polymorCorresponding

Publication no. P-2003-0408-01R 2003 The American Phytopathological Society
phisms as genetic markers. These investigations have revealed low genetic and genotypic diversity in isolates from most regions in the world. Only three multilocus genotypes were found in Australian isolates, two of A2 mating type and one of A1 mating type, with no evidence for sexual recombination (28,29). These same three multilocus genotypes were found around the world, with only three additional multilocus genotypes of A1 mating type present in global P. cinnamomi populations (30). This suggested clonal spread of P. cinnamomi through much of the world (30). South African isolates also show low genotypic diversity with isozymes (23) and comparisons of these isolates with Australian isolates by restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA techniques indicate common genotypes and a lack of evidence for sexual recombination (24), which further supports the clonal spread hypothesis. However, Papua New Guinea shows high genotypic diversity, as seven additional multilocus genotypes of A1 mating type were found there, hence it is a possible place of origin of P. cinnamomi (3,29). In recent years isozymes have been superseded by microsatellites as the marker of choice for population genetic studies because of their high degree of length polymorphism and ubiquity in eukaryote genomes. We have previously cloned and sequenced a number of microsatellite loci from P. cinnamomi (12). We have shown that they exhibit length polymorphism and studied their inheritance in the progeny of sexual crosses. These markers are particularly suitable for population genetic studies of this pathogen. This paper describes their use to test the hypothesis that P. cinnamomi is clonal in Australia and in doing so reveals the occurrence of frequent mitotic recombination within clonal lineages. MATERIALS AND METHODS Field sampling and fungal culture methods. The three infested sites chosen for intensive sampling were spread across the range of occurrence of P. cinnamomi in southwest Australia (Moore River, Buller Reserve, and Gull Rock) (Figs. 1 and 2A). The sites were dominated by Banksia woodlands with B. attenuVol. 93, No. 6, 2003 695

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