Distribution of Euascomycetes spliceosomal introns on a conservation diagram of fungal SSU rRNA overlaid on a secondary structure model of the Saccharomyces cerevisiae SSU rRNA. Spliceosomal introns are shown in large text with arrows denoting their positions. Positions with nucleotides in more than 95% of the 1042 sequences that were studied are shown as following: upper case, conserved at ≥ 95%, lower case, conserved at 90–94%, filled circle, conserved at 80–89%, and open circle, conserved at < 80%. Other regions are denoted as arcs. The numbers at the arcs show the upper and lower number of nucleotides that are found in these variable regions. The boxed regions are G-rich sequences upstream of intron insertion sites. Boxed filled circles indicate that the most frequent nucleotide at this site was a G in our alignment of 1434 fungal rRNAs that included both intron-containing and intron-less taxa.