生花植物概念简介及其名称的商榷

在上个世纪最后的20年里,系统学家应用形态性状对种子植物进行了大量的分支分析。其结果显示灭绝的五柱木属加上灭绝的本内苏铁目及尚存的买麻藤目是被子植物的姊妹群,形成一个强支,称之为生花植物支。生花植物假说对探讨被子植物起源有着重要影响,它激发人们讨论被子植物起源时间可能要提前到三叠纪甚至石炭纪,除了支持原有的真花学说外,还提出新恩格勒学说。但是,近年来对现存种子植物进行分子系统学研究的结果是：(1)拒绝接受生花植物概念;(2)买麻藤目并不是被子植物的姊妹群而是松柏目的姊妹群,甚至网结于松柏类而成为松科的姊妹群。这些结果并不使人惊讶,因为对探讨像包含许多灭绝类群的种子植物系统,决不可能是仅仅单独应用现代类群资料所能完成的。假如生花植物支是成立的,但其名称以AGPB支代替生花植物支可能较为合理。     

被子植物花的起源：假说和证据

达尔文的“令人讨厌之谜”,即被子植物的起源和早期演化,一直是植物系统学研究领域的热点。被子植物区别于其它植物类群的一个显著特征就是花,因此,解决被子植物的起源之谜很大程度上取决于对被子植物花器官起源的研究,对被子植物花器官的详尽研究已经在形态、解剖、古植物、形态发生、分子等方面积累了大量的证据,植物学家基于这些证据为被子植物花器官的起源提出了各种各样的解释。综述了迄今为止被子植物花器官起源的主要学说流派,如：真花学说、假花学说、生殖叶学说、生殖茎节学说、生花植物学说、新假花学说、古草本学说和ANITA学说等。根据研究手段和获得证据的方式。作者将被子植物花器官起源研究划分为5个阶段,并简要阐述了各个阶段的代表学说和主要研究特点。     

被子植物花的起源:假说和证据

达尔文的 令人讨厌之谜 ,即被子植物的起源和早期演化 ,一直是植物系统学研究领域的热点 .被子植物区别于其它植物类群的一个显著特征就是花 ,因此 ,解决被子植物的起源之谜很大程度上取决于对被子植物花器官起源的研究 .对被子植物花器官的详尽研究已经在形态、解剖、古植物、形态发生、分子等方面积累了大量的证据 ,植物学家基于这些证据为被子植物花器官的起源提出了各种各样的解释 .综述了迄今为止被子植物花器官起源的主要学说流派 ,如 :真花学说、假花学说、生殖叶学说、生殖茎节学说、生花植物学说、新假花学说、古草本学说和 ANITA学说等 .根据研究手段和获得证据的方式 ,作者将被子植物花器官起源研究划分为 5个阶段 ,并简要阐述了各个阶段的代表学说和主要研究特点     

马尾松Ls-rDNA 5'末端序列分析及其系统学意义

马尾松Ls-rDNA 5‘末端302个核苷酸序列已被确定,与4种裸子植物和4种被子植物及一种绿藻的同源序列进行比较分析,其所构建的Ls-rDNA系统树图表明,传统分类中的裸子植物与被子植物明显成为单系类群,支持裸子植物的两个单系谱支,即苏铁目—银杏目,买麻藤目—松柏类。Ls-rDNA 5’末端部分序列分析在种子植物高等级分类群系统进化研究中具有重要作用。     

被子植物起源研究中几种观点的思考

对被子植物起源研究中的几种观点进行了讨论。(1)由于被子植物存在着一组共同的性状,它们不可能是从不同祖先起源的,而是有着共同的祖先。被子植物是一个单源起源的类群。现存被子植物分类系统是依据包括形态学(广义)、分子系统学、古植物学和植物地理学等的综合性状建立的,只能表示出现存类群的亲缘关系并且追溯到它们最近的祖先。人们现在还不可能建立一个包括全部已绝灭的类群和现代生存类群的谱系发生系统。因此,现存被子植物分类系统只能看作是“亲缘”系统。(2)分析了用于推测被子植物起源时间的分子、化石和地理分布证据。我们认为,要确定被子植物起源时间,植物化石是一类重要证据,但化石只能说是植物本身可保存部分和当时当地所提供的化石条件的综合反映,它们不可能就是植物类群或种的起源时间。人们还必须考虑到化石本身的演化历史。应用分子钟也是一种手段,但误差比较大。如果我们除了利用上述两种资料之外,根据植物类群的现代分布格局及其形成,把植物的演化同地球的历史和板块运动联系起来,以推断它们起源的时间,这无疑会增加其可信度。通过对56个种子植物不同演化水平的重要科属地理分布的研究结果,我们曾提出被子植物的起源时间可能要追溯到早侏罗世,甚至晚三叠世。(3)分析了基于分子证据所提出的被子植物基部类群——ANITA成员(包括无油樟科Amborellaceae、睡莲科Nymphaeaceae、八角目Illiciales、早落瓣科Trimeniaceae、木兰藤科Austrobaileyaceae)的性质,讨论了ANITA成员在现代几个被子植物分类系统中的系统位置的不同观点,评价了它们的形态学(广义)性状。指出ANITA的成员由于包含大量的祖征,是属于原始的类群。但由于它们的共有衍征很少,如花粉球形,说明它们在被子植物演化早期就分道扬镳了,沿着不同的传代线分化。因此ANITA是一个源于不同传代线的复合群。     

