Molecular circumscription of the hornworts (Anthocerotophyta) based on the chloroplast DNA trnL-trnF region.

In phylogenetic trees generated from partial trnL(UAA) intron sequences, the hornworts (represented by nine species from the genera Anthoceros, Dendroceros, Megaceros, Notothylas and Phaeoceros) are resolved as a monophyletic group and are separated from the clades of mosses, liverworts and tracheophytes. A secondary structure of the trnL(UAA) intron of Anthoceros agrestis is presented, displaying the arrangement of the stem-loop regions P1-P9. Compensatory base-pair changes (coevolutionary sites) are detected in regions P4/5 and P9 within the hornwort sequences. The original homology of the most variable region, P8, cannot be detected anymore due to the extremely fast divergent evolution of this segment in the major land plant groups. Similarly, a high sequence divergence occurs in the trnL-trnF intergenic spacer. Apart from synapomorphic substitutions in the trnL(UAA) intron, the hornworts are characterised by a large P6 region consisting of many repetitive elements. The molecular data therefore support the hornworts as representing an independent land plant lineage (Anthocerotophyta). Although relationships between hornworts and the other land plant groups remain unresolved in the trnL(UAA) intron trees, it is rather unlikely that bryophytes are monophyletic in their traditional circumscription, i.e. comprising hornworts, mosses and liverworts.