and biotic stressors that may drive population losses. These losses are especially illustrated by amphibians, whose populations are declining worldwide. The causes of amphibian population declines are multifaceted and context-dependent. One major factor affecting amphibian populations is emerging infectious disease. Several pathogens and their associated diseases are especially significant contributors to amphibian population declines. These include the fungi Batrachochytrium dendrobatidis and B. salamandrivorans, and ranaviruses. In this review, we assess the effects of these three pathogens on amphibian hosts as found through experimental studies. Such studies offer valuable insights to the causal factors underpinning broad patterns reported through observational studies. We summarize key findings from experimental studies in the laboratory, in mesocosms, and from the field. We also summarize experiments that explore the interactive effects of these pathogens with other contributors of amphibian population declines. Though well-designed experimental studies are critical for understanding the impacts of disease, inconsistencies in experimental methodologies limit our ability to form comparisons and conclusions. Studies of the three pathogens we focus on show that host susceptibility varies with such factors as species, host age, life history stage, population and biotic (e.g., presence of competitors, predators) and abiotic conditions (e.g., temperature, presence of contaminants), as well as the strain and dose of the pathogen, to which hosts are exposed. Our findings suggest the importance of implementing standard protocols and reporting for experimental studies of amphibian disease.

A pathogenic skin fungus and sloughing exacerbate cutaneous water loss in amphibians

Batrachochytrium dendrobatidis (Bd) is a pathogenic fungus that causes the cutaneous, infectious disease chytridiomycosis and has been implicated in population declines of numerous anuran species worldwide. Proximate cause of death by chytridiomycosis is asystolic cardiac arrest as a consequence of severe disruption to electrolyte balance. Animals heavily infected with Bd also experience a disruption to their skin sloughing regime, indicating that core functions of the skin, such as water retention, may be severely impacted. This study examined how skin sloughing, body size and Bd infection interact to influence water loss rates in five Australian frog species: Litoria caerulea, Limnodynastes peronii, Lechriodus fletcheri, Limnodynastes tasmaniensis and Platyplectrum ornatum Rates of water loss more than doubled during sloughing in L.caerulea During active periods across all species, water loss rates were on average 232% higher in Bd infected frogs than in uninfected frogs. This indicates that dehydration stress may be a significant factor contributing to the morbidity of severely Bd infected anurans, a symptom that is then exacerbated by an increased rate of sloughing. When taking size into account, smaller and/or juvenile anurans may be more at risk from dehydration due to Bd infection, as they lose a greater amount of water and slough more frequently than adults. This may in part explain the higher mortality rates typical for small and juvenile frogs infected with Bd Understanding how Bd affects the core functions of the skin, including rates of water loss, can improve our predictions of disease outcome in amphibians.

Body size influences energetic and osmoregulatory costs in frogs infected with Batrachochytrium dendrobatidis

Sloughing maintains the skins integrity and critical functionality in amphibians. Given the behavioural, morphological and osmoregulatory changes that accompany sloughing, this process is likely to be physiologically costly. Chytridiomycosis, a cutaneous disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd), disrupts skin function and increases sloughing rates. Moreover, mortality rates from chytridiomycosis are significantly higher in juveniles and so we hypothesised that smaller individuals maybe more susceptible to chytridiomycosis because of allometric scaling effects on the energetic and osmoregulatory costs of sloughing. We measured in-vivo cutaneous ion loss rates and whole animal metabolic rate (MR) of Green tree frogs, Litoria caerulea, over a range of body sizes both infected and uninfected frogs during sloughing. Infected animals had a greater rate of ion loss and mass-specific MR during non-sloughing periods but there were no additional effects of sloughing on either of these parameters. There were also significant interactions with body size and Bd load indicating that smaller animals with higher Bd loads have greater rates of ion loss and higher energetic demands. Our results shed light on why smaller Bd-infected anurans often exhibit greater physiological disruption than larger individuals.

Gene expression differs in susceptible and resistant amphibians exposed to Batrachochytrium dendrobatidis

Chytridiomycosis, the disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has devastated global amphibian biodiversity. Nevertheless, some hosts avoid disease after Bd exposure even as others experience near-complete extirpation. It remains unclear whether the amphibian adaptive immune system plays a role in Bd defence. Here, we describe gene expression in two host species-one susceptible to chytridiomycosis and one resistant-following exposure to two Bd isolates that differ in virulence. Susceptible wood frogs (Rana sylvatica) had high infection loads and mortality when exposed to the more virulent Bd isolate but lower infection loads and no fatal disease when exposed to the less virulent isolate. Resistant American bullfrogs (R. catesbeiana) had high survival across treatments and rapidly cleared Bd infection or avoided infection entirely. We found widespread upregulation of adaptive immune genes and downregulation of important metabolic and cellular maintenance components in wood frogs after Bd exposure, whereas American bullfrogs showed little gene expression change and no evidence of an adaptive immune response. Wood frog responses suggest that adaptive immune defences may be ineffective against virulent Bd isolates that can cause rapid physiological dysfunction. By contrast, American bullfrogs exhibited robust resistance to Bd that is likely attributable, at least in part, to their continued upkeep of metabolic and skin integrity pathways as well as greater antimicrobial peptide expression compared to wood frogs, regardless of exposure. Greater understanding of these defences will ultimately help conservationists manage chytridiomycosis.