Intra-specific and inter-specific conservation of mating-type genes from the discomycete plant-pathogenic fungi Pyrenopeziza brassicae and Tapesia yallundae

In previous work, four genes involved in mating-type determination were clo ned from reference strains of Pyrenopeziza brassicae; three genes, PAD1, PM T1, and PHB1 (re-named henceforth as MAT-1-1, MAT-1-4, and MAT1-3, respecti vely), are encoded by the MAT-1 idiomorph, and one gene, PHB2 (re-named MAT -2), by the corresponding MAT-2 idiomorph. To assess MAT gene organisation within field-populations of P. brassicae, 30 field-isolates of both mating- types from different geographical locations were analysed by PCR using prim ers designed for the MAT genes of P. brassicae. The results indicate that m ating-type gene structure and organisation within these isolates is conserv ed and is consistent with the mating-type designations established by cross ing experiments. The four P. brassicae MAT genes were then used as probes a gainst gel blots of the genomic DNA of a discomycete Tapesia yallundae from the same family (Dermateaceae, order Helotiales) and one, Ascobolus sterco rarius, from a distantly related family (Ascobolaceae, order Pezizales), in order to determine whether P. brassicae MAT-gene homologs were present. MA T-specific hybridisation signals were obtained with T. yallundae using all four probes. In particular, MAT-1 DNA of T. yallundae gave a strongly hybri dising signal with MAT-1-4 (PMT1), a putative metallothionein gene found in the P. brassicae MAT-1 idiomorph but not in any other MA T idiomorph exami ned to-date. No MAT-specific hybridisation was obtained with A. stercorariu s. A fragment of the MAT-2 gene of T. yallundae was obtained by PCR using d egenerate primers designed to amplify the high-mobility group (HMG) domain present in other ascomycete MAT genes. Sequencing of this PCR product revea led similarities to MAT HMG domains from other ascomycetes with the greates t degree of similarity exhibited with P. brassicae. The T yallundae HMG-DNA sequence was shown to co-segregate with mating type (MAT-2) in progeny fro m a sexual cross.