title: Floral biology and ovule and seed ontogeny of Nymphaea thermarum, a water lily at the brink of extinction with potential as a model system for basal angiosperms.
abstract: OBJECTIVE
  Nymphaea thermarum is a member of the Nymphaeales, of one of the most ancient lineages of flowering plants. This species was only recently described and then declared extinct in the wild, so little is known about its reproductive biology. In general, the complete ontogeny of ovules and seeds is not well documented among species of Nymphaea and has never been studied in the subgenus Brachyceras, the clade to which N. thermarum belongs.
  
  
  METHODS
  Flowers and fruits were processed for brightfield, epifluorescence and confocal microscopy. Flower morphology, with emphasis on the timing of male and female functions, was correlated with key developmental stages of the ovule and the female gametophyte. Development of the seed tissues and dynamics of polysaccharide reserves in the endosperm, perisperm and embryo were examined.
  
  
  RESULTS
  Pollen release in N. thermarum starts before the flower opens. Cell walls of the micropylar nucellus show layering of callose and cellulose in a manner reminiscent of transfer cell wall patterning. Endosperm development is ab initio cellular, with micropylar and chalazal domains that embark on distinct developmental trajectories. The surrounding maternal perisperm occupies the majority of seed volume and accumulates starch centrifugally. In mature seeds, a minute but fully developed embryo is surrounded by a single, persistent layer of endosperm.
  
  
  CONCLUSIONS
  Early male and female function indicate that N. thermarum is predisposed towards self-pollination, a phenomenon that is likely to have evolved multiple times within Nymphaea. While formation of distinct micropylar and chalazal developmental domains in the endosperm, along with a copious perisperm, characterize the seeds of most members of the Nymphaeales, seed ontogenies vary between and among the constituent families. Floral biology, life history traits and small genome size make N. thermarum uniquely promising as an early-diverging angiosperm model system for genetic and molecular studies.
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, MA 02131, USA rpovilus@fas.harvard.edu.
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Povilus
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, MA 02131, USA Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, MA 02131, USA.
authors.forename: Juan M
authors.initials: JM
authors.lastname: Losada
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, MA 02131, USA Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, MA 02131, USA.
authors.forename: William E
authors.initials: WE
authors.lastname: Friedman
mesh_terms: D005075:Biological Evolution; D056727:Endangered Species; D035264:Flowers / Q000254:growth & development*; D019684:Magnoliopsida / Q000502:physiology; D030721:Nymphaea / Q000235:genetics / Q000254:growth & development*; D056771:Ovule / Q000254:growth & development; D012639:Seeds / Q000254:growth & development*
keywords: Early-diverging angiosperm; Nymphaea thermarum; Nymphaeales; embryo; endosperm; evo-devo; female gametophyte; flower biology; megagametogenesis; megasporogenesis; perisperm; protogyny; seed development; stigma