Allozyme differentiation of intersterility groups of Heterobasidion annosum isolated from conifers in the western United States
Allozyme analysis was conducted on 64 isolates from basidiocarps of Heterobasidion annosum. The isolates belonged to the "S" and "P" intersterility groups and were collected from five conifer species found in the western United States. Ten allozyme loci distributed among eight enzyme systems were examined. Intersterility groups differed at nine loci. Few alleles were common to both both intersterility groups, and only one locus, MDH-1, was monomorphic. Several loci, including MDH-2, GDH, and PGI, were diagnostic for the S or P intersterility groups, suggesting fixation for alternative or ' alleles.Distribution of alleles between intersterility groups indicates an extremely high degree of genetic divergence; Nei's genetic distance was equal to 0.926. Isolates from diseased pine trees had only alleles consistent with those of P group, and isolates from diseased true fir had S group alleles, indicating a complete association among intersterility groups, allozyme profiles, and host species. These data support a hypothesis of genetic isolation between intersterility groups in nature. Allozyme genotypes provide a rapid test for identifying intersterility groups of the fungus in North America.