Genome, Phenome, Environment, and Evolution of Land Plants
Gerrienne, Philippe , Gonez, Paul .
The early evolution of life cycles in Embryophytes: what do fossils tell us?
Embryophytes (land plants) are distinguished from their green algal ancestors by diplobiontic life cycles, i.e. alternation of multicellular gametophytic and sporophytic generations. The Bryophyte sporophyte is small and matrotrophic on the dominant gametophyte; extant vascular plants have an independent, dominant sporophyte and a reduced gametophyte. The shifts from haplobiontic to diplobiontic life cycles and from gametophytic to sporophytic dominance are most probably related with terrestrial habitats. The elaboration of the diplobiotic life cycle in embryophytes has been thoroughly discussed within the context of the Antithetic and the Homologous Theories. The Antithetic Theory proposes a green algal ancestor with a gametophyte-dominant haplobiontic life cycle, the meiosis occurring after a number of mitotic divisions of the zygote. The Homologous Theory suggests a green algal ancestor with alternation of isomorphic generations, with a gradual reduction of the gametophyte and, parallely, an increasing complexity of the sporophyte. Cladistic studies strongly support the Antithetic Theory in repeatedly identifying charophycean green algae as the closest relatives of land plants. In recent years, exceptionally well-preserved gametophytes have been described from the Rhynie Chert (Lower Devonian, 410 million years ago). Those gametophytes consist of erect axes with archegonia or antheridia, generally borne within distal gametangiophores. Some are tracheophytes. This fossil evidence supports the Homologous Theory. Correspondence with four sporophytes has been proposed, and the complete life cycles of those plants have now been reconstructed. All demonstrate an alternation of more or less isomorphic generations. On this basis, it is today currently admitted that alternation of isomorphic generations is the plesiomorphic condition among all early land plants, including basal tracheophytes. Here we review the existing evidence for early land plant gametophytes. We also discuss some recently discovered plants preserved as compression-fossils and interpreted as gametophytes. Finally, we suggest an alternative scenario for the early evolution of embryophyte life cycles.
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1 - University of Liège, Geology, Paleobotany, Paleopalynology & Micropaleontology, Bat. B18 Sart Tilman, Liege, 4000, Belgium
Presentation Type: Symposium or Colloquium Presentation
Location: Ballroom 2/Cliff Lodge - Level B
Date: Monday, July 27th, 2009
Time: 4:15 PM