DNA from 124 strains of Heterobasidion annosum was amplified using the core sequence of the M13 minisatellite region as primer. The strains were derived from 11 S-, 8 P- and 1 F-intersterility group populations originating in Scandinavia, Germany and Italy. Following electrophoresis the banding patterns of all isolates were compared and 23 fragments were scored. Average band-sharing indices (ABSI) were used to compare genetic diversity within and between populations. Genetic similarity of populations decreased with increasing geographical distance in the S group. In both the S and P intersterility groups, ABSI values were higher within than among populations (70.3 +/- 2.3 S.D. and 66.4 +/- 2.2, respectively in the S group and 77.0 +/- 2.3 and 75.0 +/- 2.4, respectively in the P group) indicating local differentiation. The higher values in the P than in the S group indicate that the P group is less variable. ABSI was much lower between S and P group populations (49.2 +/- 2.5) than within either group. This result demonstrates the large divergence between these two intersterility groups. In the F population ABSI was higher (68 +/- 3.1) than for the comparisons with the S- (50.1 +/- 1.6) and P-groups (44.0 +/- 2.4). By using discriminant analysis, the agreement between geographical population affiliation and that inferred from banding patterns of amplified DNA was 20.8% within the S group, and 14.9% within the P group. Chi-square test of the discriminant analysis indicated regional differentiation in the S but not in the P intersterility group. The agreement between methods was 98.4% when strains were classified to intersterility group by pairing tests or amplified DNA profiles. Results are consistent with a high degree of exchange between populations of the same intersterility group but low exchange between intersterility groups.
