You’d have to be living under a rock – as some amphibians do – to not be aware of the massive extinction facing our vertebrate friends living within aquatic habitats. Researchers still don’t fully understand what is causing the amphibian mass-extinction – stress from habitat loss, increased chemical concentrations in the environment, and an auto-immune degrading infection have all been proposed. What is known is that the chytrid fungus Batrachochytrium dendrobatidis – opportunistic or not – is infecting and killing a large number of amphibians.
What is not fully understood about B. dendrobatidis is its pathogenicity and what mechanisms it employs to cause infection. A recent paper, “Species-Specific Chitin-Binding Module 18 Expansion in the Amphibian Pathogen Batrachochyrium dendrobatidis”, published in the mBio journal by John Abramyam & Jason Stajich at UC Riverside, begins to address this pathogenicity. As the authors point out – more than 100,000 species of fungi have been described to date and very few of them are pathogenic. This means that the ability to be pathogenic is derived from somewhere: genome expansion events, gene family duplication and diversification events – and we’re only starting to understand horizontal gene transfer events in fungi. This paper addresses the expansion of a gene family across two B. dendrobatidis genomes that are associated with pathogenicity.
When comparing the genomes of B. dendrobatidis with the genomes from other chytrid fungi there has been an expansion of genes within the family Carbohydrate-Binding Module Family 18 (CBM18). The CBM18 family is a large group of proteins that have been implicated in other fungal pathogenic infections on both plants and animals. The authors here question whether this interesting lineage specific expansion of CBM18 in B. dendrobatidis could be associated with the virulence of its pathogenicity on amphibians.
The authors used the CBM18 protein family domain HMM to search across the B. dendrobatidis genomes and found an increase in the number of domains when comparing it to genome of its closest relative. When constructing phylogenetic trees of the CBM18 gene family, three monophyletic and strongly supported clades emerged. When focusing on divergence of the protein domains, the authors determined that individual domain groups were monophyletic and showed a general pattern with regards to their genome locations.
More specifically, clades of the CBM18 family appears to possess different gene functions, some of which appear to be similar to lectins (LL), tyrosinase/catechol oxidases (TL), and chitin deacetylases (DL). The function of these genes has yet to be experimentally determined, but the authors make some deductions based on DNA sequences. The lectin-like genes may be involved in the sequestering of chitin, which could then be disrupting the amphibian immune response. The tyrosinase/catechol oxidase gene family is associated with melanin synthesis, which could be disrupting the electron transport of the infected amphibians. Lastly, chitin deacetylases may be involved in suppressing defense mechanisms in place to suppress the fungal infection of the host. The authors plan to continue to elucidate the pathogenicity of B. dendrobatidis in an attempt to understand the ecology and evolution of its infection on amphibians.