木兰藤科系统位置评述

木兰藤科(Austrobaileyaceae)含1属2种,是系统学上最孤立的科之一。其花粉类似于最古老的被子植物化石之一:晚白垩世的棒纹粉。最新的分子系统发育研究结果表明,木兰藤科是现存被子植物的基部类群之一,其对于被子植物的起源与早期进化的研究具有重要价值。被子植物(有花植物)的起源和辐射一直是植物学家关注的热点。有关木兰藤科的系统位置一直存在争议。本文对该科系统位置的研究历史与现状进行评述。     

金鱼藻科系统位置评述

金鱼藻科(ceratophyllaceae)含1属7种,广布全世界。形态学研究显示,金鱼藻科具有许多难以解释的性状,与其它类群无法比较;最新的分子系统发育研究显示,金鱼藻科是现存被子植物的基部类群之一;有关金鱼藻科的系统位置存在争议,被子植物(有花植物)的起源与辐射一直是植物系统学家关注的热点。本文对该科系统位置的研究历史与现状进行评述。     

被子植物系统学中花发育研究的进展及对今后研究的思考

从花发育研究的方法、花发育与被子植物花部结构的多样性、花发育与被子植物的系统发育以及 花发育的分子遗传学等四个方面对近年来被子植物系统学中花发育研究的主要进展作一综述,例举了 一些重要结果。同时,对该领域今后研究的方向和应注意的一些问题作了简要评论。作者认为植物的 形态结构可以看作是一个时空过程,在系统学研究中对花部性状的分析和认识应该树立动态的观点。 今后应该从动态的角度开展被子植物花的发生和发育以及性状在不同类群间的比较等方面的广泛研究,并加强对在被子植物花的起源和演化中起重要作用的花部同源异型现象的发育过程的观察。     

腺齿木科系统位置评述

腺齿木科(Trimeniaceae)含2属5种。形态学研究显示腺齿木科具有许多原始性状。最新的分子系统发育研究显示,腺齿木科是现存被子植物的重要基部类群之一。但有关腺齿木科的系统位置存在争议。被子植物(有花植物)的起源与辐射一直是植物系统学家关注的热点。对该科系统位置的研究历史与现状进行评述。     

单子叶植物科级分类阶元形态性状分支分析

基于93个形态形状,采用13个被子植物基部类群做为外类群,对49个单子叶植物科级分类阶元进行了分支系统学分析。经过简约性分析,得到了1684棵同等最大简约分支树。严格一致树的分支结构图表明：1）古草本类植物和单子叶植物是姐妹群关系;2）具有网状脉的类群,薯蓣科,菝葜科,百部科是单子叶植物的最基部类群。由于性状状态间存在着较多的平行和逆转进化,这在一定程度上影响了系统发育重建的准确性;所选择的性状状态之间的演化很可能是平行的、多次的或者是特化的状态,因此这样复杂的演化关系的探索关键在于找到一些能确切反映其系统演化关系的形态性状。目前很难通过简约化的形态分支分析来解开整个单子叶植物的起源和演化之谜。为了避开对系统学分析造成干扰的误导性状,形态数据结合DNA序列分析很可能是必需的。     

