title: Chromosome behavior at the base of the angiosperm radiation: karyology of Trithuria submersa (Hydatellaceae, Nymphaeales).
abstract: •
  
  
  OBJECTIVE
  Hydatellaceae are minute annual herbs with potential as a model system for studying early angiosperm evolution, but their karyology and ploidy levels are almost unknown. We investigated these aspects of Trithuria submersa, a widespread species that we show to be amenable to extended vegetative propagation.•
  
  
  METHODS
  We cultivated plants of T. submersa in vitro after developing and optimizing culture conditions. We estimated genome size using flow cytometry, counted chromosome numbers using root-meristem squashes after Feulgen staining, and examined meiotic chromosome behavior using microsporocytes.•
  
  
  RESULTS
  We developed methods to reliably germinate seeds of T. submersa and to propagate them vegetatively in critical thermo- and photoperiod regimes on 1/2 Murashige-Skoog (MS) medium with vitamins and 2% sucrose solidified with 0.7% agar-agar. Seedling growth requires the medium be supplemented with activated charcoal. The mean nuclear DNA content of T. submersa sporophytes is 2C = 2.74 pg (∼2.68 Gbp). The sporophytic chromosome number is 2n = 56 with a bimodal complement, which may suggest an allopolyploid origin. Some of the largest chromosomes lack a recognizable constriction, which relates to a highly unusual and irregular chromosome behavior. Microsporocytes undergo reduced and asynchronous meioses that show a modified intermediate cell division with a nucleus division by fractional postreduction, indicating partially inverted microsporogenesis.•
  
  
  CONCLUSIONS
  In vitro cultivation and karyological assessment of T. submersa open new opportunities for investigating early-divergent angiosperms. The remarkably different meiotic behavior exhibits new insights into a potentially ancestral microsporogenesis.
authors.affiliation: Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.
authors.forename: Ralf G
authors.initials: RG
authors.lastname: Kynast
authors.affiliation: Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.
authors.forename: Jeffrey A
authors.initials: JA
authors.lastname: Joseph
authors.affiliation: Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.
authors.forename: Jaume
authors.initials: J
authors.lastname: Pellicer
authors.affiliation: Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Ramsay
authors.affiliation: Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3AB, UK.
authors.forename: Paula J
authors.initials: PJ
authors.lastname: Rudall
mesh_terms: D005075:Biological Evolution; D002467:Cell Nucleus; D032461:Chromosomes, Plant; D003470:Culture Media; D018744:DNA, Plant / Q000032:analysis*; D035264:Flowers; D059646:Genome Size; D018745:Genome, Plant; D018525:Germination; D019684:Magnoliopsida / Q000235:genetics*; D008540:Meiosis; D008938:Mitosis; D018517:Plant Roots; D011123:Polyploidy; D012098:Reproduction; D036226:Seedlings / Q000235:genetics
chemical_list: D003470:Culture Media; D018744:DNA, Plant
keywords: Hydatellaceae; Nymphaeales; Trithuria submersa; chromosome behavior; chromosome number; genome size; in vitro culture; post-reductional meiosis