Mating-type gene structure and spatial distribution of Didymella tanaceti in pyrethrum fields

Tan spot of pyrethrum (Tanacetum cinerariifolium) is caused by the ascomycete, Didymella tanaceti. To assess the evolutionary role of ascospores in the assumed asexual species, the structure and arrangement of mating-type (MAT) genes were examined. A single MAT1-1 or MAT1-2 idiomorph was identified in all isolates examined, indicating the species is heterothallic. The idiomorphs were flanked upstream and downstream by regions encoding pyridoxamine phosphate oxidase-like and DNA lyase-like proteins, respectively. A multiplex MAT-specific PCR assay was developed and used to genotype 325 isolates collected within two transects in each of four fields in Tasmania, Australia. The ratio of mating-types in each transect were consistent with a 1:1 ratio. The spatial distribution of the mating-types within each transect was random, for all except one transect for MAT1-1 isolates, indicating that clonal patterns of each mating-type were absent. However, evidence of a reduced selection pressure on MAT1-1 isolates was observed, with a second haplotype of the MAT1-1-1 gene identified in 4.4% of MAT1-1 isolates. In vitro crosses between isolates with opposite mating-types failed to produce ascospores. While the sexual morph could not be induced, the occurrence of both mating-types in equal frequencies suggested a cryptic sexual mode of reproduction may occur within field populations.