# Comparative Genomics Reveals Intra and Inter Species Variation in the Pathogenic Fungus Batrachochytrium dendrobatidis

## TL;DR

This study compares the genomes of different lineages of a deadly amphibian fungus to understand genetic differences that may affect pathogenicity.

## Contribution

The study provides high-quality genomes for the Bd-BRAZIL lineage and identifies lineage-specific gene families and pangenome structure in Batrachochytrium dendrobatidis.

## Key findings

- The core genome of B. dendrobatidis includes 6,278 gene families, while accessory genomes differ between Bd-BRAZIL and Bd-GPL lineages.
- Copy number differences were found in gene families related to pathogenicity between Bd-BRAZIL and Bd-GPL strains.
- Reference genome selection impacts RNAseq transcript recovery, highlighting the importance of diverse genomic resources.

## Abstract

The Global Panzootic Lineage (GPL) of Batrachochytrium dendrobatidis (Bd) has been described as a main driver of amphibian extinctions. Pathogen studies have benefited from three Bd-GPL strain genomes, but identifying the genetic and molecular features that distinguish the B. dendrobatidis lineages requires additional high-quality genomes from diverse lineages. We sequenced and assembled genomes with Oxford Nanopore Technologies to produce assemblies of three Bd-BRAZIL isolates and one nonpathogen outgroup species Polyrhizophydium stewartii. The Bd-BRAZIL assembly sizes ranged between 22.0 and 26.1 Mb with 8,495 to 8,620 predicted protein-coding genes. We sought to categorize the pangenome of the species by identifying homologous genes across the sampled genomes as either being core and present in all strains, or accessory and shared among strains in a lineage, an analysis that has not yet been conducted on B. dendrobatidis and its lineages. We identified a core genome consisting of 6,278 gene families, and an accessory genome of 202 Bd-BRAZIL and 172 Bd-GPL specific gene families. We discovered copy number differences in pathogenicity gene families: M36 Peptidases, Crinkler Necrosis genes, Aspartyl Peptidases, Carbohydrate-Binding Module-18 genes, and S41 Proteases, between Bd-BRAZIL and Bd-GPL strains. Comparison of B. dendrobatidis and two closely related saprophytic species identified differences in protein sequence and domain counts for M36 and CBM18 families respectively. Our pangenome analysis of lineage-specific gene content led us to explore how the selection of the reference genome affects recovery of RNAseq transcripts when comparing different strains. We tested the hypothesis that genomic variation among Bd-GPL and Bd-BRAZIL lineages can impact transcript count data by comparing results with our new Bd-BRAZIL genomes as the reference genomes. Our analysis examines the genomic variation between strains in Bd-BRAZIL and Bd-GPL and offers insights into the application of these high-quality reference genomes resources for future studies.