Molecular phylogenies of the marchantioid liverwort genus Riccia were reconstructed using independent nuclear and plastid data sets in the hope of better understanding relative age, relationships and character evolution in this taxonomically puzzling and putatively ancient radiation. Cladistic analyses are based on three nucleotide sequence alignments: 1) partial nuclear-encoded Large Subunit rDNA (LSU rDNA), 2) the plastid-encoded trnL-F region and 3) a data set consisting of combined nuclear and plastid alignments. Alignment ambiguous regions of each alignment were culled. Independently-derived strict consensus topologies based on 17-18 species of Riccia representing 5/8 of subgenera and 13/19 of sections (sensu Schuster 1992) are largely congruent; combining the data results in a strict consensus tree with even higher overall bootstrap and decay support. A comparison of the nuclear and plastid trees reveals that five specific clades are common to both; moreover, in the combined strict consensus topology, six clades are indicated with bootstrap support greater than 75%. All analyses (nuclear, plastid and combined) affirm the biological reality of a monophyletic Riccia (rooted on outgroups Ricciocarpos and Oxymitra). Striking morphological divergence within well-supported terminal clades, suggests a propensity in Riccia for volatile morphology that is not reflected in the underlying genetic history; characters emphasized in prior systematic treatments are apparently unstable and therefore perhaps largely unreliable for the purpose of discriminating phylogenetically meaningful higher-level intrageneric taxa.

Key words: character evolution, liverworts, Marchantiales, Riccia, volatile morphology