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First published online August 13, 2009; doi:10.3732/ajb.0900033 American Journal of Botany 96: 1581-1593 (2009) © 2009 Botanical Society of America, Inc.

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Seed fertilization, development, and germination in Hydatellaceae (Nymphaeales): Implications for endosperm evolution in early angiosperms¹

² Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3AB, UK ³ The University of Western Australia, Crawley, WA 6009, Australia ⁴ Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK ⁵ Department of Higher Plants, Biological Faculty, Moscow State University 119991, Moscow, Russia

ABSTRACT

New data on endosperm development in the early-divergent angiosperm Trithuria (Hydatellaceae) indicate that double fertilization results in formation of cellularized micropylar and unicellular chalazal domains with contrasting ontogenetic trajectories, as in waterlilies. The micropylar domain ultimately forms the cellular endosperm in the dispersed seed. The chalazal domain forms a single-celled haustorium with a large nucleus; this haustorium ultimately degenerates to form a space in the dispersed seed, similar to the chalazal endosperm haustorium of waterlilies. The endosperm condition in Trithuria and waterlilies resembles the helobial condition that characterizes some monocots, but contrasts with Amborella and Illicium, in which most of the mature endosperm is formed from the chalazal domain. The precise location of the primary endosperm nucleus governs the relative sizes of the chalazal and micropylar domains, but not their subsequent developmental trajectories. The unusual tissue layer surrounding the bilobed cotyledonary sheath in seedlings of some species of Trithuria is a belt of persistent endosperm, comparable with that of some other early-divergent angiosperms with a well-developed perisperm, such as Saururaceae and Piperaceae. The endosperm of Trithuria is limited in size and storage capacity but relatively persistent.

Key Words: angiosperm evolution • embryo • endosperm • Hydatellaceae • seed development • synchrotron • Trithuria • waterlilies

Received for publication 28 January 2009. Accepted for publication 14 April 2009.

FOOTNOTES

¹ The authors thank R. Bateman for critically reading the manuscript and T. Macfarlane for help with fieldwork in Australia. F. Marone and M. Stampanoni (Tomcat Beamline, Swiss Light Source, Paul Scherrer Institute) and S. Joomun (Royal Holloway, University of London) provided assistance with synchrotron x-ray tomographic microscopy, and the Swiss Light Source and EU provided time and funding to work there. The research was partly supported by a 2007 CoSyst grant.

⁶ Author for correspondence (e-mail: p.rudall{at}kew.org)

⁷ Present address: Museum of Paleontology, University of Michigan, 1109 Geddes Road, Ann Arbor, Michigan 48109 USA

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