Molecular Systematic Studies Of Suillus And Related Taxa Of The Boletaceae Using Mitochondrial Dna.

Abstract

Mapping studies were performed to evaluate the mode, tempo, and phylogenetic significance of mitochondrial DNA (mtDNA) evolution in 15 species of Suillus and five species from four related genera of the Boletaceae (Basidiomycotina, Boletales). Comparisons within species of Suillus and between two closely related species of the genus revealed that insertions and deletions are the major form of genomic change, whereas most restriction sites are conserved. Among the 15 species of Suillus, mtDNAs vary in size from 36 to 121 kb, differ in sequence order by only one major rearrangement, and have diverged in nucleotide sequence within the large subunit ribosomal RNA gene region by a maximum of 2.9%. Three additional mtDNA fragment orders exist in three of related genera examined. Two of the three orders can be transformed into the predominant Suillus order by either one or two single step rearrangements. The fourth requires two or three rearrangements to be converted to any of the others. The minimum estimates of nucleotide divergence within the large subunit ribosomal RNA gene region vary from 8.3% to 11% in comparisons between Suillus and these related species. We conducted cladistic analyses of restriction-site and size differences within the mitochondrial ribosomal RNA gene regions. The results are highly congruent with the hypothesized sequence of mtDNA rearrangements and provide evidence for an expansion of the mitochondrial genome within Suillus. Structural and sequence changes in mtDNA provide phylogenetically informative characters that allow us to resolve several systematic controversies within the Boletaceae. A surprisingly close relationship between the false truffle Rhizopogon subcaerulescens (Hymenogastrales) and Suillus was revealed. Both taxa share identical fragment orders within their mitochondrial genomes and also share most restriction sites in their mitochondrial ribosomal RNA genes. Significant variation exists with respect to both features when Suillus and Rhizopogon are compared to three related genera of the Boletaceae. This finding demonstrates an acceleration of morphological divergence during the evolution of R. subcaerulescens from Suillus.