Regional and local population structure of the pioneer wood-decay fungus Trichaptum abietinum.

The population structure of 11 Fennoscandian geographic populations of the pioneer wood-decay basidiomycete Trichaptum abietinum was assessed with PCR-RFLPs, intersequence simple repeats (ISSRs) and mating studies. The three codominant PCR-RFLP markers (1) internal transcribed spacer 2 (nrDNA), (2) glyceraldehyde-3-phosphate dehydrogenase and (3) translation elongation factor 1α showed that genotype distributions in most cases (94%) agreed with Hardy-Weinberg expectations and that random association of alleles occurred across loci. The molecular data suggest that T. abietinum is a highly outcrossing fungus that regularly proliferates and spreads by sexual spores. Interstock mating reactions suggest a high number of mating factors among individuals and that biological barriers to gene flow are nonexistent in the region. The three PCR-RFLP loci gave an overall F(ST) = 0.03, indicating a low level of genetic differentiation and presumably high gene flow among the geographic populations. The ISSR markers revealed no systematic substructuring and the among-population variance component was low (6.1%) in AMOVA. However, all PCR-RFLP and most ISSR markers (7/12) showed significant deviation from the null hypothesis of an even distribution of allele frequencies across the 11 geographic populations. Allele frequencies varied in an apparently random manner, suggesting that genetic drift might be an important structuring factor in T. abietinum. The spatial small-scale distribution of heterokaryons on three selected substrate units (logs) showed that most isolates represented discrete individuals and that a number of genets (19) may occupy a single log. The small-scale genotype distributions (within logs) were in agreement with panmictic Hardy-Weinberg expectations.