Evolutionary Developmental Biology (Evo-Devo)
Tomescu, Alexandru MF .
Microphylls, megaphylls, and the evolution of leaf development.
Leaves of vascular plants share four defining features (vascularization, determinacy, bilateral symmetry, definite arrangement), yet they are not homologous. The historical distinction between microphylls and megaphylls, originally linked to the major phylogenetic divide between zosterophyllophytes (lycophytes) and trimerophytes (euphyllophytes), is not free of ambiguities. Definitions contrasting microphylls and megaphylls as mutually exclusive morphological concepts are inconsistent, highlighting a disconnection between morphology and phylogeny. Although modern phylogenies support the lycophyte-euphyllophyte divide, they have not provided unequivocal answers on precursor structures and evolutionary processes that generated the lycophyte leaf, and are supporting conflicting hypotheses of euphyllophyte phylogeny which nevertheless indicate several independent megaphyll origins. Proposed ‘partial homology’ of megaphylls at the level of precursor structures (branching systems of undifferentiated axes) does not imply homology of the different types of ‘megaphyllous’ leaves, and lacks unequivocal support at the level of developmental mechanisms. If they are to be retained, the microphyll and megaphyll concepts need to be redefined or they are bound to perpetuate confusion. Integration of phylogenetic and developmental data is expected to illuminate these issues. Developmental anatomy of extant euphyllophytes indicates megaphyll homoplasy. The number and origin of cells recruited into leaf primordia need further exploration as features potentially congruent with the lycophyte-euphyllophyte dichotomy. Molecular mechanisms controlling leaf development include: KNOX-ARP interactions, shared among polysporangiophytes, but modulated differently between lineages, even across euphyllophytes; HD-ZIPIII genes which appear to have different functions in lycophytes and euphyllophytes; YABBY genes involved in leaf development in seed plants but apparently not in other ‘megaphyllous’ euphyllophytes. Studies of leaf development regulation in key extant taxa (homosporous lycophytes, eusporangiate ferns, Equisetum, Psilotum) are needed to bring resolution to the evolution of leaf development, and so are studies of fossils that could identify morpho-anatomical fingerprints of developmental regulatory mechanisms in extinct lineages for which molecular data are unavailable.
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1 - Humboldt State University, Department of Biological Sciences, Arcata, California, 95521, USA
Presentation Type: Oral Paper:Papers for Topics
Date: Wednesday, July 30th, 2008
Time: 10:00 AM