A diverse population of introns in the nuclear ribosomal genes off ericoidmycorrhizal fungi includes elements with sequence similarity to endonuclease-coding genes

Ericoid mycorrhizal fungi form symbioses with the roots of members of the E ricales. Although only two genera have been identified in culture, the taxo nomic diversity of ericoid symbionts is certainly wider. Genetic variation among 40 ericoid fungal isolates was investigated in this study. PCR amplif ication of the nuclear small-subunit ribosomal DNA (SSU rDNA) and of the in ternal transcribed spacer (ITS), followed by sequencing, led to the discove ry of DNA insertions of various sizes in the SSU rDNA of most isolates. The y reached sizes of almost 1,800 bp and occurred in up to live different ins ertion sites. Their positions and sizes were generally correlated with morp hological and ITS-RFLP grouping of the isolates, although some insertions w ere found to be optional among isolates of the same species, and insertions were not always present in all SSU rDNA repeats within an isolate. Most in sertions were identified as typical group I introns, possessing the conserv ed motifs characteristic of this group. However, other insertions lack thes e motifs and form a distinct group that includes other fungal ribosomal int rons. Alignments with almost 70 additional sequences from fungal nuclear SS U rDNA introns indicate that introns inserted at the same site along the rD NA gene are generally homologous, but they also suggest the possibility of some horizontal transfers. Two of the ericoid fungal introns showed strong homology with a conserved motif found in endonuclease genes from nuclear rD NA introns.