Too Little, Too Late: Amphibian Responses to Chytrid Fungus Across Time and Space

264 pagesFungal pathogens pose a threat to biodiversity worldwide, and none have had a more disastrous effect on wildlife than those within the chytrid genus, Batrachochytrium. The sister species, Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal), have been responsible for population declines and extirpations in more than 500 amphibian species. Despite decades of research, many open questions about host-pathogen interactions within this system remain. Most notably, how hosts differentially respond to endemic and invasive chytrid lineages and to coinfection are open areas of research. In this dissertation I address these topics using a combination of field sampling, Bd and Bsal exposure trials, and functional and population genomics. In chapter 1, I compare patterns of gene expression both within and between two Bd lineages, endemic Bd-Brazil and invasive Bd-GPL, identifying key genes that may account for differences in virulence. I build on this foundation by then examining differential host responses to Bd-Brazil and Bd-GPL in chapter 2. Focusing on the pumpkin toadlet, Brachycephalus pitanga, I demonstrate that host responses to Bd-GPL are both delayed and dysregulated compared to Bd-Brazil, with which the host has coevolved. In chapter 3, I examine the evolutionary effects of a Bd epizootic on amphibian hosts. Although a population of Emerald glass frogs, Espadarana prosoblepon, declined during the epizootic, population genetic diversity and differentiation were surprisingly unchanged, suggesting that the population is likely persisting with enzootic Bd due to long-range dispersal and gene flow. Whereas Bd has reached stable enzootic states in many regions, Bsal is actively invading new habitats, and its potential impact on North American salamander taxa could be catastrophic. In chapters 4 and 5, I focus explicitly on host responses to Bsal. I first quantify the differential effects of Bsal and Bd coinfection versus single infection on immune processes of the eastern newt, Notophthalmus viridescens. This research demonstrates that coinfection compromises immune pathways active against Bsal alone, which could have profound consequences on host survival given high preexisting Bd prevalence throughout the species’ range. Finally, in chapter 5 I test for intraspecific variation in eastern newt susceptibility to Bsal, finding that there is a positive correlation between latitude and host survival, with southern populations exhibiting increased mortality. Collectively, the results presented in this dissertation refine our understanding of host responses to chytrid fungus and offer insight into potential avenues for disease mitigation.2023-06-0