Dissecting the Pyrenophora tritici-repentis (tan spot of wheat) pangenome

Abstract

We sequenced the genome of a global collection (40 isolates) of the fungus Pyrenophora tritici-repentis (Ptr), a major foliar pathogen of wheat and model for the evolution of necrotrophic pathogens. Ptr exhibited an open-pangenome, with 43% of genes in the core set and 57% defined as accessory (present in only a subset of isolates), of which 56% were singleton genes (present in only one isolate). A clear distinction between pathogenic and non-pathogenic genomes was observed in size, gene content, and phylogenetic relatedness. Chromosomal rearrangements and structural organization, specifically around the effector coding genes, were explored further using the annotated genomes of two isolates sequenced by PacBio RS II and Illumina HiSeq. The Ptr genome exhibited major chromosomal rearrangements, including chromosomal fusion, translocation, and segment duplications. An intraspecies translocation of ToxA, the necrosis-inducing effector-coding gene, was facilitated within Ptr via a 143 kb Starship transposon (dubbed Horizon). Additionally, ToxB, the gene encoding the chlorosis-inducing effector, was clustered as three copies on a 294 kb transposable element in a ToxB-producing isolate. ToxB and its carrying transposon were missing from the ToxB non-coding reference isolate, but the homolog toxb and the transposon were both present in another non-coding isolate. The Ptr genome also appears to exhibit a one-compartment organization, but may still possess a two-speed genome that is facilitated by copy-number variation as reported in other fungal pathosystems. IMPORTANCEPtr is one of the most destructive wheat pathogens worldwide. Its genome is a mosaic of present and absent effectors, and serves as a model for examining the evolutionary processes behind the acquisition of virulence in necrotrophs and disease emergence. In this work, we took advantage of a diverse collection of pathogenic Ptr isolates with different global origins and applied short- and long-read sequencing technologies to dissect the Ptr genome. This study provides comprehensive insights into the Ptr genome and highlights its structural organization as an open pangenome with one-compartment. In addition, we identified the potential involvement of transposable elements in genome expansion and the movement of virulence factors. The ability of effector-coding genes to shuffle across chromosomes on large transposons was illustrated by the intraspecies translocation of ToxA and the multi-copy ToxB. In terms of gene contents, the Ptr genome exhibits a large percentage of orphan genes, particularly in non-pathogenic or weakly-virulent isolates.