Molecules, Morphology, Fossils, and the Relationship of Angiosperms and Gnetales

Section of Evolution and Ecology, University of California, Davis, California, 95616, USA.
Molecular Phylogenetics and Evolution (Impact Factor: 3.92). 07/1998; 9(3):448-62. DOI: 10.1006/mpev.1998.0506
Source: PubMed


Morphological analyses of seed plant phylogeny agree that Gnetales are the closest living relatives of angiosperms, but some studies indicate that both groups are monophyletic, while others indicate that angiosperms are nested within Gnetales. Molecular analyses of several genes agree that both groups are monophyletic, but differ on whether they are related. Conflicts among morphological trees depend on the interpretation of certain characters; when these are analyzed critically, both groups are found to be monophyletic. Conflicts among molecular trees may reflect the rapid Paleozoic radiation of seed plant lines, aggravated by the long branches leading to extant taxa. Trees in which angiosperms are not related to Gnetales conflict more with the stratigraphic record. Even if molecular data resolve the relationships among living seed plant groups, understanding of the origin of angiosperm organs will require integration of fossil taxa, necessarily using morphology.

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Available from: James A. Doyle, May 13, 2014
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    • "The extant Ephedraceae (Ephedra) usually occupy a basal position in phylogenetic trees of the Gnetales (Figure 1d) while Gnetum and Welwitschia are more derived and are more closely related to each other than either is to Ephedra[14,17-23,27-33]. Ephedra has scale-like, linear and 2 (—3) parallel-veined leaves which are often opposite and decussate at nodes and connate at the base into a sheath (e.g., E. equisetina Bunge, E. distachya L., E. sinica Stapf, E. pedunculata Engelm. "
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    ABSTRACT: Background The extant Gnetales include three monotypic families, namely, Ephedraceae (Ephedra), Gnetaceae (Gnetum), and Welwitschiaceae (Welwitschia), all of which possess compound female cones that comprise a main axis and 1 to multiple pairs/whorls of bracts subtending a female reproductive unit or having lower pairs/whorls of bracts sterile. However, the evolutionary origin of such a reproductive architecture in Gnetales is controversial in the light of the competing anthophyte versus gnetifer hypotheses of seed plant relationships. Hence, macrofossils demonstrating the structure of compound female cones of the Gnetales should be important to decipher the early evolution of the order. Results A new ephedroid plant Chengia laxispicata gen. et sp. nov. is described from the Early Cretaceous Yixian Formation of western Liaoning, Northeast China. The fossil represents a part of a leafy shooting system with reproductive organs attached. The main shoot bears internodes and swollen nodes, from which lateral branches arise oppositely. Reproductive organs consist of female spikes terminal to twigs or axillary to linear leaves. Spikes are loosely arranged, having prominent nodes and internodes. Bracts of the spikes are decussately opposite and comprise 4—8 pairs of bracts. Each bract subtends an ellipsoid seed. Seeds are sessile, with a thin outer envelope and a distal micropylar tube. Conclusions Chengia laxispicata gen. et sp. nov. provides a missing link between archetypal fertile organs in the crown lineage of the Gnetales and compound female cones of the extant Ephedraceae. Combined with a wealth of Ephedra and ephedroid macrofossils from the Early Cretaceous, we propose a reduction and sterilization hypothesis that the female cone of the extant Ephedraceae may have stemmed from archetypal fertile organs in the crown lineage of the Gnetales. These have undergone sequentially intermediate links similar to female cones of Cretaceous Siphonospermum, Chengia, and Liaoxia by reduction and sterilization of the lower fertile bracts, shortenings of internodes and peduncles as well as loss of reproductive units in all inferior bracts. The basal family Ephedraceae including Ephedra of the extant Gnetales was demonstrated to have considerable diversity by the Early Cretaceous, so an emended familial diagnosis is given here. The Jehol Biota in Northeast China and adjacent areas contains a plethora of well-preserved macrofossils of Ephedra and ephedroids that show different evolutionary stages including primitive and derived characters of Ephedraceae, so Northeast China and adjacent areas may represent either the centre of origination or one of the centres for early diversification of the family.
    Full-text · Article · Mar 2013 · BMC Evolutionary Biology
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    • "In an analysis of ∼1,200 genes but only 16 taxa (de la Torre-Bárcena et al. 2009), angiosperms were basal in seed plants and Gnetales were sister to other gymnosperms (Figure 4d ). Trees of the last two types are less consistent with stratigraphy than either anthophyte or gnepine trees because Gnetales and angiosperms are the last two seed plant lines to appear in the fossil record (Doyle 1998a, Burleigh & Mathews 2004). These conflicts among trees do not inspire confidence in molecular data (cf. "
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    ABSTRACT: Molecular data on relationships within angiosperms confirm the view that their increasing morphological diversity through the Cretaceous reflected their evolutionary radiation. Despite the early appearance of aquatics and groups with simple flowers, the record is consistent with inferences from molecular trees that the first angiosperms were woody plants with pinnately veined leaves, multiparted flowers, uniovulate ascidiate carpels, and columellar monosulcate pollen. Molecular data appear to refute the hypothesis based on morphology that angiosperms and Gnetales are closest living relatives. Morphological analyses of living and fossil seed plants that assume molecular relationships identify glossopterids, Bennettitales, and Caytonia as angiosperm relatives; these results are consistent with proposed homologies between the cupule of glossopterids and Caytonia and the angiosperm bitegmic ovule. Jurassic molecular dates for the angiosperms may be reconciled with the fossil record if the first angiosperms we...
    Full-text · Article · May 2012 · Annual Review of Earth and Planetary Sciences
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    • "Ovules across seed plants are probably homologous, given that analyses of morphological data from both living and extinct taxa have supported their monophyly (e.g. Crane, 1985; Nixon et al., 1994; Rothwell & Serbet, 1994; Doyle, 1998; Hilton & Bateman, 2006; Doyle, 2008). However, it remains unclear how integuments, which are variable in number across seed plants, are related to one another and, similarly, what is the correspondence among ovule-bearing structures of different clades. "
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    ABSTRACT: The study of developmental genetics is providing insights into how plant morphology can and does evolve, and into the fundamental nature of specific organs. This new understanding has the potential to revise significantly the way we think about seed plant evolution, especially with regard to reproductive structures. Here, we have sought to take a step in bridging the divide between genetic data and critical fields such as paleobotany and systematics. We discuss the evidence for several evolutionarily important interpretations, including the possibility that ovules represent meristematic axes with their own type of lateral determinate organs (integuments) and a model that considers carpels as analogs of complex leaves. In addition, we highlight the aspects of reproductive development that are likely to be highly labile and homoplastic, factors that have major implications for the understanding of seed plant relationships. Although these hypotheses may suggest that some long-standing interpretations are misleading, they also open up whole new avenues for comparative study and suggest concrete best practices for evolutionary analyses of development.
    Full-text · Article · Mar 2012 · New Phytologist
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