PHYLOGENETIC RELATIONSHIPS IN SEED PLANTS

Abstract— The phylogenetic relationships of nineteen extant and fossil seed plants are considered. Analysis of 31 characters produced ten topologically similar and equally parsimonious cladograms. A strict consensus tree derived from these cladograms places Lyginopteris as the sister taxon to the other seed plants included. Within this clade all the taxa considered, except medullosans and cycads, form a single monophyletic group defined by the presence of flattened seeds and saccate pollen ("platy-sperms"). Relationships between medullosans, cycads, and "platysperms" were not resolved, but within the "platysperm" clade conifers and cordaites ( Cordaixylon, Mesoxylon ) + Ginkgo form a monophyletic group ("coniferophytes"). The "higher platysperms" (glossopterids, Caytonia , corystosperms, Bennettitales, Pentoxylon , Gnetales, and angiosperms) are also monophyletic, but their relationship to "coniferophytes," peltasperms, and Callistophyton is unresolved. Pentoxylon is placed as sister taxon to the Bennettitales, and together they form the sister group to a clade in which Gnetales and angiosperm are sister taxa. The Bennettitales + Pentoxylon + Gnetales + angiosperms ("anthophytes") form a monophyletic sister group to the corystosperms. This analysis is compared with current classifications of seed plants. It does not support a close relationship between Bennettitales and cycads, it provides no evidence for seed plant polyphyly, and it strongly suggests that the current concept of seed ferns has little value in a phylogenetic context.     

Is the anthophyte hypothesis alive and well? New evidence from the reproductive structures of Bennettitales

Bennettitales is an extinct group of seed plants with reproductive structures that are similar in some respects to both Gnetales and angiosperms, but systematic relationships among the three clades remain controversial. This study summarizes characters of bennettitalean plants and presents new evidence for the structure of cones and seeds that help clarify relationships of Bennettitales to flowering plants, Gnetales, and other potential angiosperm sister groups. Bennettitales have simple mono- or bisporangiate cones. Seeds are borne terminally on sporophylls. They have a unique structure that includes a nucellus with a solid apex, no pollen chamber, and a single integument, and they are clearly not enclosed by a cupule or other specialized structures. Such features differ substantially from Gnetales, flowering plants, and the seed fern Caytonia, providing no compelling evidence for the origin of the angiospermous carpel. Cladistic tests were performed to assess the strength of the "anthophyte hypothesis" and possible relationships of Bennettitales, Gnetales, and Caytonia to flowering plants. Our results do not support the anthophyte hypothesis for the origin of angiosperms by a transformation of fertile organs that were already aggregated into a cone or flower-like structure. However, the anthophyte topology of the seed plant tree continues to be supported by morphological analyses of living and extinct taxa.     

Phylogeny of seed plants based on evidence from eight genes

Relationships among the five groups of extant seed plants (cycads, Ginkgo, conifers, Gnetales, and angiosperms) remain uncertain. To explore relationships among groups of extant seed plants further and to attempt to explain the conflict among molecular data sets, we assembled a data set of four plastid (cpDNA) genes (rbcL, atpB, psaA, and psbB), three mitochondrial (mtDNA) genes (mtSSU, coxI, and atpA), and one nuclear gene (18S rDNA) for 19 exemplars representing the five groups of living seed plants. Analyses of the combined eight-gene data set (15?772 base pairs/taxon) with maximum parsimony (MP), maximum likelihood (ML), and Bayesian approaches reveal a gymnosperm clade that is sister to angiosperms. Within the gymnosperms, a conifer clade includes Gnetales as sister to Pinaceae. Cycads and Ginkgo are either successive sisters to this conifer clade (including Gnetales) or a clade that is sister to conifers and Gnetales. All analyses of the mtDNA partition and ML analyses of the nuclear partition yield very similar topologies. However, MP analyses of the combined cpDNA genes place Gnetales as sister to all other seed plants with strong bootstrap support, whereas ML and Bayesian analyses of the cpDNA data set place Gnetales as sister to Pinaceae. Maximum parsimony and ML analyses of first and second codon positions of the cpDNA partiation also place Gnetales as sister to Pinaceae. In contrast, MP analyses of third codon positions place Gnetales as sister to other seed plants, although ML analyses of third codon positions place Gnetales with Pinaceae. Thus, most of the discrepancies in seed plant topologies involve third codon positions of cpDNA genes. The likelihood ratio (LR) and Shimodaira-Hasegasa (SH) tests were applied to the cpDNA data. The preferred topology based on the LR test is that Gnetales are sister to Pseudotsuga. The SH test based on first and second codon and all three codon positions indicated that there is no significant difference between the best topology (Gnetales sister to Pseudotsuga) and Gnetales sister to a conifer clade. However, there is a significant difference between the best topology and topologies in which Gnetales are sister to the rest of the seed plants or Gnetales sister to angiosperms.     

