CHROMOSOME SIZE POLYMORPHISM IN LEPTOSPHAERIA-MACULANS

Chromosome size polymorphism among nine isolates of Leptosphaeria macu lans was studied by hybridization of homologous and heterologous DNA f ragments to chromosomes separated by contour-clamped homogeneous elect ric field (CHEF) electrophoresis. Four of the fungal isolates were hig hly virulent and three were weakly virulent in Brassica plants; the re maining two originated from the cruciferous weed Thlaspi arvense. Ther e was a high degree of chromosome size polymorphism, and no two isolat es had the same karyotype. However, three general patterns could be di stinguished by differences in size range, distribution of bands within the size ranges, and hybridization to probes. These three banding pat terns corresponded to the three pathogenicity groups. The four highly virulent isolates had 12-14 chromosomal bands with estimated sizes ran ging 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 isolat es 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 chrom osome. The weakly virulent and Thlaspi isolates had a higher proportio n of small chromosomes, while those of the highly virulent isolates we re more evenly distributed throughout the size range. Most of the DNA fragments used as probes hybridized to a single chromosome in each iso late and to chromosomes of similar sizes (+/- 0.5 Mb) within a pathoge nicity group, but often to a chromosome with a very different size in isolates from the other pathogenicity groups. Hybridization to many ch romosomes by DNA fragments cloned from one of the highly virulent isol ates indicated the presence of repetitive sequences specific for highl y virulent isolates. The results indicate that these pathogenicity gro ups are very different from each other and perhaps different species.