Journal Article

Seedling Diversity in Hydatellaceae: Implications for the Evolution of Angiosperm Cotyledons

Dmitry D. Sokoloff, Margarita V. Remizowa, Terry D. Macfarlane, Renee E. Tuckett, Margaret M. Ramsay, Anton S. Beer, Shrirang R. Yadav and Paula J. Rudall

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 101, issue 1, pages 153-164
Published in print January 2008 | ISSN: 0305-7364
Published online November 2007 | e-ISSN: 1095-8290 | DOI: http://dx.doi.org/10.1093/aob/mcm274
Seedling Diversity in Hydatellaceae: Implications for the Evolution of Angiosperm Cotyledons

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Background and Aims

Cotyledon number has long been a primary morphological feature distinguishing monocots from other angiosperms. Recent placement of Hydatellaceae near the early-divergent angiosperm order Nymphaeales, rather than in the monocot order Poales, has prompted reassessment of seedling morphology in this poorly known family.

Methods

Seedlings of six species representing all eco-geographical groups of Hydatellaceae are described using light and scanning electron microscopy.

Key Results

Two seedling types were discovered. Material examined of Trithuria submersa, T. bibracteata, T. austinensis and T. filamentosa possess a transparent bilobed sheathing structure that surrounds the main axis below the first foliage leaf. The seed coat is attached to the sheathing structure. Seedlings of Trithuria lanterna and T. konkanensis lack a sheathing structure, and the seed coat is attached to a short, narrow lateral outgrowth on the main axis of the seedling.

Conclusions

The sheathing structure that is present in seedlings of some Hydatellaceae could be homologized with the two united cotyledons of water lilies. It also resembles the single cotyledon of some monocots, and hence demonstrates a possible pathway of the origin of a monocot-like embryo, though no homology is implied. The sheathing structure is reduced in Trithuria lanterna and T. konkanensis, and the short, narrow outgrowth of its seedling could represent a single cotyledon. This synapomorphy suggests that the only Indian species of Hydatellaceae, T. konkanensis, is closer to the northern Australian T. lanterna than to the south-western Australian T. bibracteata.

Keywords: Seedling; cotyledon; monocot; dicot; Hydatellaceae; Trithuria

Journal Article.  5832 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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