Herptile gut microbiomes: a natural system to study multi-kingdom interactions between filamentous fungi and bacteria.

Reptiles and amphibians (herptiles) are some of the most endangered and threatened species on the planet and numerous conservation strategies are being implemented with the goal of ensuring species recovery. Little is known, however, about the gut microbiome of wild herptiles and how it relates to the health of these populations. Here, we report results from the gut microbiome characterization of both a broad survey of herptiles, and the correlation between the fungus Basidiobolus, and the bacterial community supported by a deeper, more intensive sampling of Plethodon glutinosus, known as slimy salamanders. We demonstrate that bacterial communities sampled from frogs, lizards, and salamanders are structured by the host taxonomy and that Basidiobolus is a common and natural component of these wild gut microbiomes. Intensive sampling of multiple hosts across the ecoregions of Tennessee revealed that geography and host:geography interactions are strong predictors of distinct Basidiobolus operational taxonomic units present within a given host. Co-occurrence analyses of Basidiobolus and bacterial community diversity support a correlation and interaction between Basidiobolus and bacteria, suggesting that Basidiobolus may play a role in structuring the bacterial community. We further the hypothesis that this interaction is advanced by unique specialized metabolism originating from horizontal gene transfer from bacteria to Basidiobolus and demonstrate that Basidiobolus is capable of producing a diversity of specialized metabolites including small cyclic peptides.IMPORTANCEThis work significantly advances our understanding of biodiversity and microbial interactions in herptile microbiomes, the role that fungi play as a structural and functional members of herptile gut microbiomes, and the chemical functions that structure microbiome phenotypes. We also provide an important observational system of how the gut microbiome represents a unique environment that selects for novel metabolic functions through horizontal gene transfer between fungi and bacteria. Such studies are needed to better understand the complexity of gut microbiomes in nature and will inform conservation strategies for threatened species of herpetofauna.

Cross-species gut microbiota transplantation predictably affects host heat tolerance.

The gut microbiome is known to influence and have regulatory effects in diverse physiological functions of host animals, but only recently has the relationship between host thermal biology and gut microbiota been explored. Here, we examined how early-life manipulations of the gut microbiota in larval amphibians influenced their critical thermal maximum (CTmax) at different acclimation temperatures. We stripped the resident microbiome from egg masses of wild-caught wood frogs (Lithobates sylvaticus) via an antibiotic wash, and then inoculated the eggs with pond water (control), no inoculation, or the intestinal microbiota of another species that has a wider thermal tolerance - green frogs (Lithobates clamitans). We predicted that this cross-species transplant would increase the CTmax of the recipient wood frog larvae relative to the other treatments. In line with this prediction, green frog microbiome-recipient larvae had the highest CTmax while those with no inoculum had the lowest CTmax. Both the microbiome treatment and acclimation temperature significantly influenced the larval gut microbiota communities and α-diversity indices. Green frog microbiome-inoculated larvae were enriched in Rikenellaceae relative to the other treatments, which produce short-chain fatty acids and could contribute to greater energy availability and enhanced heat tolerance. Larvae that received no inoculation had a higher relative abundance of potentially pathogenic Aeromonas spp., which negatively affects host health and performance. Our results are the first to show that cross-species gut microbiota transplants alter heat tolerance in a predictable manner. This finding has repercussions for the conservation of species that are threatened by climate change and demonstrates a need to further explore the mechanisms by which the gut microbiota modulate host thermal tolerance.

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 (CTmax), compared to uninfected controls. Ranavirus infection did not reduce CTmax and there was a positive relationship between CTmax 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 CTmax 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 CTmax was observed, this suggests that infected hosts would not be under greater risk of 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 (CTmax ). A hardening treatment (sub-critical temperature exposure) was applied 2 h before the CTmax 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 CTmax 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.

Captivity and Animal Microbiomes: Potential Roles of Microbiota for Influencing Animal Conservation.

