Abstract
Background and AimsThe gymnosperm order Cycadales is pivotal to our understanding of seed-plant phylogeny because of its phylogenetic placement close to the root node of extant spermatophytes and its combination of both derived and plesiomorphic character states. Although widely considered a ‘living fossil’ group, extant cycads display a high degree of morphological and anatomical variation. We investigate stomatal development in Zamiaceae to evaluate variation within the order and homologies between cycads and other seed plants.MethodsLeaflets of seven species across five genera representing all major clades of Zamiaceae were examined at various stages of development using light microscopy and confocal microscopy.Key ResultsAll genera examined have lateral subsidiary cells of perigenous origin that differ from other pavement cells in mature leaflets and could have a role in stomatal physiology. Early epidermal patterning in a ‘quartet’ arrangement occurs in Ceratozamia, Zamia and Stangeria. Distal encircling cells, which are sclerified at maturity, are present in all genera except Bowenia, which shows relatively rapid elongation and differentiation of the pavement cells during leaflet development.ConclusionsStomatal structure and development in Zamiaceae highlights some traits that are plesiomorphic in seed plants, including the presence of perigenous encircling subsidiary cells, and reveals a clear difference between the developmental trajectories of cycads and Bennettitales. Our study also shows an unexpected degree of variation among subclades in the family, potentially linked to differences in leaflet development and suggesting convergent evolution in cycads.
Highlights
The extant gymnosperms, though relatively species-poor compared with angiosperms, display a high degree of morphological disparity that reflects a long evolutionary history, extending as far back as the Devonian period of the Palaeozoic era
The gymnosperm order Cycadales is pivotal for our understanding of seed-plant phylogeny because of its phylogenetic placement close to the root node of extant spermatophytes and its combination of both derived and plesiomorphic character states
Leaflets of seven species across five genera representing all major clades of Zamiaceae were examined at various stages of development using light microscopy and confocal t microscopy. rip Key Results All genera examined have lateral subsidiary cells of perigenous origin that differ from other pavement cells in mature leaflets and could have a role in stomatal physiology
Summary
The extant gymnosperms, though relatively species-poor compared with angiosperms, display a high degree of morphological disparity that reflects a long evolutionary history, extending as far back as the Devonian period of the Palaeozoic era. Among the four gymnosperm lineages that include living representatives, the cycad lineage is highly significant in studies of character evolution because of its phylogenetic placement close to the root node of the t spermatophyte clade, either as sister to all other extant gymnosperms in most molecular analyses rip The gymnosperm order Cycadales is pivotal for our understanding of seed-plant phylogeny because of its phylogenetic placement close to the root node of extant spermatophytes and its combination of both derived and plesiomorphic character states. Conclusions: Stomatal structure and development in Zamiaceae highlights some traits that are plesiomorphic in seed plants, including the presence of perigenous encircling subsidiary cells, and shows a clear difference between the developmental trajectories of cycads and Bennettitales. Cycadales is pivotal for our understanding of seed-plant phylogeny because of its phylogenetic placement close to the root node of extant spermatophytes and its combination of both derived and plesiomorphic character states. We investigate stomatal development in Zamiaceae to evaluate variation within the order and homologies between cycads and other seed plants
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