Phylogenetic analysis of LSU and SSU r DNA group I introns of lichen photobionts associated with the genera X anthoria and X anthomendoza ( T eloschistaceae, lichenized A scomycetes)

We studied group I introns in sterile cultures of selected groups of lichen photobionts, focusing on T rebouxia species associated with X anthoria s. lat. (including X anthomendoza spp.; lichen‐forming ascomycetes). Group I introns were found inserted after position 798 ( E scherichia coli numbering) in the large subunit ( LSU ) r RNA in representatives of the green algal genera T rebouxia and A sterochloris . The 798 intron was found in about 25% of X anthoria photobionts including several reference strains obtained from algal culture collections. An alignment of LSU ‐encoded r DNA intron sequences revealed high similarity of these sequences allowing their phylogenetic analysis. The 798 group I intron phylogeny was largely congruent with a phylogeny of the internal transcribed spacer region, indicating that the insertion of the intron most likely occurred in the common ancestor of the genera T rebouxia and A sterochloris . The intron was vertically inherited in some taxa, but lost in others. The high‐sequence similarity of this intron to one found in C hlorella angustoellipsoidea suggests that the 798 intron was either present in the common ancestor of T rebouxiophyceae, or that its present distribution results from more recent horizontal transfers, followed by vertical inheritance and loss. Analysis of another group I intron shared by these photobionts at small subunit position 1512 supports the hypothesis of repeated lateral transfers of this intron among some taxa, but loss among others. Our data confirm that the history of group I introns is characterized by repeated horizontal transfers, and suggests that some of these introns have ancient origins within C hlorophyta.