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Peter K. Endress 2,4 and James A. Doyle 3 American Journal of Botany 96(1): 22–66. 2009. R ECONSTRUCTING THE ANCESTRAL ANGIOSPERM FLOWER AND ITS INITIAL SPECIALIZATIONS 1 Peter K. Endress 2,4 and James A. Doyle 3 2 Institute of Systematic Botany, University of Zurich, Zollikerstrasse 107, 8008 Zurich, Switzerland; and 3 Department of Evolution and Ecology, University of California, Davis, California 95616 USA Increasingly robust understanding of angiosperm phylogeny allows more secure reconstruction of the fl ower in the most recent common ancestor of extant angiosperms and its early evolution. The surprising emergence of several extant and fossil taxa with simple fl owers 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 fl oral evolution in an- giosperms by parsimony optimization of morphological characters on phylogenetic trees derived from morphological and molecu- lar 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 protrud- ing 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 fl owers, whorled vs. spiral fl oral phyllotaxis, presence vs. absence of tepal differentiation, anatropous vs. orthotropous ovules. Our results indicate that the simple fl owers of the newly recognized basal groups are reduced rather than primitively simple. Key words: ancestral fl owers; angiosperm phylogeny; ANITA grade; Archaefructus ; basal angiosperms; Ceratophyllum ; Chloranthaceae; fl ower evolution; Hydatellaceae; water plants. The question of the structure and biology of the ancestral lecular results, have linked glossopterids, Pentoxylon , Bennet- angiosperms, and especially their fl owers, is an enduring riddle. titales, and Caytonia , with or without Gnetales, with angiosperms Although we are continually gaining new insights from new ( Bateman et al., 2006 ; Doyle, 2006 ; Friis et al., 2007 ; Frohlich fossils and new studies on phylogeny, morphology, and devel- and Chase, 2007 ), but there is no general agreement that any of opmental genetics in extant plants, we are still far from a fi nal these taxa are related to angiosperms. The question of still answer. There are gaps at different levels. First is the uncer- closer angiosperm stem relatives is still a void because there are tainty concerning which other seed plants are the closest rela- no fossils that undisputedly represent this part of the tree. tives of angiosperms, particularly extinct groups because most Fortunately, there has been much more progress in recon- molecular analyses indicate that no living group of gymno- struction of the fi rst crown group angiosperms. Recent work on sperms is any closer to angiosperms than any other. Second, early fossil angiosperms (reviewed by Doyle, 2001, and Friis even if known fossils can be recognized as angiosperm stem et al., 2006 ) and on extant “ ANITA grade ” angiosperms relatives, all such groups are morphologically well removed ( Endress, 2001 , 2008a) has provided new insights. Problems at from angiosperms, so there is still a major gap that can only be this level have become easier to tackle thanks to analyses of liv- fi lled by the discovery of closer stem relatives. Third is the ing angiosperms, particularly using molecular data, which have problem of the original morphology and early evolutionary dif- clarifi ed relationships within the crown group with a degree of ferentiation of crown group angiosperms. precision and statistical confi dence barely imaginable two de- Identifi cation of seed plant relatives of the angiosperms has cades ago. These analyses have consistently rooted the an- been one of the most contentious issues in plant systematics and giosperm phylogenetic tree among the ANITA lines, namely evolution, both before and after the introduction of phyloge- Amborella , Nymphaeales, and Austrobaileyales ( Mathews and netic methods ( Crane, 1985 ; Doyle and Donoghue, 1986 ; Nixon Donoghue, 1999 ; Parkinson et al., 1999 ; Qiu et al., 1999 ; Renner, et al., 1994 ; Doyle, 1994 , 1996 ). Molecular analyses contradict 1999 ; Soltis et al., 1999, 2000 ; Barkman et al., 2000 ; Graham one of the few points on which morphological analyses agreed, and Olmstead, 2000 ; Zanis et al., 2002 ), which has focused at- that Gnetales are the closest living relatives of angiosperms tention on these taxa as particularly likely to yield insights on ( Donoghue and Doyle, 2000 ; Burleigh and Mathews, 2004 ; the fi rst angiosperms ( Doyle and Endress, 2000 ; Endress and Soltis et al., 2005 ), but they say nothing about fossil relatives. Igersheim, 2000a , b ; Endress, 2001 , 