| This dissertation consists of three main subproject manuscripts. In manuscript 1, preliminary molecular phylogenies of the Marchantiales are presented. The marchantioid sample includes 10 carpocephalate taxa and 24 acarpocephalate taxa (emphasizing Riccia), Monoclea, Sphaerocarpos, and Riella. Three Metzgeriales (Fossombronia, Pellia and Blasia), the hornwort Anthoceros, four mosses and Coleochaete are also sampled. Cladistic analyses are based on three culled nucleotide sequence alignments: (1) partial nuclear-encoded Large Subunit rDNA; (2) the plastid-encoded trnL-region and (3) combined data. Relative rate tests reveal significant heterogeneity in the nuclear LSU rDNA data. Lunularia positions as the most basal of sampled Marchantiopsida; Sphaerocarpales, Marchantia and Corsinia represent early diverging lines. Monophyletic Aytoniaceae, Cleveaceae and Riccia are indicated. Topologies imply that extant acarpocephalate taxa are derived from carpocephalate forms. Monoclea positions well within Marchantiales sensu stricto. A well-supported long branch unites all sampled Marchantiopsida and isolates this clade from other liverworts and bryophytes. An unresolved marchantioid polytomy follows the well-supported basal nodes. This polytomy may correspond to an explosive radiation of taxa coincident with extreme conditions and ecological reorganizations of the Permo-Triassic. In manuscript 2, focused analyses of Riccia are presented. Nuclear, plastid and combined data strict consensus topologies based on 17-18 species of Riccia (representing 5/8 of subgenera) are largely congruent with respect to terminal groups; basal resolution is poor, the possible signature of an explosive initial species radiation during the Permo-Triassic. Unexpected placement of several taxa is well-supported suggesting a propensity in Riccia for volatile morphology not reflected in the underlying genetic history. In manuscript 3, an alternative hypothesis is articulated to explain the origin of a marchantialean complex thallus from a Sphaerocarpos- or Geothallus-like model. The complex thallus is envisioned to have originated from a transitional form with a highly regularized, bilaterally-symmetrical reticulum of fused dorsal lappets. This lappet-modular hypothesis is largely derived from the concepts of Burgeff (1943, Verlag von Gustav Fischer, Jena) and Doyle (1962, University of California Publications in Botany 33:185-368) and attempts to reconcile the novel observations of both workers. |
