Evolution of Double Fertilization and Endosperm Development in Basal Angiosperms

随着维管植物和种子植物的起源，开花植物（被子植物）的起源代表了过去4.5亿年中陆地植物三个最重要的进化辐射之一。被子植物有超过250，000种现存物种，是最大和最多样化的植物群。 巧合的是，我们对开花植物的起源和早期进化史的了解，比对许多相当古老的陆生植物群体的了解要少。 事实上，达尔文关于开花植物的起源和早期进化的“令人憎恶的奥秘”，继续挑战着进化生物学家。 100年来，“双受精”过程产生一个二倍体胚胎和一个称为胚乳的三倍体胚胎营养组织，这被认为是被子植物的一个定义性特征。 虽然胚乳是被子植物普遍存在的特征，但精子参与第二次受精以产生胚乳的行为仅在相对少量的单子叶植物和真双子叶植物分类群中被仔细记录。 很明显，从最近的系统发育分析的基础被子植物（和植物受精生物学文献的调查），第二次受精事件，长期以来一直被认为是一个重要的和定义功能的显花植物，还没有得到最终的记录和特点，在任何pupestrian基础被子植物。 这在我们对“原始”被子植物受精生物学最基本方面的知识中提出了一个巨大的缺口。 拟议的研究旨在确定第二次受精事件是否与基部被子植物胚乳组织的起始有关;或者某些基部被子植物的胚乳组织是否严格起源于母体。 现代细胞生物学方法将被用来研究和文件的存在或不存在的双受精在一个广泛的横截面的基础被子植物谱系。 如果发现双受精作用不是被子植物的突触形态（而是被子植物的一个子集的突触形态），这将对胚乳进化的假设产生重大影响，以及关于开花植物中这种独特的胚胎营养组织可能的适应价值（关键创新地位）的想法。 拟议的研究的第二个组成部分，旨在确定胚乳发育模式的多样化在开花植物的早期进化辐射的发展基础。 在过去的世纪里，胚乳的结构被分为三种类型（游离核、细胞核、日形核）。 然而，从来没有分析过一种模式转变为另一种模式的发展基础。 因此，很少有人知道的关键字符转换和相关的修改的基本方面的发展（即有丝分裂和胞质分裂，极性，对称性和细胞自主命运的关系）。对胚乳发育的系统发育研究将为解释胚乳的多样性和进化提供新的范式。 在“原始”有花植物中，为了解受精生物学和胚乳发育的比较和进化方面而进行的共同努力以前从未被尝试过。 因此，对被子植物的早期进化和繁殖模式的多样化几乎一无所知。 这种生殖功能的比较分析，从关键的基础被子植物谱系将解决重要的问题的字符分布和进化显着的进化辐射显着的开花植物，并将标志着第一次明确的尝试，以限制最基本的方面受精生物学和胚乳发育在最早的被子植物。

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.