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5 - Changing views of flower evolution and new questions
- Edited by Livia Wanntorp, Swedish Museum of Natural History, Louis P. Ronse De Craene, Royal Botanic Garden Edinburgh
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- Book:
- Flowers on the Tree of Life
- Published online:
- 07 October 2011
- Print publication:
- 22 September 2011, pp 120-141
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Summary
Flowers in phylogenetic and evolutionary studies
The role of flowers in evolutionary biology has changed in the past 20 years, as the major foci are constantly changing with new approaches and better understanding of evolutionary processes. The revolution of molecular phylogenetics and molecular developmental genetics produced a trend in flower studies away from phylogenetics and towards evolution. In turn, the discovery of many well-preserved Cretaceous fossil flowers led to a new trend in flower studies towards phylogenetics, because fossil flowers do not provide DNA. The following three current fields of flower structural studies may be distinguished:
Comparative morphological analysis of flowers – Many new major angiosperm clades have been recognized by molecular phylogenetic studies since Chase et al. (1993), as surveyed in APG (1998, 2009), Stevens (2001 onwards) and Soltis et al. (2005). These new clades need now to be critically studied comparatively in their structure and biology as they are largely unknown (e.g. Endress and Matthews, 2006; Endress, 2010a).
Morphology for phylogenetic studies – Flowers were generally used for phylogenetic studies in the era before the molecular revolution. In the past 20 years, phylogenetics has concentrated on molecular approaches, which yield more results in a shorter time than morphology. However, morphological phylogenetic analyses are still performed and yield interesting results, either alone or in combination with molecular analyses (at higher systematic levels, e.g. Nandi et al., 1998; Doyle and Endress, 2000, or lower levels, e.g. Carillo-Reyes et al., 2008; Sweeney, 2008). There has been a pessimistic attitude towards the use of morphological features in phylogenetics because of too much homoplasy (e.g. Givinish and Sytsma, 1997; Patterson and Givnish, 2002; Givnish, 2003; Scotland et al., 2003) and difficulties in scoring structural characters (Stevens, 2000). This is true if superficial structural features that are easy to spot are used (e.g. tepals large and showy versus small and inconspicuous, or fruits capsules versus berries, or storage organs rhizomes versus bulbs). However, morphology encompasses much more than such features. It can be expected that as our knowledge of flowers increases, there will be a resurgence in morphological phylogenetic analyses. In addition, the more fossil flowers become available, the more important morphological phylogenetic analyses will become (e.g. Friis et al., 2009; Doyle and Endress, 2010). There are not only many more fossil flowers available than 20 years ago, but there are also new techniques to reconstruct their morphology: the use of microtome section series (Schönenberger, 2005) and tomography (Friis et al., 2009). The search for and the detection of new structural patterns of interest is a continuing challenge. Characters and character states ‘cannot be defined but need to be discussed,’ as Wagner (2005) put it, meaning that definitions need to be constantly evaluated and updated to fit the current knowledge with each change in the phylogenetic framework. New knowledge on phylogeny (and evolution) continuously creates a new basis for discussion. Of course, if morphological characters are used for phylogenetic studies, this also means the necessity of repeated reciprocal illumination (see also Kelly and Stevenson, 2005). ‘Tree-thinking’ has been encouraged in evolutionary studies (O’Hara, 1988; Donoghue and Sanderson, 1992). This is of course also relevant for the focus on structural features, including the construction of morphological matrices for phylogenetic studies. The more detailed a tree under reconstruction already is and the more detailed our knowledge about the distribution of traits on this tree is, the better we can judge the quality of characters and character states to be scored.
Morphology for evolutionary studies – The new phylogenetic results can now be used to study the evolution of flowers on a much more solid basis than was possible before. A general result is that many features are more evolutionarily flexible than previously assumed. A number of examples are surveyed in this study. Rarely is a character more stable than previously assumed at macrosystematic level; such an exception are features of ovules (Endress, 2003, 2005a, 2010). However, such flexibility is not randomly distributed through the larger clades. Given features are more concentrated (but not universal) in a certain clade than in another one. Why is this so? Answers can be expected from better knowledge of the genetic systems that operate in the development of such features (e.g. Borowsky, 2008; Melzer et al., 2008). Thus, homoplasy in structure is pervasive, much more common than earlier imagined and is a fascinating aspect of flower evolution (e.g. Cantino, 1985; Endress, 1996). For more evolutionary aspects of flower morphology, see Endress (1994, 2003, 2005b, 2006).
