Distribution and evolution of variable group-I introns in the small ribosomal subunit of North Atlantic Porphyra (Bangiales, Rhodophyta)

Within the Bangialean red algae the multi-copy small subunit nuclear ribosomal gene (SSU) has been shown to contain group-I introns at positions corresponding to 516 and 1506. Using the polymerase chain reaction (PCR) with primers flanking the 1506 intron we found that this intron was present in some copies of the rDNA in several North Atlantic Porphyra species. However, when the combination of a flanking and internal primer was used to amplify across the intron the percentages of individuals containing the intron increased. The likely explanation for these results was that, when flanking primers were used for PCR, template competition favoured the amplification of the intronless rDNA fragments. A combination of internal and flanking primers used to screen for the 516 intron amplified the intron in every accession of five taxa (Porphyra dioica, P. leucosticta, P. suborbiculata, P. umbilicalis and P. yezoensis) and in most samples of P. amplissima, P. linearis, and P. purpurea. When PCR primers were optimized for the 1506 intron and SSU of P. umbilicalis, the intron was detected in 28/28 samples. Such observations suggest that the 1506 intron is ubiquitous in Porphyra and is present in some, but not all, copies of the ribosomal DNA (rDNA) repeat. Multiple size 1506 introns were detected in individuals of P. umbilicalis. Four of the larger size variants were shown to encode a putative His-Cys box that was associated with mobility of other group-I introns. Our observations suggest that the 1506 group-I intron may still be mobile within the Porphyra lineage.