Vertical evolution and intragenic spread of lichen-fungal group I introns

One family within the Euascomycetes (Ascomycota), the lichen-forming Physciaceae, is particularly rich in nuclear ribosomal [r]DNA group I introns. We used phylogenetic analyses of group I introns and lichen-fungal host cells to address four questions about group I intron evolution in lichens, and generally in all eukaryotes: 1) Is intron spread in the lichens associated with the intimate association of the fungal and photosynthetic cells that make up the lichen thallus? 2) Are the multiple group I introns in the lichen-fungi of independent origins, or have existing introns spread into novel sites in the rDNA? 3) If introns have moved to novel sites, then does the exon context of these sites provide insights into the mechanism of intron spread? and 4) What is the pattern of intron loss in the small subunit rDNA gene of lichen-fungi? Our analyses show that group I introns in the lichen-fungi and in the lichen-algae (and lichenized cyanobacteria) do not share a close evolutionary relationship, suggesting that these introns do not move between the symbionts. Many group I introns appear to have originated in the common ancestor of the Lecanorales, whereas others have spread within this lineage (particularly in the Physciaceae) putatively through reverse-splicing into novel rRNA sites. We suggest that the evolutionary history of most lichen-fungal group I introns is characterized by rare gains followed by extensive losses in descendants, resulting in a sporadic intron distribution. Detailed phylogenetic analyses of the introns and host cells are required, therefore, to distinguish this scenario from the alternative hypothesis of widespread and independent intron gains in the different lichen-fungal lineages.