Molecular Evolution of the trnTUGU‐trnFGAA Region in Bryophytes

Abstract: Structure, variability, and molecular evolution of the trnT‐F region in the Bryophyta (mosses and liverworts) is analyzed based on about 200 sequences of the trnT‐L spacer and trnL 5′ exon, 1000 sequences of the trnL intron, and 800 sequences of the trnL 3′ exon and trnL‐F spacer, including comparisons of lengths, GC contents, sequence similarities, and functional elements. Mutations occurring in the trnL 5′ and 3′ exons, including compensatory base pair changes, and a transition in the trnL anticodon in Takakia lepidozioides, are discussed. All three non‐coding regions display a mosaic structure of highly variable elements (V1 ‐ V3 in the trnT‐L spacer, V4/V5 corresponding to stem‐loop regions P6/P8 in the trnL intron, and V6/V7 in the trnL‐F spacer) and more conserved elements. In the trnL intron this structure is a consequence of the defined secondary structure necessary for correct splicing, whereas in both spacers conserved regions are restricted to promoter elements. At least the highly variable regions in the trnT‐L spacer and stem‐loop region P8 of the trnL intron seem to evolve independently in the major bryophyte lineages and are therefore not suitable for high taxonomic level phylogenetic reconstructions. In mosses, a trend of length reduction towards the more derived lineages is observed in all three non‐coding regions. GC contents are mostly linked to sequence variability, with the conserved regions being more GC rich and the more variable AT rich. The lowest GC values (< 10 %) are found in the trnT‐L spacer of mosses. In addition to two putative sigma70‐type promoters in the trnT‐L spacer, a third putative promoter is present in the trnL‐F spacer, although trnL and trnF are assumed to be co‐transcribed. Consensus sequences are provided for the ‐35 and ‐10 sequences of the major bryophyte lineages. The third promoter is part of a hairpin secondary structure, whose loop region is highly homoplastic in mosses due to an inversion occurring independently in non‐related taxa, even at the intraspecific level.