Molecular toolkit unlocks life cycle of the panzootic amphibian pathogen Batrachochytrium dendrobatidis

Abstract

Amphibians are facing an extinction crisis that threatens up to 50% of all species (1, 2). Uniquely, a pathogenic fungus, Batrachochytrium dendrobatidis (Bd), is now recognized as a proximate driver of these declines (3). The pathogen's widespread global host range in >400 species of amphibian on 5 continents gives unfortunate candidacy for this being the most destructive emergence of infectious disease ever witnessed. Bd is a basal fungal lineage in the Chytridiomycota. These fungi are characteristically aquatic and flagellate, and the lack of any chytrid pathogens of vertebrates has led to the group remaining poorly characterized relative to other fungi, both at the taxonomic and molecular level. As a consequence, the mechanisms utilized by Bd to infect and cause disease in amphibians remain shrouded in mystery. The recent sequencing of two Bd genomes has created an opportunity to leverage comparative genomics and molecular biology to unlock the life cycle of this secretive fungus. The report by Rosenblum et al. (4) in a recent issue of PNAS represents the first use of this genomic information by using whole-genome arrays to compare patterns of global gene expression for two stages of Bd, the sessile sporangium and the infectious flagellate zoospore. This new study has shown that >55% of the ≈9,000 genes in the Bd genome are undergoing differential expression between these two stages. Mining the predicted function of these genes by identifying similarities with genes of known function in other species (gene ontology) has provided the first clues into the higher mechanisms that orchestrate Bd growth, infection, and pathogenicity in amphibians.