Leaf Ontogeny and Ontology

The evolution and development of leaves have been debated for more than a century. There is general agreement that the leaves of lycophytes (microphylls) and the leaves of ferns and seed plants (megaphylls) evolved independently. However there is little agreement on the number of times megaphylls evolved particularly within the ferns. In addition, there are several theories to describe the evolution of leaves in vascular plants. This has resulted in additional terminology to describe leaf-like organs. Developmental genomic studies hold much promise to move the debate on leaf evolution. We analyzed the Class III HD Zip genes from across the ferns and lycophytes and identified the duplication and gene loss events of this gene family in vascular plants. We also analyzed the expression of these genes in key fern and lycophyte species. Osmunda regalis, Psilotum nudum and Selaginella moellendorffii. We found that there is a conservation in a leaf developmental mechanism between ferns and seed plants.  In addition, our expression analyses in the problematic fern species, Psilotum nudum, provide support for the interpretation of these structures as leaves as opposed to enations. Finally, our expression analyses indicate that the ancestral role of Class III HD Zip genes was in sporangia initiation and development and that these key regulators were co-opted into leaf development differently in lycophytes and euphyllophytes. These results provide a catalyst to shift the debate on leaf evolution and evolution and development and will be discussed in the context of plant ontologies.