Variable sizes of introns in the SSU rDNA in three species of Roccella (Arthoniales, Euascomycetes)

Three species of the lichenized fungus Roccella (Arthoniales, Euascomycetes), R. canariensis, R. tuberculata and R. montagnei, were found to possess insertions in the first half of the nuclear SSU rDNA. Both the number and the type of these insertions varied between and within species. At position 516 the insertions belonged to group-IC1 introns with a characteristic secondary structure and conserved sequences. At three positions, 300, 330 and 393, the insertions were short and lacked the characteristic features of group-I introns. The splicing-site junctions of these short introns resembled those of mRNA introns. Furthermore, the conserved sequence flanking the branch-point A of mRNA introns was also present. Extraction of RNA and sequencing of cDNA confirmed that the insertions from positions 300, 330 and 516 had been spliced from the pre-rRNA during maturation. At position 393 the insertion had been spliced from R. canariensis, whereas in R. montagnei the insertion was still present in the rRNA.