COS 171-1 - Understanding the role of Tetrodotoxin on skin microbiome composition and chytrid fungal infection in rough-skinned newts (Taricha granulosa).

Abstract: Chytridiomycosis is an amphibian disease caused by fungal etiological agents Batrachochytrium dendrobatidis (Bd) and sister species, B. salamandrivorans (Bsal). Chytridiomycosis has been linked to numerous population declines and species extinctions. However, some amphibian species and populations are less susceptible to severe infections leading to declines due to a variety of intrinsic host and external factors. Host-associated skin microbes play an important role in host defense, inhibiting pathogens by competing for resources and producing antifungal metabolites. Aiding amphibian defense against predation, parasites, and infectious disease are mucous and granular glands. Mucous (serous) glands create glycosylated mucins and mucopolysaccharides used in preventing desiccation from water loss and facilitating diverse microbiomes, which may affect disease outcomes. Granular (poison) glands release toxins to deter predation and parasites in response to epinephrine or norepinephrine stress cues. Tetrodotoxin (TTX) is a potent neurotoxin found throughout marine taxa but limited to Anura and Caudata among land vertebrates. Studies have shown a negative relationship between TTX concentration and Bd infection intensity but impacts of TTX concentration on microbiome diversity and subsequent pathogen dynamics remain relatively unanswered. Here we examine effects of TTX concentrations on Bd infection intensity and skin microbiome in rough-skinned newts (Taricha granulosa, n = 90) collected across six populations in the Cascade Mountains and surrounding area. Individual infection intensity ranged from 0.0 to 1.7x10⁴ zoospore equivalents (mean ± SE = 772.7 ± 216.743). TTX concentration ranged from 0.061 to 6.718mg/cm2 skin (mean ± SE = 1.939 ± 0.2372 mg TTX/cm2 skin). Between populations, there was no significant difference in zoospore equivalents (one-way ANOVA: df = 5, F value = 1.637, P = 0.159), but there was a significant difference in TTX levels (one-way ANOVA: df = 5, F value = 77.38, P < 0.0001). We found no correlation between TTX concentrations and infection intensity (GLM Quasi-Poisson: df = 88, SE ± 0.1342, t value = 1.046, P = 0.298); however, our ongoing investigations into TTX concentration and microbial diversity may reveal interactions of complex micro-ecosystem processes amphibians utilize in defense against Bd. Elucidating evolutionary relationships of host-pathogen-microbiome dynamics in vulnerable groups is necessary to further conservation efforts in even distantly related taxa by clarifying physiological and microbiome responses to antagonism.