Evidence For Lateral Transfer Of An Ie Intron Between Fungal And Red Algal Small Subunit Rrna Genes

A previous study of the North American biogeography of the red algal genus Hildenbrandia noted the presence of group I introns in the nuclear small subunit (SSU) rRNA gene of the marine species H. rubra (Sommerf.) Menegh. Group IC1 introns have been previously reported at positions 516 and 1506 in the nuclear SSU RNA genes in the Bangiales and Hildenbrandiales. However, the presence of an unclassified intron at position 989 in a collection of H. rubra from British Columbia was noted. This intron is a member of the IE subclass and is the first report of this intron type in the red algae. Phylogenetic analyses of the intron sequences revealed a close relationship between this IE intron inserted at position 989 and similar fungal IE introns in positions 989 and 1199. The 989 IE introns formed a moderately to well-supported clade, whereas the 1199 IE introns are weakly supported. Unique structural helices in the P13 domain of the 989 and 1199 IE introns also point to a close relationship between these two clades and provide further evidence for the value of secondary structural characteristics in identifying homologous introns in evolutionarily divergent organisms. The absence of the 989 IE intron in all other red algal nuclear SSU rRNA genes suggests that it is unlikely that this intron was vertically inherited from the common ancestor of the red algal and fungal lineages but rather is the result of lateral transfer between fungal and red algal nuclear SSU rRNA genes.