Evolution of Double Fertilization and Endosperm Development in Basal Angiosperms

FRIEDMAN9816107与维管植物和种子植物的起源一样，开花植物(被子植物)的起源代表了过去4.5亿年来陆地植物最重要的三次进化辐射之一。被子植物有超过25万个现存物种，是进化出的最大和最多样化的植物群体。矛盾的是，我们对开花植物的起源和早期进化史的了解比我们对许多相当古老的陆地植物群体的了解要少。事实上，达尔文关于开花植物起源和早期进化的“令人讨厌的谜团”继续挑战着进化生物学家。100年来，产生二倍体胚胎和三倍体胚胎营养组织(称为胚乳)的“双受精”过程一直被认为是被子植物的一个定义特征。虽然胚乳是被子植物普遍存在的一个特征，但精子参与第二次受精以产生胚乳的情况在相对较少的单子叶和真子叶分类群中被仔细地记录下来。从最近对基生被子植物的系统发育分析(和植物受精生物学文献的综述)可以清楚地看出，第二次受精事件长期以来被认为是开花植物的一个重要和决定性的特征，但在任何假定的基生被子植物中还没有确凿的证据和特征。这在我们对以“原始”被子植物为特征的受精生物学的最基本方面的了解上存在着巨大的差距。这项拟议的研究试图确定第二次受精事件是否与基生被子植物中胚乳组织的启动有关；或者某些基生被子植物中的胚乳组织是否严格地起源于母体。现代细胞生物学方法将被用来研究和记录在基本被子植物谱系的广泛横截面上存在或不存在双受精现象。如果发现双受精不是被子植物的同体(而是被子植物的子集的同体)，这将对胚乳进化的假说以及关于这种独特的开花植物胚胎营养组织可能的适应价值(关键创新状态)的想法产生重大影响。拟议研究的第二部分试图确定开花植物早期进化辐射期间胚乳发育模式多样化的发育基础。在过去的一个世纪里，三种类型分类被用来划分胚乳结构(例如，游离核、细胞性、日耳型)。然而，从来没有对一种模式转变为另一种模式的发展基础进行分析。因此，对关键性状的转变和发育基本方面的相关修改(即有丝分裂和胞质分裂的关系、极性、对称性和细胞自主命运)知之甚少。在基本被子植物的系统发育谱上研究胚乳发育将为解释胚乳的多样性和进化提供新的范式。在“原始”开花植物中，以前从未尝试过理解受精生物学和胚乳发育的比较和进化方面的共同努力。因此，对被子植物繁殖模式的早期进化和多样化几乎一无所知。这种对主要基本被子植物谱系生殖特征的比较分析将解决与开花植物显着的进化辐射相关的特征分布和进化的重要问题；并将标志着首次明确地尝试限制最早的被子植物受精、生物学和胚乳发育的最基本方面。

FRIEDMAN9816107Along with the origins of vascular plants and seed plants, the origin of flowering plants (angiosperms) represents one of the three most significant evolutionary radiations of land plants during the last 450 million years. With over 250,000 extant species, angiosperms are the largest and most diverse group of plants to have evolved. Paradoxically, we know less about the origin and early evolutionary history of flowering plants than we do about many considerably older groups of land plants. Indeed Darwin's "abominable mystery" of the origin and early evolution of flowering plants, continues to challenge evolutionary biologists. For 100 years, a "double fertilization" process that yields a diploid embryo and a triploid embryo-nourishing tissue called endosperm has been considered a defining feature of angiosperms. Although endosperm is known to be a ubiquitous feature of angiosperms, the participation of a sperm in a second act of fertilization to produce endosperm has been carefully documented in only a relatively small number of derived monocot and eudicot taxa. It is clear from recent phylogenetic analyses of basal angiosperms (and a survey of the plant fertilization biology literature) that a second fertilization event, long assumed to be an important and defining feature of flowering plants, has yet to be conclusively documented and characterized in any putatively basal angiosperm. This presents an enormous gap in our knowledge of the most basic aspects of fertilization biology that characterize "primitive" angiosperms. The proposed research seeks to determine whether a second fertilization event is associated with the initiation of endosperm tissue in basal angiosperms; or whether endosperm tissue in some basal angiosperms is strictly maternal in origin. Modern cell biological approaches will be used to study and document the presence or absence of double fertilization in a broad cross section of basal angiosperm lineages. If it is discovered that double fertilization is not a synapomorphy of angiosperms (and is instead a synapomorphy of a subset of angiosperms), this will have a significant impact on hypotheses of endosperm evolution, and ideas concerning the possible adaptive value (key innovation status) of this unique embryo-nourishing tissue in flowering plants. A second component of the proposed research seeks to determine the developmental basis for the diversification of endosperm developmental patterns during the early evolutionary radiation of flowering plants. Three typological categories have been used to circumscribe endosperm structure (e.g. free nuclear, cellular, helobial) for the past century. However, there has never been an analysis of the developmental basis for transformations of one pattern into another. Thus, little is known about key character transitions and the associated modifications of basic aspects of development (i.e. relationship of mitosis and cytokinesis, polarity, symmetry, and cell autonomous fates). Study of endosperm development across the phylogenetic spectrum of basal angiosperms will lead to new paradigms for the interpretation of the diversification and evolution of endosperm. A concerted effort to understand comparative and evolutionary aspects of fertilization biology and endosperm development in "primitive" flowering plants has never been before been attempted. As a consequence, virtually nothing is known of the early evolution and diversification of patterns of reproduction among angiosperms. This comparative analysis of reproductive features from key basal angiosperm lineages will address important issues of character distribution and evolution associated with the remarkable evolutionary radiation of flowering plants; and will mark the first explicit attempt to circumscribe the most basic aspects of fertilization biology and endosperm development in the earliest angiosperms.