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Review
, 108 (4), 589-98

Cabomba as a Model for Studies of Early Angiosperm Evolution

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Review

Cabomba as a Model for Studies of Early Angiosperm Evolution

Aurelie C M Vialette-Guiraud et al. Ann Bot.

Abstract

Background: The angiosperms, or flowering plants, diversified in the Cretaceous to dominate almost all terrestrial environments. Molecular phylogenetic studies indicate that the orders Amborellales, Nymphaeales and Austrobaileyales, collectively termed the ANA grade, diverged as separate lineages from a remaining angiosperm clade at a very early stage in flowering plant evolution. By comparing these early diverging lineages, it is possible to infer the possible morphology and ecology of the last common ancestor of the extant angiosperms, and this analysis can now be extended to try to deduce the developmental mechanisms that were present in early flowering plants. However, not all species in the ANA grade form convenient molecular-genetic models.

Scope: The present study reviews the genus Cabomba (Nymphaeales), which shows a range of features that make it potentially useful as a genetic model. We focus on characters that have probably been conserved since the last common ancestor of the extant flowering plants. To facilitate the use of Cabomba as a molecular model, we describe methods for its cultivation to flowering in the laboratory, a novel Cabomba flower expressed sequence tag database, a well-adapted in situ hybridization protocol and a measurement of the nuclear genome size of C. caroliniana. We discuss the features required for species to become tractable models, and discuss the relative merits of Cabomba and other ANA-grade angiosperms in molecular-genetic studies aimed at understanding the origin of the flowering plants.

Figures

Fig. 1.
Fig. 1.
Phylogenetic position of Cabomba. Schematic phylogeny based on Bremer et al. (2009) and related internet resources. Approximate species numbers are given in parentheses.
Fig. 2.
Fig. 2.
Morphology of Cabomba caroliniana. (A) Erect stems carrying highly dissected, submerged leaves, seen from above. (B) Three successive floating leaves (1–3) emerging from the inflorescence apex, each subtending a flower bud present in the tissue at the centre. (C) Side view of a newly opened flower, supported above the water on its long pedicel by a floating leaf. (D) Open flower showing its trimerous, whorled arrangement. (E) A mature fruit, partially degraded under water to release the seeds. Scale bars = 1 cm.
Fig. 3.
Fig. 3.
Scanning electron micrographs of Cabomba caroliniana flowers growing at Kew. (A) Undissected flower buds, showing two preanthetic buds enclosed by tepals, and two early-stage buds (indicated by white arrows) not yet enclosed by tepals. (B) Early developmental stage showing early stamen, gynoecium and tepal primordia. (C) Later stage showing early carpel differentiation. (D–F) Successive developmental stages showing organ development and elongation; nectary developing on inner tepal in (F). (G) Inner tepal of anthetic flower showing nectaries. Abbreviations: c, carpel primordium or carpel; g, gynoecium primordium; n, nectary; s, stamen primordium or stamen; t, tepal. Scale bars: (A, F, G) = 200 µm, (B–E) = 100 µm.
Fig. 4.
Fig. 4.
Functional annotation of the Cabomba flower EST database. Sequences are annotated using Blast2GO to categorize ‘Gene Ontology level 2 terms’ matching with at least 100 sequences for: biological processes (A), molecular functions (B) and cellular components (C). Numbers of ESTs in each category are shown in parentheses.
Fig. 5.
Fig. 5.
In situ hybridization of a HISTONE4 riboprobe to a Cabomba aquatica shoot apical meristem. Actively dividing cells that strongly express HISTONE4 are shown by dark blue staining. The same specimen is shown at three magnifications, taken using brightfield illumination in conjunction with 4× (A) and 10× (B) objective lenses, and Nomarski contrast optics in conjunction with a 20× objective lens (C). Cell outlines and nuclei are clearly visible at high magnification. Scale bars = 400 µm. Full technical details are provided in Supplementary Data (available online).

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