During the ongoing biodiversity crisis, captive conservation and breeding programs offer a refuge for species to persist and provide source populations for reintroduction efforts. Unfortunately, captive animals are at a higher disease risk and reintroduction efforts remain largely unsuccessful. One potential factor in these outcomes is the host microbiota which includes a large diversity and abundance of bacteria, fungi, and viruses that play an essential role in host physiology. Relative to wild populations, the generalized pattern of gut and skin microbiomes in captivity are reduced alpha diversity and they exhibit a significant shift in community composition and/or structure which often correlates with various physiological maladies. Many conditions of captivity (antibiotic exposure, altered diet composition, homogenous environment, increased stress, and altered intraspecific interactions) likely lead to changes in the host-associated microbiome. To minimize the problems arising from captivity, efforts can be taken to manipulate microbial diversity and composition to be comparable with wild populations through methods such as increasing dietary diversity, exposure to natural environmental reservoirs, or probiotics. For individuals destined for reintroduction, these strategies can prime the microbiota to buffer against novel pathogens and changes in diet and improve reintroduction success. The microbiome is a critical component of animal physiology and its role in species conservation should be expanded and included in the repertoire of future management practices.

Experimental inoculation trial to determine the effects of temperature and humidity on White-nose Syndrome in hibernating bats.

Disease results from interactions among the host, pathogen, and environment. Inoculation trials can quantify interactions among these players and explain aspects of disease ecology to inform management in variable and dynamic natural environments. White-nose Syndrome, a disease caused by the fungal pathogen, Pseudogymnoascus destructans (Pd), has caused severe population declines of several bat species in North America. We conducted the first experimental infection trial on the tri-colored bat, Perimyotis subflavus, to test the effect of temperature and humidity on disease severity. We also tested the effects of temperature and humidity on fungal growth and persistence on substrates. Unexpectedly, only 37% (35/95) of bats experimentally inoculated with Pd at the start of the experiment showed any infection response or disease symptoms after 83 days of captive hibernation. There was no evidence that temperature or humidity influenced infection response. Temperature had a strong effect on fungal growth on media plates, but the influence of humidity was more variable and uncertain. Designing laboratory studies to maximize research outcomes would be beneficial given the high costs of such efforts and potential for unexpected outcomes. Understanding the influence of microclimates on host-pathogen interactions remains an important consideration for managing wildlife diseases, particularly in variable environments.

Microbiome mediation of animal life histories via metabolites and insulin-like signalling.

The regulatory pathways by which gut microbiota potentially shape host life histories remain largely untested, however, a constellation of research suggests that gut bacteria likely have significant effects on their hosts via metabolites. In this article we review known and hypothesized pathways by which gut microbiota influence host life histories through interfacing with the neuroendocrine system, with a focus on the insulin-like growth factor (IGF) signalling pathway. Bacterially derived metabolites including short-chain fatty acids (SCFAs), polyamines, and peptides likely impact host life histories as metabolic substrates, essential nutrients, and via molecular signalling with well-studied neuroendocrine pathways. The hypothalamus-pituitary axis and insulin-like signalling (ILS) pathways are central regulatory networks for development, growth, reproductive maturity, reproduction, and senescence and are likely targets for tests of how gut bacterial metabolites shape host life histories. SCFAs in particular, as metabolites derived from bacterial fermentation, are implicated as significant microbiome signalling molecules shown to interface with the ILS pathway as well as to bind to receptors on neuroendocrine and peripheral nervous tissues. For example, experimental increases of SCFA production have been shown to affect IGF-1 levels in circulation and are associated with robust development, growth, reproduction, and delayed senescence. Finally, emerging -omics approaches are providing integrative ways to test and detail the potential diverse ways in which gut microbiota interact with their hosts and the likely important roles they play in shaping host life-history responses to varied environmental conditions.

Taxonomy, not locality, influences the cloacal microbiota of two nearctic colubrids: a preliminary analysis.

