Position of multiple insertions in SSU rDNA of lichen-forming fungi. Molecular Biology and Evolution 12

Lichen-forming fungi. in symbiotic associations with algae. frequently have nuclear small subunit ribosomal DNA (SSU rONA) longer than the 1.800 nucleotides typical for eukaryotes. The lichen-forming ascomycetous fungus Lecanora clic\persa contains insertions at eight distinct positions of its SSU rONA: the lichen-forming fungi Calicililn tricolor and Porpidia al/stlliala each contain one insertion. Inseliions are not limited to fungi that form lichens: the lichen ally Alycocalicililn alboniRntm also contains two insertions. Of the II insertion positions now reported for lichen-forming fungi and this ally. 6 positions are known only from lichen-forming fungi. Including the 4 newly reported in this study, insertions are now known from at least 17 positions among all reported SSU rONA sequences. Insertions. most of which are Group I introns, are reported in fungal and protistan lineages and occur at corresponding positions in genomes as phylogenetica11y distant as the nuclei of fungi, green algae. and red algae. Many of these positions are exposed in the mature rRNA tertiary structure and may be subject to independent insertion ofintrons. Insertion of introns, accompanied by their sporadic loss, accounts for the scattered distribution of insertions observed within the SSU rONA of these diverse organisms