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Chromosome behavior at the base of the angiosperm radiation: Karyology of Trithuria submersa (Hydatellaceae, Nymphaeales)

2014; Wiley; Volume: 101; Issue: 9 Linguagem: Inglês

10.3732/ajb.1400050

1537-2197

Ralf G. Kynast, Jeffrey A. Joseph, Jaume Pellicer, Margaret Ramsay, Paula J. Rudall,

Plant Diversity and Evolution

• Premise of the study: 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. • Key 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 2 n = 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.