AOBPreview originally published online on March 30, 2008
Annals of Botany 2008 101(7):941-956; doi:10.1093/aob/mcn032
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Comparative Ovule and Megagametophyte Development in Hydatellaceae and Water Lilies Reveal a Mosaic of Features Among the Earliest Angiosperms
1 Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey, TW9 3DS, UK
2 Department of Higher Plants, Biological Faculty, Moscow State University, 119991 Moscow, Russia
3 New York Botanical Garden, Bronx, New York 10458, USA
4 Western Australian Herbarium, Science Division, Department of Environment & Conservation, Brain Street, 6258 Manjimup, WA, Australia
5 The University of Western Australia, Crawley, WA 6009 and Botanic Gardens and Parks Authority, Fraser Avenue, West Perth, WA 6005, Australia
6 Shivaji University, Vidyanagar, Kolhapur 416 004, India
* For correspondence. E-mail p.rudall{at}kew.org
Received: 29 November 2007 Returned for revision: 10 January 2008 Accepted: 12 February 2008 Published electronically: 30 March 2008
Background and Aims: The embryo sac, nucellus and integuments of the early-divergent angiosperms Hydatellaceae and other Nymphaeales are compared with those of other seed plants, in order to evaluate the evolutionary origin of these characters in the angiosperms.
Methods: Using light microscopy, ovule and embryo sac development are described in five (of 12) species of Trithuria, the sole genus of Hydatellaceae, and compared with those of Cabombaceae and Nymphaeaceae.
Key Results: The ovule of Trithuria is bitegmic and tenuinucellate, rather than bitegmic and crassinucellate as in most other Nymphaeales. The seed is operculate and possesses a perisperm that develops precociously, which are both key features of Nymphaeales. However, in the Indian species T. konkanensis, perisperm is relatively poorly developed by the time of fertilization. Perisperm cells in Trithuria become multinucleate during development, a feature observed also in other Nymphaeales. The outer integument is semi-annular (‘hood-shaped’), as in Cabombaceae and some Nymphaeaceae, in contrast to the annular (‘cap-shaped’) outer integument of some other Nymphaeaceae (e.g. Barclaya) and Amborella. The megagametophyte in Trithuria is monosporic and four-nucleate; at the two-nucleate stage both nuclei occur in the micropylar domain. Double megagametophytes were frequently observed, probably developed from different megaspores of the same tetrad. Indirect, but strong evidence is presented for apomictic embryo development in T. filamentosa.
Conclusions: Most features of the ovule and embryo sac of Trithuria are consistent with a close relationship with other Nymphaeales, especially Cabombaceae. The frequent occurrence of double megagametophytes in the same ovule indicates a high degree of developmental flexibility, and could provide a clue to the evolutionary origin of the Polygonum-type of angiosperm embryo sac.
Key words: Embryo sac, megagametophyte, ovule, Hydatellaceae, Trithuria
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