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theses

Amphibians face a suite of challenges to survival including predation, pollution, habitat loss, and infectious diseases. Over the last several decades, amphibian populations have been severely declining as a result of a combination of these factors. A looming threat caused by an emerging fungal pathogen has highlighted the potential importance of the amphibian cutaneous microbiome in mediating the effects of disease. In the first two chapters of my dissertation, my research focus is on understanding the diversity and function of the amphibian cutaneous microbiome in the context of resistance against this pathogen. In the last chapter, I shift my focus to exploring the role of phenotypic plasticity in protecting tadpoles of a near-threatened species from a natural predator.
In chapter 1, I used culture-based techniques to study bacteria isolated from green frog skin that inhibit the growth of the fungal pathogen, Batrachochytrium dendrobatidis (Bd). Despite bacteria being classified as the same operational taxonomic unit (OTU, ~bacterial species) based on 16S rRNA sequencing, I found differences in Bd inhibition capabilities among isolates. This suggests that phylogenetic relatedness alone is not a reliable predictor of whether or not a bacterial isolate can prevent Bd growth. Furthermore, I found unique communities of anti-Bd bacteria among three populations of green frogs, suggesting functional redundancy of Bd inhibition across populations.
Because not all bacteria are readily culturable, I used culture-independent techniques in chapter 2 to explore variation in amphibian skin microbiomes among frog species, sampling sites, and individuals with and without Bd present on their skin. Through sampling skin microbiomes of six frog species, I found significant differences among frog skin microbiomes across species and sites, but not between Bd-positive and Bd-negative individuals. Additionally, putative anti-Bd OTUs made up a third of bacterial abundance among host-associated communities, and several putative anti-Bd OTUs were strongly associated with frogs based on their abundance and prevalence. The presence of anti-Bd OTUs may be offering frogs protection against Bd and may partially explain why several of the host species sampled are asymptomatic carriers of Bd. Overall, these results suggest that skin-associated microbial communities reflect host species and the environment, but not Bd status among the frogs studied here.
In my final chapter, I described phenotypic changes in tadpoles of Hyla andersonii exposed to the odonate predator Anax junius. Predator-exposed tadpoles developed darker and deeper tail fins. This response likely increases tadpole survival due to the "lure effect," where conspicuous tail morphology attracts predator attacks toward the tail, which can be regrown, and away from the vulnerable head. Given that these findings are consistent with previous documentation of conspicuous tail coloration in hylid tadpoles, I propose that this provides evidence of an adaptive syndrome among hylid tadpoles, where tadpoles develop conspicuous tails in the presence of odonate predators.
This dissertation has provided new insight on the defensive strategies employed by amphibians against their natural enemies and will help inform conservation plans in this time of significant amphibian declines.

ETD_10237

Ph.D.

Includes bibliographical references

doi:10.7282/t3-qzdm-3g05

ETD doctoral

The author owns the copyright to this work.