Pathogenicity in Chytrid Fungi

The chytrid fungus Batrachochytrium dendrobatidis (Bd) is an emerging pathogen that has been implicated in decimating amphibian populations around the world. Bd is the only member of an ancient group of fungi (called the Chytridiomycota) that is known to attack vertebrates. The question of how an amphibian-killing fungus evolved from non-pathogenic ancestors is vital to protecting the world's remaining amphibians from Bd.

Researchers sequenced the genome of Bd's closest known relative - a non-pathogenic chytrid named Homolaphlyctis polyrhiza (Hp). They compared the genomes of Bd, Hp and 18 additional fungi to identify what makes Bd unique, and identified a large number of Bd-specific genes, a gene set that contains a number of possible pathogenicity factors. In particular, this paper describes a large number of protease genes in the Bd genome and show that these genes were duplicated after the divergence of Bd and Hp from their common ancestor. Studying Bd's pathogenesis in an evolutionary context provides new evidence for the role of protease genes in Bd's ability to kill amphibians.


Genomic Transition to Pathogenicity in Chytrid Fungi. (2011) PLoS Pathog 7(11): e1002338. doi:10.1371/journal.ppat.1002338
Understanding the molecular mechanisms of pathogen emergence is central to mitigating the impacts of novel infectious disease agents. The chytrid fungus Batrachochytrium dendrobatidis (Bd) is an emerging pathogen of amphibians that has been implicated in amphibian declines worldwide. Bd is the only member of its clade known to attack vertebrates. However, little is known about the molecular determinants of - or evolutionary transition to - pathogenicity in Bd. Here we sequence the genome of Bd's closest known relative - a non-pathogenic chytrid Homolaphlyctis polyrhiza (Hp). We first describe the genome of Hp, which is comparable to other chytrid genomes in size and number of predicted proteins. We then compare the genomes of Hp, Bd, and 19 additional fungal genomes to identify unique or recent evolutionary elements in the Bd genome. We identified 1,974 Bd-specific genes, a gene set that is enriched for protease, lipase, and microbial effector Gene Ontology terms. We describe significant lineage-specific expansions in three Bd protease families (metallo-, serine-type, and aspartyl proteases). We show that these protease gene family expansions occurred after the divergence of Bd and Hp from their common ancestor and thus are localized to the Bd branch. Finally, we demonstrate that the timing of the protease gene family expansions predates the emergence of Bd as a globally important amphibian pathogen.

On the origin of chytridiomycosis

One of the more puzzling aspects of the emergence of amphibian chytridiomycosis has been that, whereas epizootics have been widely observed, many susceptible amphibian communities apparently coexist alongside Bd with no evidence of disease. This new paper shows that the Bd genotype is also an important epidemiological determinant.

In addition, a commentary in New Scientist suggests that the global amphibian trade not only spread the lethal chytrid fungus, but may have created the disease in the first place: Frog-killer disease was born in trade.


Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalized hypervirulent recombinant lineage. PNAS USA November 7, 2011
Batrachochytrium dendrobatidis (Bd) is a globally ubiquitous fungal infection that has emerged to become a primary driver of amphibian biodiversity loss. Despite widespread effort to understand the emergence of this panzootic, the origins of the infection, its patterns of global spread, and principle mode of evolution remain largely unknown. Using comparative population genomics, we discovered three deeply diverged lineages of Bd associated with amphibians. Two of these lineages were found in multiple continents and are associated with known introductions by the amphibian trade. We found that isolates belonging to one clade, the global panzootic lineage (BdGPL) have emerged across at least five continents during the 20th century and are associated with the onset of epizootics in North America, Central America, the Caribbean, Australia, and Europe. The two newly identified divergent lineages, Cape lineage (BdCAPE) and Swiss lineage (BdCH), were found to differ in morphological traits when compared against one another and BdGPL, and we show that BdGPL is hypervirulent. BdGPL uniquely bears the hallmarks of genomic recombination, manifested as extensive intergenomic phylogenetic conflict and patchily distributed heterozygosity. We postulate that contact between previously genetically isolated allopatric populations of Bd may have allowed recombination to occur, resulting in the generation, spread, and invasion of the hypervirulent BdGPL leading to contemporary disease-driven losses in amphibian biodiversity.

Remember, remember the amphibians on the 5th of November!

The Royal Horticultural Society, The Wildlife Trusts and Amphibian and Reptile Conservation are asking organisers of bonfire night celebrations to look out for amphibians before they light their fires.
"People tend to check for hedgehogs in the wood they have gathered for their fire," says Andrew Halstead, Principal RHS Entomologist. "But it is equally likely that toads, frogs and newts will have found shelter in these piles and might be missed. Toads and frogs play an important role as predators in the garden and should be encouraged."

via ARC

Antiviral Immunity in Amphibians

Although a variety of virus species can infect amphibians, diseases caused by ranaviruses (RVs) have become prominent, and are a major concern for biodiversity, agriculture and international trade. The relatively recent and rapid increase in prevalence of RV infections, the wide range of host species infected by RVs, the variability in host resistance among population of the same species and among different developmental stages, all suggest an important involvement of the amphibian immune system. Nevertheless, the roles of the immune system in the etiology of viral diseases in amphibians are still poorly investigated. We review here the current knowledge of antiviral immunity in amphibians, focusing on model species such as the frog Xenopus and the salamander (Ambystoma tigrinum), and on recent progress in generating tools to better understand how host immune defenses control RV infections, pathogenicity, and transmission.

Antiviral Immunity in Amphibians. Viruses. 2011; 3(11):2065-2086