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(American Journal of Botany. 2009;96:22-66.) doi: 10.3732/ajb.0800047 © 2009 Botanical Society of America, Inc.

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Reconstructing the ancestral angiosperm flower and its initial specializations¹

² Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland ³ Department of Evolution and Ecology, University of California, Davis, California 95616 USA

ABSTRACT

Increasingly robust understanding of angiosperm phylogeny allows more secure reconstruction of the flower in the most recent common ancestor of extant angiosperms and its early evolution. The surprising emergence of several extant and fossil taxa with simple flowers near the base of the angiosperms—Chloranthaceae, Ceratophyllum, Hydatellaceae, and the Early Cretaceous fossil Archaefructus (the last three are water plants)—has brought a new twist to this problem. We evaluate early floral evolution in angiosperms by parsimony optimization of morphological characters on phylogenetic trees derived from morphological and molecular data. Our analyses imply that Ceratophyllum may be related to Chloranthaceae, and Archaefructus to either Hydatellaceae or Ceratophyllum. Inferred ancestral features include more than two whorls (or series) of tepals and stamens, stamens with protruding adaxial or lateral pollen sacs, several free, ascidiate carpels closed by secretion, extended stigma, extragynoecial compitum, and one or several ventral pendent ovule(s). The ancestral state in other characters is equivocal: e.g., bisexual vs. unisexual flowers, whorled vs. spiral floral phyllotaxis, presence vs. absence of tepal differentiation, anatropous vs. orthotropous ovules. Our results indicate that the simple flowers of the newly recognized basal groups are reduced rather than primitively simple.

Key Words: ancestral flowers • angiosperm phylogeny • ANITA grade • Archaefructus • basal angiosperms • Ceratophyllum • Chloranthaceae • flower evolution • Hydatellaceae • water plants

Received for publication 7 February 2008. Accepted for publication 12 September 2008.

FOOTNOTES

¹ The authors thank E. M. Friis and M. Frohlich for useful discussions and suggestions that improved the manuscript. J.A.D. thanks P. Garnock-Jones and the School of Biological Sciences, Victoria University of Wellington, for facilities and a supportive environment during preparation of this paper. This work was facilitated by travel support from the NSF Deep Time Research Coordination Network (RCN0090283).

⁴ Author for correspondence (e-mail: pendress{at}systbot.uzh.ch)

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