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Microsatellite and Minisatellite Analysis of Leptosphaeria maculans in Australia Reveals Regional Genetic Differentiation

The population genetic structure of the fungal pathogen Leptosphaeria maculans was determined in Australia using six microsatellite and two minisatellite markers. Ascospores were sampled from Brassica napus stubble in disease nurseries and commercial fields in different sites over 2 years. The 13 subpopulations of L. maculans exhibited high gene (H = 0.393 to 0.563) and genotypic diversity, with 357 haplotypes identified among 513 isolates. Although the majority of genetic variation was distributed within subpopulations (85%), 10% occurred between the regions of eastern and Western Australia, and 5% within regions. F_ST analysis of subpopulation pairs also showed the east-west genetic differentiation, whereas factorial correspondence analysis separated Western Australian subpopulations from eastern ones. Bayesian model-based population structure analyses of multilocus haplotypes inferred three distinct populations, one in Western Australia and an admixture of two in eastern Australia. These two regions are separated by 1,200 km of arid desert that may act as a natural barrier to gene flow, resulting in differentiation by random genetic drift. The genetic differentiation of L. maculans isolates between eastern and Western Australia means that these regions can be treated as different management units, and reinforces the need for widespread disease nurseries in each region to screen breeding lines against a range of genetic and pathogenic populations of L. maculans.