2004 , 2006, 2008a; Fried- Several recent studies, some of which take into account mo- man and Williams, 2003 , 2004 ; Williams and Friedman, 2004 ; Friedman, 2006 ; Endress and Doyle, 2007 ). The main uncer- tainty is whether Amborella and Nymphaeales form two succes- 1 Manuscript received 7 February 2008; revision accepted 12 September 2008. sive branches or a clade ( Barkman et al., 2000 ), with some The authors thank E. M. Friis and M. Frohlich for useful discussions and recent support for the latter hypothesis from mitochondrial genes suggestions that improved the manuscript. J.A.D. thanks P. Garnock-Jones ( Qiu et al., 2006 ), but the former supported by recent analyses of and the School of Biological Sciences, Victoria University of Wellington, entire plastid genomes ( Jansen et al., 2007 ; Moore et al., 2007 ). for facilities and a supportive environment during preparation of this paper. This work was facilitated by travel support from the NSF Deep Time An alternative rooting based on plastid genomes of fewer taxa, Research Coordination Network (RCN0090283). with grasses the sister group of all other angiosperms ( Goremykin 4 Author for correspondence (e-mail: [email protected]) et al., 2003 ), appears to be an artifact of low taxon sampling and long branch attraction ( Degtjareva et al., 2004 ; Soltis and Soltis, doi:10.3732/ajb.0800047 2004 ; Stefanovic et al., 2004 ; Leebens-Mack et al., 2005 ). 22 January 2009] Endress and Doyle — Ancestral flowers 23 All other angiosperms form a strongly supported clade, named could be said to be typical at a relatively “ basal ” level of angio- Mesangiospermae by Cantino et al. (2007) , but relationships sperms (groups other than monocots and eudicots, or “ Magno- among several lines in this clade remain poorly resolved, prob- liidae ” in the paraphyletic sense of Takhtajan, 1964 ), but ably as a result of very rapid radiation ( Moore et al., 2007 ). One because they were scattered in different taxa (e.g., anther open- important area of current uncertainty is the position of Chloran- ing by valves, spiral fl oral phyllotaxis, inner staminodes, trim- thaceae, which have been the subject of much discussion be- erous fl owers), it was possible to entertain several alternative cause of their extremely simple fl owers. Combined analyses of models for the ancestral fl ower (e.g., Endress, 1986a ). However, morphological and molecular data ( Doyle and Endress, 2000 ) especially since 1999, a more precise discussion is possible be- and some molecular studies ( Qiu et al., 2005 ; Duvall et al., cause phylogenetic reconstructions are generally more advanced, 2006 ; Mathews, 2006 ) have placed Chloranthaceae at the base and specifi cally the topology of the basal grade of extant angio- of mesangiosperms, but they are nested within mesangiosperms sperms is well supported and can be used as a basis for discus- in most molecular trees, including most of those found in analy- sions on evolution. As emphasized by Crisp and Cook (2005) , ses of complete plastid genomes ( Jansen et al., 2007 ; Moore it cannot be assumed that single low-diversity “ basal ” lines are et al., 2007 ). Suggestions that fl owers of Chloranthaceae were plesiomorphic in any given character, but when several lines primitive based on the abundance of apparently related fossils branch sequentially below the vast bulk of a clade, as is appar- in the Early Cretaceous (reviewed by Eklund et al., 2004 ; Friis ently the case for angiosperms, and these lines share the same et al., 2006 ) have faded with fi rm establishment of the basal character state, this state can be reconstructed by parsimony ANITA grade, but if Chloranthaceae are sister to the remaining analysis as ancestral. We took advantage of the new evidence mesangiosperms they could still be relevant to reconstruction on rooting in an analysis of basal angiosperms (including basal of the original fl ower and its initial modifi cations. monocots and basal eudicots), in which we used parsimony op- Comparative studies of fl oral developmental genetics repre- timization on a tree based on morphological data and rbcL , sent another growing fi eld that promises to provide new insights atpB , and 18S rDNA sequences ( Soltis et al., 2000 ) to estimate on early fl oral evolution. Such studies have already been used for ancestral states and trace character evolution ( Doyle and interpolations between angiosperms and other living seed plants Endress, 2000 ). In later articles we concentrated on implica- and within angiosperms
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