The emergence of Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal), two fungal pathogens responsible for amphibian declines and extinctions worldwide, led to 20 yr of intensive research focused on revealing mechanisms underlying infection, disease progression, and host outcome. Genetic and genomic studies in this system have contributed to our understanding of the origin and genetic diversity within these pathogens, their genetic architecture, and mechanisms used for host exploitation and host immune response. Functional genomic studies, specifically differential gene expression studies, have been used to characterize both pathogen and host mechanisms of exploitation and defense as well as host–pathogen interactions and the influence of biotic and abiotic environmental factors on disease outcomes. Here, we summarize the results of functional genomic studies in the amphibian–chytrid host–pathogen system, with a focus on identifying common patterns, and also examine the factors that contribute to the high variability in disease outcomes observed in nature. We find substantial variation in differential gene expression among lineages of Bd and between Bd and Bsal, pointing to some potential virulence factors for these pathogens. On the host side, we find variation in immune response among species with different levels of resistance to chytrid infections. Finally, we find significant effects of temperature and coinfection on host responses, underscoring the important role that abiotic and biotic factors play in modulating immune response.

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