4 - Tracing the early evolutionary diversification of the angiosperm flower
- Edited by Livia Wanntorp, Swedish Museum of Natural History, Louis P. Ronse De Craene, Royal Botanic Garden Edinburgh
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- Book:
- Flowers on the Tree of Life
- Published online:
- 07 October 2011
- Print publication:
- 22 September 2011, pp 88-119
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Summary
Introduction
The origin of the angiosperm flower and its subsequent evolution have been major topics of discussion and controversy for over a century. Because so many of the distinctive synapomorphies of angiosperms involve the flower, its origin and the homologies of its parts are closely tied to the vexed problem of the origin of angiosperms as a group. From a phylogenetic point of view, the origin of angiosperms involves two related problems: identification of the closest outgroups of angiosperms, which may clarify homologies of their distinctive features with structures seen in other plants, and rooting of the angiosperm phylogenetic tree and identification of its earliest branches, which may allow reconstruction of the flower in the most recent common ancestor of living angiosperms. It is this second topic that we address in this chapter (for the first study, see Frohlich and Chase, 2007; Doyle, 2008). This task has become much easier in the past ten years, thanks to molecular phylogenetics.
Ideas on the ancestral flower have varied greatly since early in the last century. Two extremes were euanthial theories, which postulated that the flower was a simple strobilus that was originally bisexual and had many free parts (Arber and Parkin, 1907), and pseudanthial theories, which assumed that the first angiosperms had unisexual flowers with few parts, as in ‘Amentiferae’ (now mostly Fagales), which were later grouped to form bisexual flowers (Wettstein, 1907; review in Friis and Endress, 1990). Later variations on the pseudanthial theory proposed that the angiosperms were polyphyletic (Meeuse, 1965, 1975), while recognition of chloranthoid pollen, leaves and flowers in the Early Cretaceous fossil record (Muller, 1981; Upchurch, 1984; Walker and Walker, 1984; Friis et al., 1986; Pedersen et al., 1991; Eklund et al., 2004) contributed to suggestions that Chloranthaceae, which combine putatively primitive wood and monosulcate pollen with extremely simple flowers, often consisting of just one stamen or one carpel, might provide another model for the ancestral flower (Endress, 1986b; Taylor and Hickey, 1992).
8 - Comparative floral structure and development of Nitrariaceae (Sapindales) and systematic implications
- Edited by Livia Wanntorp, Swedish Museum of Natural History, Louis P. Ronse De Craene, Royal Botanic Garden Edinburgh
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- Book:
- Flowers on the Tree of Life
- Published online:
- 07 October 2011
- Print publication:
- 22 September 2011, pp 181-217
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Summary
Introduction
For the last 20 years, the development and improvement of molecular methods, based mostly on the comparison of DNA sequences, have been increasingly successful in reconstructing the phylogenetic tree of plants at all hierarchical levels. Consequently, they have contributed greatly to the recent improvement of angiosperm systematics. In addition, they have shown that earlier classifications, based mostly on plant vegetative and reproductive structures, had sometimes been misled by homoplastic characters, and a number of orders and families have had to be newly circumscribed or even newly established (e.g. APG, 1998, 2003, 2009; Stevens, 2001 onwards). These new results provide a novel basis for comparative structural studies to characterize the newly recognized clades and to evaluate clades that have only limited molecular support. However, because such comparative studies are time-consuming and the systematic classification has different hierarchical levels, they can only be done in a stepwise fashion (for eudicots, e.g. Matthews and Endress, 2002, 2004, 2005a, b, 2006, 2008; von Balthazar et al., 2004; Schönenberger and Grenhagen, 2005; Endress and Matthews, 2006; Ronse De Craene and Haston, 2006; von Balthazar et al., 2006; Bachelier and Endress, 2008, 2009; Janka et al., 2008; Schönenberger, 2009; von Balthazar and Schönenberger, 2009; Schönenberger et al., 2010).
As part of such a comparative approach, we studied the floral structure of Nitrariaceae, a small family which has been recently reclassified in Sapindales (APG, 2009). Nitrariaceae comprise four genera and around 15 species (Stevens, 2001 onwards; APG, 2009). They are native to arid and semi-arid regions of the Old World and are small to medium-sized shrubs (Nitraria; Engler, 1896a, b; Bobrov, 1965; Noble and Whalley, 1978), perennial herbs (Peganum and Malacocarpus; Engler, 1896a, 1931; El Hadidi, 1975) or small annual herbs of only a few centimetres height (Tetradiclis; Engler, 1896b, 1931; Hamzaoglu et al., 2005). In earlier classifications, the position and the mutual affinities of these genera varied tremendously, depending on the weight an author gave either to their vegetative or their reproductive features (Takhtajan, 1969, 1980, 1983, 2009; El Hadidi, 1975; Dahlgren, 1980; Cronquist, 1981, 1988; see Sheahan and Chase, 1996 for a detailed review of classifications). Because none of the traditional classifications was entirely satisfactory, however, most authors followed Engler’s influential work (1896a, b, 1931) and Nitraria, Tetradiclis and Peganum (including Malacocarpus) remained for a long time in their own subfamilies in Zygophyllaceae (Nitrarioideae, Tetradiclidoideae and Peganoideae; for more details of the history of classification, see Sheahan and Chase, 1996).