Title Characterisation of the amphibian pathogenic fungus Batrachochytrium dendrobatidis supercontig 5

Abstract

Amphibian species have been experiencing a massive global decline during the past few decades. Chytridiomycosis, caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd), is a cutaneous infectious disease that leads to irregular epidermal hyperplasis and hyperkeratosis in amphibians. It is believed to be one of the major causes of the amphibian population declines and is also the first chytrid fungus known to parasitise vertebrates. This global amphibian pandemic led to mass mortality and local or complete extinction of amphibian species. This project is primarily focused on establishing the multi-locus sequence typing (MLST) approach in a single chromosomal region (supercontig 5). Five strains sampled from North America, Central America and Australia were included in the study. They are JEL197, JE1225, JEL253, JEL308 and P16. This allows intra-chromosomal comparisons among strains by providing the strain-specific allelic profiles. The data will be integrated with the sequences from the two available sequenced strains, JEL423 and JAM081. The allelic profiles are composed of 34 loci across the supercontig 5 on which the intein BdeRPC2 is embedded. The raw sequence data of the two sequenced strains were also examined to establish the genotypic structure of the Bd supercontig 5 in these strains. The resulting allelic profiles show low genetic variation in the Bd genome, fixed heterozygosity in some loci among strains and little genotype-geography correlation, which are all consistent with the previous studies. Therefore Bd is believed to be primarily clonally reproduced. It is believed that the fungus has recently emerged as a novel pathogen which has been introduced into na'ive amphibian populations. Genotypic differentiations were observed on the Bd supercontig 5 and these variations, with a great probability, have been generated by loss of heterozygosity through mitotic recombination. A 40 Kb region showing several recombination events (a recombination hotspot) was also observed on the Bd supercontig 5. In this region three breakpoints were identified in the two sequenced strains and a minisatellite was also observed. The results demonstrate that all seven Bd strains that have been studied are very closely related suggesting they are derived from a recent common ancestor.