BACKGROUND: The gut microbiota is an emerging frontier in wildlife research and its importance to vertebrate health and physiology is becoming ever more apparent. Reptiles, in particular snakes, have not received the same attention given to other vertebrates and the composition of their wild gut microbiome remains understudied. The primary goal of this work was to describe the cloacal microbiota of two Colubrids, the Eastern Gartersnake (Thamnophis sirtalis sirtalis) and the Northern Watersnake (Nerodia sipedon sipedon), and if their cloacal microbiota differed as well as if it did between a wetland and upland population of the former species. METHODS AND RESULTS: We utilized next-generation sequencing of cloacal swabs-a non-destructive proxy for the gut microbiota. The cloacal microbiome of Eastern Gartersnakes (N = 9) was like those of other snakes being comprised of Proteobacteria, Bacteroidetes, and Firmicutes, while that of Northern Watersnakes (N = 6) was dominated by Tenericutes. Seven microbial operational taxonomic units (OTUs), all members of Proteobacteria, were shared among all individuals and were indicative of a core microbiome in Eastern Gartersnakes, but these OTUs were not particularly relevant to Northern Watersnakes. The latter had greater OTU richness than did Eastern Gartersnakes, and habitat did not have any apparent effect on the microbial community composition in Eastern Gartersnakes. CONCLUSIONS: Our findings suggest host taxonomy to be a determining factor in the cloacal microbiota of snakes and that Tenericutes are associated with aquatic habitats. This is the first report to examine the cloacal microbiome of these species and provides a useful foundation for future work to build upon.

Eurythermic Sprint and Immune Thermal Performance and Ecology of an Exotic Lizard at Its Northern Invasion Front.

AbstractThermal performance of immunity has been relatively understudied in ectotherms, especially in the context of invasive species or in relation to other fitness-related traits and thermoregulatory patterns in the field. For reptiles, thermal biology is a primary factor determining physiological performance and population viability, and suboptimal thermal conditions may limit the expansion of exotic species along the edges of their invasion fronts. This study examined thermoregulatory ecology and thermal performance of immunity and sprinting in a population of Mediterranean geckos (Hemidactylus turcicus) at the northern edge of their invasion front in a temperate zone of the United States. In the field, we quantified temperatures of geckos of varied age classes in relation to air, wall, and refugia temperatures. We also quantified temperature-dependent sprint performance and immune function in field-collected geckos to detail thermal performance patterns that may contribute to the capacity for this species to invade cool climates. Although body temperature (Tb) of wild-caught geckos correlated with wall temperature, average Tb exhibited wide distributions, suggesting eurythermy. Furthermore, the thermal performance of immune swelling responses to phytohemagglutinin injections and sprinting was optimized over a similarly wide temperature range that overlapped with the field Tb's that suggest eurythermy in this species. The wide thermal performance breadths in these traits could buffer against variation in factors such as pathogen exposure and environmental temperatures that could otherwise suppress functional performance. Thus, eurythermy of sprint and immune performance may facilitate the invasive potential of H. turcicus.

Effects of urbanization on the population structure of freshwater turtles across the United States.

Landscape-scale alterations that accompany urbanization may negatively affect the population structure of wildlife species such as freshwater turtles. Changes to nesting sites and higher mortality rates due to vehicular collisions and increased predator populations may particularly affect immature turtles and mature female turtles. We hypothesized that the proportions of adult female and immature turtles in a population will negatively correlate with landscape urbanization. As a collaborative effort of the Ecological Research as Education Network (EREN), we sampled freshwater turtle populations in 11 states across the central and eastern United States. Contrary to expectations, we found a significant positive relationship between proportions of mature female painted turtles (Chrysemys picta) and urbanization. We did not detect a relationship between urbanization and proportions of immature turtles. Urbanization may alter the thermal environment of nesting sites such that more females are produced as urbanization increases. Our approach of creating a collaborative network of scientists and students at undergraduate institutions proved valuable in terms of testing our hypothesis over a large spatial scale while also allowing students to gain hands-on experience in conservation science.