1. UBC Botanical Garden and Centre for Plant Research (Faculty of Land and Food Systems), Centre for Biodiversity Research, and Department of Botany, 2357 Main Mall, The University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
2. Section of Evolution and Ecology, University of California, Davis, California 95616, USA
3. Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland
4. The Arnold Arboretum, 22 Divinity Avenue, Harvard University, Cambridge, Massachusetts 02138, USA
5. Royal Botanic Gardens, Mrs Macquaries Road, Sydney, New South Wales 2000, Australia
6. Present address: Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada.

Correspondence to: Sean W. Graham¹ Correspondence and requests for materials should be addressed to S.W.G. (Email: swgraham@interchange.ubc.ca).

Abstract

Although the relationship of angiosperms to other seed plants remains controversial¹, great progress has been made in identifying the earliest extant splits in flowering-plant phylogeny, with the discovery that the New Caledonian shrub Amborella trichopoda, the water lilies (Nymphaeales), and the woody Austrobaileyales constitute a basal grade of lines that diverged before the main radiation in the clade^{2, 3, 4, 5, 6, 7, 8}. By focusing attention on these ancient lines, this finding has re-written our understanding of angiosperm structural and reproductive biology, physiology, ecology and taxonomy^{9, 10, 11, 12}. The discovery of a new basal lineage would lead to further re-evaluation of the initial angiosperm radiation, but would also be unexpected, as nearly all of the 460 flowering-plant families have been surveyed in molecular studies¹⁰. Here we show that Hydatellaceae, a small family of dwarf aquatics that were formerly interpreted as monocots, are instead a highly modified and previously unrecognized ancient lineage of angiosperms. Molecular phylogenetic analyses of multiple plastid genes and associated noncoding regions from the two genera of Hydatellaceae identify this overlooked family as the sister group of Nymphaeales. This surprising result is further corroborated by evidence from the nuclear gene phytochrome C (PHYC), and by numerous morphological characters. This indicates that water lilies are part of a larger lineage that evolved more extreme and diverse modifications for life in an aquatic habitat than previously recognized.

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