Genetic relationships in an international collection of Puccinia horiana isolates based on newly identitfied molecular markers and demonstration of recombination

The obligate biotrophic pathogen Puccinia horiana is the causal agent of chrysanthemum white rust. Although P. horiana is a quarantine organism, it was able to spread to most chrysanthemum producing regions in the world since the 1960's, however, the transfer routes are largely obscure. An extremely low level of allelic diversity was observed in a geographically diverse set of eight isolates using Complexity Reduction of Polymorphic Sequences (CRoPS) technology. Only 184 of the 16,196 contigs (1.1 %) showed one or more single nucleotide polymorphisms (SNPs). Thirty two SNPs and one simple sequence repeat (SSR) were translated into molecular markers and used to genotype 45 isolates originating from North and South America, Asia and Europe. Phylogenetic clustering was in most cases related to geographic origin, indicating local establishment. The European isolates mostly grouped in two major populations that may relate to the two historic introductions previously reported. However, evidence of recent geographical transfer was also observed, including transfer events between Europe and South America and between South-East Asia and Europe. In contrast with the presumed clonal propagation of this microcyclic rust, strong indications of marker recombination were observed, presumably as a result of anastomosis, karyogamy and somatic meiosis. Recombination and transfer also explain the geographical dispersal of specific markers. A near-to-significant correlation between the genotypic data and previously obtained pathotype data was observed and one marker was associated withthe most virulent pathotype group. In combination with a fast SNP detection method, the markers presented here will be helpful tools to further elucidate the transfer pathways and local survival of this pathogen.

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