ORCID

https://orcid.org/0000-0001-9082-4633

Date of Award

2021

Document Type

Thesis

Degree Name

MS in Biology

Department

Biological Sciences

First Advisor

Juan Santos

Second Advisor

Dianella Howarth

Third Advisor

Javier Juarez

Abstract

Chytridiomycosis, a disease caused by the fungus Batrachochytrium dendrobatidis (Bd), is a major concern for Amphibian populations. Over the past thirty years, global declines related to Bd infection have been observed, particularly in neotropical amphibians. Although Bd is endemic to many amphibian species, a significant proportion of amphibian populations have shown evidence of resistance to the pathogen. The precise reasoning as to why there is resistance variation across amphibian taxa remains to be elucidated, but many hypotheses have been suggested. In particular, immunogenetic variations in the innate immune system among amphibians are potential indicators to this dilemma. Toll-Like Receptors (TLRs), a family of innate immune receptors that recognize pathogens, are crucial in the first line of defense against foreign invaders and regulate both the innate and adaptive immune response. TLRs have been shown to be upregulated as a consequence of Bd infection and have also been confirmed to recognize fungal pathogens. Thus, TLRs are likely to play a key role in the defense against Bd. The aims of this study are to sequence the TLR repertoire of amphibians with an emphasis on neotropical taxa, as well as to provide the selection landscapes of all TLRs expressed in the amphibians studied. We preformed RNA extractions of a diverse array of amphibians and also used TLR sequences available from the ncbi. The species under analysis belonged to Anurans, Caudata, and Gymnophiona. We carried out transcriptome assemblies and annotated genes encoding TLRs, as well as phylogenetic techniques to align and provide selection landscapes of amphibian TLRs. For receptors involved in Bd infection, the hypothesis is that positively selected sites (PSSS) in codons associated with non-redundant functions can lead to defective changes that impact the immune response. With our results, we provided evidence of positive selection occurring on TLRs which is indicative of pathogen-mediated evolution, which could have implications in resistance to Bd. Our work provides a platform for future research in chytridiomycosis resistance and is also the largest attempted transcriptome analysis of amphibian TLRs to date.

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