Vegetative and reproductive innovations of early land plants: implicationsfor a unified phylogeny

As the oldest extant lineages of land plants, bryophytes provide a living l aboratory in which to evaluate morphological adaptations associated with ea rly land existence. In this paper we examine reproductive and structural in novations in the gametophyte and sporophyte generations of hornworts, liver worts, mosses and basal pteridophytes. Reproductive features relating to sp ermatogenesis and the architecture of motile male gametes are overviewed an d evaluated from an evolutionary perspective. Phylogenetic analyses of a da ta set derived from spermatogenesis and one derived from comprehensive morp hogenetic data are compared with a molecular analysis of nuclear and mitoch ondrial small subunit rDNA sequences. Although relatively small because of a reliance on water for sexual reprodu ction, gametophytes of bryophytes are the most elaborate of those produced by any land plant. Phenotypic variability in gametophytic habit ranges from leafy to thalloid forms with the greatest diversity exhibited by hepatics. Appendages, including leaves, slime papillae and hairs, predominate in liv erworts and mosses, while hornwort gametophytes are strictly thalloid with no organized external structures. Internalization of reproductive and veget ative structures within mucilage-filled spaces is an adaptive strategy exhi bited by hornworts. The formative stages of gametangial development are sim ilar in the three bryophyte groups, with the exception that in mosses apica l growth is intercalated into early organogenesis, a feature echoed in moss sporophyte ontogeny. A monosporangiate, unbranched sporophyte typifies bryophytes, but developme ntal and structural innovations suggest the three bryophyte groups diverged prior to elaboration of this generation. Sporophyte morphogenesis in hornw orts involves non-synchronized sporogenesis and the continued elongation of the single sporangium, features unique among archegoniates. In hepatics, e longation of the sporophyte seta and archegoniophore is rapid and requires instantaneous wall expandability and hydrostatic support. Unicellular, spir alled elaters and capsule dehiscence through the formation of four regular valves are autapomorphies of liverworts. Sporophytic sophistications in the moss clade include conducting tissue, stomata, an assimilative layer and a n elaborate peristome for extended spore dispersal. Characters such as stom ata and conducting cells that are shared among sporophytes of mosses, hornw orts and pteridophytes are interpreted as parallelisms and not homologies. Our phylogenetic analysis of three different data sets is the most comprehe nsive to date and points to a single phylogenetic solution for the evolutio n of basal embryophytes. Hornworts are supported as the earliest divergent embryophyte clade with a moss/liverwort clade sister to tracheophytes. Amon g pteridophytes, lycophytes are monophyletic and an assemblage containing f erns, Equisetum and psilophytes is sister to seed plants. Congruence betwee n morphological and molecular hypotheses indicates that these data sets are tracking the same phylogenetic signal and reinforces our phylogenetic conc lusions. It appears that total evidence approaches are valuable in resolvin g ancient radiations such as those characterizing the evolution of early em bryophytes. More information on land plant phylogeny can be found at: http: //www.science.siu.edu/landplants/index.html.