Error, bias, and long-branch attraction in data for two chloroplast photosystem genes in seed plants

Sequences of two chloroplast photosystem genes, psaA and psbB, together comprising about 3,500 bp, were obtained for all five major groups of extant seed plants and several outgroups among other vascular plants. Strongly supported, but significantly conflicting, phylogenetic signals were obtained in parsimony analyses from partitions of the data into first and second codon positions versus third positions. In the former, both genes agreed on a monophyletic gymnosperms, with Gnetales closely related to certain conifers. In the latter, Gnetales are inferred to be the sister group of all other seed plants, with gymnosperms paraphyletic. None of the data supported the modern "anthophyte hypothesis," which places Gnetales as the sister group of flowering plants. A series of simulation studies were undertaken to examine the error rate for parsimony inference. Three kinds of errors were examined: random error, systematic bias (both properties of finite data sets), and statistical inconsistency owing to long-branch attraction (an asymptotic property). Parsimony reconstructions were extremely biased for third-position data for psbB. Regardless of the true underlying tree, a tree in which Gnetales are sister to all other seed plants was likely to be reconstructed for these data. None of the combinations of genes or partitions permits the anthophyte tree to be reconstructed with high probability. Simulations of progressively larger data sets indicate the existence of long-branch attraction (statistical inconsistency) for third-position psbB data if either the anthophyte tree or the gymnosperm tree is correct. This is also true for the anthophyte tree using either psaA third positions or psbB first and second positions. A factor contributing to bias and inconsistency is extremely short branches at the base of the seed plant radiation, coupled with extremely high rates in Gnetales and nonseed plant outgroups.     

The evolutionary split of Pinaceae from other conifers: evidence from an intron loss and a multigene phylogeny.

The second intron in the mitochondrial gene nad1 was surveyed using PCR, DNA sequencing, or Southern hybridization in 323 species (313 genera, 212 families) of seed plants. The intron was absent in all 22 species (22 genera, 8 families) of non-Pinaceae conifers studied, in Welwitschia mirabilis, and in seven angiosperms. Whereas absence of the intron in seven angiosperms and Welwitschia is likely due to seven independent losses when evaluated against the recently published multigene phylogenies, the lack of the intron in all non-Pinaceae conifers can be best explained by a single loss. These data suggest that the non-Pinaceae conifers represent a monophyletic group. We also conducted a phylogenetic analysis of seed plants using a combined data set of the partial exon and intron sequences of nad1 generated from this study and published sequences of mitochondrial cox1 and small subunit (SSU) rDNA, chloroplast rbcL, and nuclear 18S rDNA. The results supported the split of conifers into two groups: Pinaceae and non-Pinaceae conifers. The Gnetales were sister to Pinaceae, in agreement with the conclusion from other recent molecular phylogenetic studies that refute the anthophyte hypothesis.     

Morphogenesis of the reproductive shoots of Welwitschia mirabilis and Ephedra distachya (Gnetales), and its evolutionary implications

For decades, Gnetales appeared to be closely related to angiosperms, the two groups together forming the anthophyte clade. At present, molecular studies negate such a relationship and give strong support for a systematic position of Gnetales within or near conifers. However, previous interpretations of the male sporangiophores of Gnetales as pinnate with terminal synangia conflict with a close relationship between Gnetales and conifers. Therefore, we investigated the morphogenesis of the male reproductive structures of Welwitschia mirabilis and Ephedra distachya by SEM and light microscopy. The occurrence of reduced apices to both halves of the antherophores of W. mirabilis gives strong support for the assumption that the male ‘flowers’ of W. mirabilis represent reduced compound cones. We assume that each half of the antherophore represents a lateral male cone that has lost its subtending bract. Although both halves of the antherophores of Ephedra distachya lack apical meristems, the histological pattern of the developing antherophores supports interpreting them as reduced lateral male cones as well. Therefore, the male sporangiophores of Gnetales represent simple organs with terminal synangia. Although extant conifers do not exhibit terminal synangia, similar sporangiophores are reported for some Cordaitales, the hypothetical sister group of conifers. Moreover, several Paleozoic conifers exhibit male cones with terminal sporangia or synangia. Therefore, we propose that conifers, Cordaitales and Gnetales originated from a common ancestor that displayed simple sporangiophores with a terminal cluster of sporangia.     

