PHYLOGENETIC EVIDENCE FOR HORIZONTAL TRANSMISSION OF GROUP-I INTRONS IN THE NUCLEAR RIBOSOMAL DNA OF MUSHROOM-FORMING FUNGI

Group I introns were discovered inserted at the same position in the n uclear small-subunit ribosomal DNA (nuc-ssu-rDNA) in several species o f homobasidiomycetes (mushroom-forming fungi). Based on conserved intr on sequences, a pair of intron-specific primers was designed for PCR a mplification and sequencing of intron-containing rDNA repeats. Using t he intron-specific primers together with flanking rDNA primers, a PCR assay was conducted to determine presence or absence of introns in 39 species of homobasidiomycetes. Introns were confined to the genera Pan ellus, Clavicorona, and Lentinellus. Phylogenetic analyses of nuc-ssu- rDNA and mitochondrial ssu-rDNA sequences suggest that Clavicorona and Lentinellus are closely related, but that Panellus is not closely rel ated to these. The simplest explanation for the distribution of the in trons is that they have been twice independently gained via horizontal transmission, once on the lineage leading to Panellus, and once on th e lineage leading to Lentinellus and Clavicorona. BLAST searches using the introns from Panellus and Lentinellus as query sequences retrieve d 16 other similar group I introns of nuc-ssu-rDNA and nuclear large-s ubunit rDNA (nuc-lsu-rDNA) from fungal and green algal hosts. Phylogen etic analyses of intron sequences suggest that the mushroom introns ar e monophyletic, and are nested within a clade that contains four other introns that insert at the same position as the mushroom introns, two from different groups of fungi and two from green algae. The distribu tion of host lineages and insertion sites among the introns suggests t hat horizontal and vertical transmission, homing, and transposition ha ve been factors in intron evolution. As distinctive, heritable feature s of nuclear rDNAs in certain lineages, group I introns have promise a s phylogenetic markers. Nevertheless, the possibility of horizontal tr ansmission and homing also suggest that their use poses certain pitfal ls.