CHROMOSOME SIZE POLYMORPHISM IN LEPTOSPHAERIA-MACULANS

Chromosome size polymorphism among nine isolates of Leptosphaeria maculans was studied by hybridization of homologous and heterologous DNA fragments to chromosomes separated by contour-clamped homogeneous electric field (CHEF) electrophoresis. Four of the fungal isolates were highly virulent and three were weakly virulent in Brassica plants; the remaining two originated from the cruciferous weed Thlaspi arvense. There was a high degree of chromosome size polymorphism, and no two isolates had the same karyotype. However, three general patterns could be distinguished by differences in size range, distribution of bands within the size ranges, and hybridization to probes. These three banding patterns corresponded to the three pathogenicity groups. The four highly virulent isolates had 12-14 chromosomal bands with estimated sizes ranging from 0.7 to 3.7 Mb. The three weakly virulent isolates had 11 or 12 bands with sizes between 0.8 and 2.7 Mb, and the two Thlaspi isolates had 14 and 16 bands with sizes between 1.0 and 3.2 Mb. Some of the bands were brighter than average and may represent more than one chromosome. The weakly virulent and Thlaspi isolates had a higher proportion of small chromosomes, while those of the highly virulent isolates were more evenly distributed throughout the size range. Most of the DNA fragments used as probes hybridized to a single chromosome in each isolate and to chromosomes of similar sizes (+/- 0.5 Mb) within a pathogenicity group, but often to a chromosome with a very different size in isolates from the other pathogenicity groups. Hybridization to many chromosomes by DNA fragments cloned from one of the highly virulent isolates indicated the presence of repetitive sequences specific for highly virulent isolates. The results indicate that these pathogenicity groups are very different from each other and perhaps different species.