Paleobotánica

Int. J. Plant Sci. 169(7):816–843. 2008.
Ó 2008 by The University of Chicago. All rights reserved.
1058-5893/2008/16907-0002$15.00 DOI: 10.1086/589887

INTEGRATING MOLECULAR PHYLOGENETIC AND PALEOBOTANICAL
EVIDENCE ON ORIGIN OF THE FLOWER
James A. Doyle1
Department of Evolution and Ecology, University of California, Davis, California 95616, U.S.A.

Inferences on the origin of theangiosperm flower require consideration of other seed plants, especially fossils.
Molecular data favor a relationship of Gnetales to conifers rather than to angiosperms, and both alternatives are
equally parsimonious in terms of the morphological data set presented here. However, if molecular relationships
among extant taxa are accepted, morphology still associates glossopterids, Pentoxylon,Bennettitales, and
Caytonia with angiosperms. Bennettitales had flowerlike structures, but if Caytonia is sister to angiosperms,
aggregation of fertile parts probably occurred independently in Bennettitales and angiosperms. These results and
developmental genetic data are consistent with homology of the angiosperm bitegmic ovule with the cupule of
glossopterids and Caytonia, while the carpel couldrepresent a leaf and a cupule-bearing axillary branch. Origin of
an adaxial cross zone could produce a uniovulate, ascidiate carpel, as in living basal angiosperms. Stamens may
represent similar units bearing two microsynangia. However, ovulate structures of Pentoxylon and Bennettitales
are more difficult to interpret, and any homologue of the carpel wall in Caytonia is unclear. Further progress mayrequire better understanding of homologies in known fossils and/or recognition of closer stem relatives of
angiosperms. A proposed Cretaceous stem relative, Archaefructus, is more likely a crown-group angiosperm
related to Hydatellaceae (Nymphaeales).
Keywords: angiosperms, seed plants, paleobotany, phylogeny, flower, ovule.
Online enhancement: appendix.

Introduction

and formallydesignated Mesangiospermae by Cantino et al.
(2007). The main uncertainty is whether Amborella and Nymphaeales form two successive lines or a clade, with more data
favoring the former arrangement (Zanis et al. 2002; Soltis
et al. 2005; Moore et al. 2007). Using parsimony to optimize
characters on a tree based on sequences of three genes and
morphology, Doyle and Endress (2000) presented a list ofestimated ancestral states for 108 characters, such as vesselless
wood; simple, ovate leaves with pinnate venation and chloranthoid teeth; unilacunar two-trace nodes; undifferentiated
perianth; stamens with adaxial microsporangia; several ascidiate carpels; and fleshy, indehiscent fruits. Pollen characters
were further refined and evaluated by Doyle (2005), floral
phyllotaxis by Endress and Doyle(2007), and leaf architecture
by Doyle (2007). Ronse De Craene et al. (2003) presented a
similar analysis of floral characters. It can be expected that
such results will be improved by integration of the increasing
numbers of Early Cretaceous floral fossils, many described by
Friis et al. (2006).
The second question—what sort of structures in earlier
plants the flower came from, how they weretransformed into
the perianth, stamens, and carpels, and how they were assembled into the flower—can be approached phylogenetically by
asking what the closest outgroups of angiosperms are. Examination of the seed- and pollen-bearing organs in these plants
and optimization of the relevant characters on a cladogram
may provide an improved picture of the precursor structures
for the flower at theclosest outgroup node. This ‘‘bottom-up’’
approach (Bateman et al. 2006) is therefore a question of seed
plant phylogeny and the place of angiosperms in it, and because

Reconstruction of the first flower and its origin—what the
angiosperm flower and its component parts came from—are
two separate but related questions. In this article, I approach
the second question with phylogenetic methods,...