Perspective: the origin of flowering plants and their reproductive biology--a tale of two phylogenies

Recently, two areas of plant phylogeny have developed in ways that could not have been anticipated, even a few years ago. Among extant seed plants, new phylogenetic hypotheses suggest that Gnetales, a group of nonflowering seed plants widely hypothesized to be the closest extant relatives of angiosperms, may be less closely related to angiosperms than was believed. In addition, recent phylogenetic analyses of angiosperms have, for the first time, clearly identified the earliest lineages of flowering plants: Amborella, Nymphaeales, and a clade that includes Illiciales/ Trimeniaceae/Austrobaileyaceae. Together, the new seed plant and angiosperm phylogenetic hypotheses have major implications for interpretation of homology and character evolution associated with the origin and early history of flowering plants. As an example of the complex and often unpredictable interplay of phylogenetic and comparative biology, we analyze the evolution of double fertilization, a process that forms a diploid embryo and a triploid endosperm, the embryo-nourishing tissue unique to flowering plants. We demonstrate how the new phylogenetic hypotheses for seed plants and angiosperms can significantly alter previous interpretations of evolutionary homology and firmly entrenched assumptions about what is synapomorphic of flowering plants. In the case of endosperm, a solution to the century-old question of its potential homology with an embryo or a female gametophyte (the haploid egg-producing generation within the life cycle of a seed plant) remains complex and elusive. Too little is known of the comparative reproductive biology of extant nonflowering seed plants (Gnetales, conifers, cycads, and Ginkgo) to analyze definitively the potential homology of endosperm with antecedent structures. Remarkably, the new angiosperm phylogenies reveal that a second fertilization event to yield a biparental endosperm, long assumed to be an important synapomorphy of flowering plants, cannot be conclusively resolved as ancestral for flowering plants. Although substantive progress has been made in the analysis of phylogenetic relationships of seed plants and angiosperms, these efforts have not been matched by comparable levels of activity in comparative biology. The consequence of inadequate comparative biological information in an age of phylogenetic biology is a severe limitation on the potential to reconstruct key evolutionary historical events.     

Seed plant phylogeny: Demise of the anthophyte hypothesis?

Recent molecular phylogenetic studies indicate, surprisingly, that Gnetales are related to conifers, or even derived from them, and that no other extant seed plants are closely related to angiosperms. Are these results believable? Is this a clash between molecules and morphology?     

Evolution of Reproductive Organs in Land Plants

LEAFY gene is the positive regulator of the MADS-box genes in flower primordia. The number of MADS-box genes presumably increased by gene duplications before the divergence of ferns and seed plants. Most MADS-box genes in ferns are expressed similarly in both vegetative and reproductive organs, while in gymnosperms, some MADS-box genes are specifically expressed in reproductive organs. This suggests that (1) the increase in the number of MADS-box genes and (2) the subsequent recruitment of some MADS-box genes as homeotic selector genes were important for the evolution of complex reproductive organs. The phylogenetic tree including both angiosperm and gymnosperm MADS-box genes indicates the loss of the A-function genes in the gymnosperm lineage, which is presumably related to the absence of perianths in extant gymnosperms. Comparison of expression patterns of orthologous MADS-box genes in angiosperms, Gnetales, and conifers supports the sister relationship of Gnetales and conifers over that of Gnetales and angiosperms predicted by phylogenetic trees based on amino acid and nucleotide sequences. Received 30 July 1999/ Accepted in revised form 9 September 1999     

Fossils and seed plant phylogeny reanalyzed

In a cladistic analysis of Recent seed plants, Loconte and Stevenson (1990) obtained results that conflict with our 1986 analysis of both extant and fossil groups and argued that fossil data had led us to incorrect conclusions. To explore this result and the general influence of fossils on phylogeny reconstruction, we assembled new “Recent” and “Complete” (extant plus fossil) data sets incorporating new data, advances in treatment of characters, and those changes of Loconte and Stevenson that we consider valid. Our Recent analysis yields only one most parsimonious tree, that of Loconte and Stevenson, in which conifers are linked with Gnetales and angiosperms (anthophytes), rather than with Ginkgo, as in our earlier Recent and Complete analyses. However, the shortest trees derived from our Complete analysis show five arrangements of extant groups, including that of Loconte and Stevenson and our previous arrangements, suggesting that the result obtained from extant taxa alone may be misleading. This increased ambiguity occurs because features that appear to unite extant conifers and anthophytes are seen as convergences when fossil taxa are interpolated between them. All trees found in the Complete analysis lead to inferences on character evolution that conflict with those that would be drawn from Recent taxa alone (e.g., origin of anthophytes from plants with a “seed fern” morphology). These results imply that conclusions on many aspects of seed plant phylogeny are premature; new evidence, which is most likely to come from the fossil record, is needed to resolve the uncertainties.