Comparative genome analysis indicates rapid evolution of pathogenicity genes in Colletotrichum tanaceti

Abstract

Colletotrichum tanaceti is an emerging foliar fungal pathogen of pyrethrum (Tanacetum cinerariifolium), posing a threat to the global pyrethrum industry. Despite being reported consistently from field surveys in Australia, the molecular basis of pathogenicity of C. tanaceti on pyrethrum is unknown. Herein, the genome of C. tanaceti (isolate BRIP57314) was assembled de novo and annotated using transcriptomic evidence. The inferred pathogenicity gene suite of C. tanaceti comprised a large array of genes encoding secreted effectors, proteases, CAZymes and secondary metabolites. Comparative analysis of its CAZyme pathogenicity profiles with those of closely related species suggested that C. tanaceti had additional hosts to pyrethrum. The genome of C. tanaceti had a high repeat content and repetitive elements were located significantly closer to genes inferred to influence pathogenicity than other genes. These repeats are likely to have accelerated mutational and transposition rates in the genome, resulting in a rapid evolution of certain CAZyme families in this species. The C. tanaceti genome consisted of a gene-sparse, A-T rich region facilitating a "two-speed" genome. Pathogenicity genes within this region were likely to have a higher evolutionary rate than the core genome. This "two-speed" genome phenomenon in certain Colletotrichum spp. was hypothesized to have caused the clustering of species based on the pathogenicity genes, to deviate from taxonomy. With the large repertoire of pathogenicity factors that can potentially evolve rapidly in response to control measures, C. tanaceti may pose a high-risk to global pyrethrum production. Knowledge of the pathogenicity genes will facilitate future research in disease management of C. tanaceti and other Colletotrichum spp..