Evolution and development of land plant embryos

陆地植物大约在4.7亿年前从水生绿色藻类进化而来。从水到陆地的过渡是由一些特征的进化促进的，这些特征允许在相对恶劣的陆地环境中生长和生存。这些获得的特征之一是胚胎，这是一个非常基本的特征，所有的陆地植物都被统称为“胚植物”。分子和形态分析表明，轮藻是与陆生植物关系最密切的藻类类群。轮藻纲绿色藻类展示了一系列的身体计划，从单细胞到高度分支的多细胞结构，但这些都是在生命周期的单倍体（配子体）世代中发现的。轮藻生命周期的二倍体（孢子体）阶段是不变的，并且在两个单倍体交配类型之间受精后形成单细胞合子，然后第一次合子细胞分裂是“减数分裂”-使两个子细胞中的染色体数目减半以形成单倍体孢子，所述单倍体孢子萌发以形成新的配子体。相比之下，所有陆地植物中的合子都经历多次“有丝分裂”细胞分裂，保留每个子细胞中的染色体数目，并产生多细胞二倍体胚胎，发育成孢子体。孢子体最初经历营养阶段，其可以持续数天（例如在苔藓中）或数年（例如在树木中），然后转化为生殖模式并经历减数分裂以产生单倍体配子体。因此，胚胎的进化需要通过一种机制来促进受精卵的有丝分裂和/或延迟减数分裂，在受精卵形成和新配子体的产生之间插入营养阶段。在苔藓的生命周期中的主要转变的五个监管机构先前已被确定的突变体分析[1-5]。通过转基因分析，我们的目标是表征这些调节器在轮藻中的祖先作用，并确定它们在胚胎发育中的作用在苔藓和地钱之间保守的程度。此外，我们的目标是确定新的监管机构的胚胎图案，通过分析的数据集，区分基因表达的轮藻单细胞合子从那些在早期胚胎的苔藓和/或苔类植物。在胚胎中表达但在单细胞合子中不表达的五个基因的功能将通过苔藓和地钱中的转基因分析来测试。结合起来，这些结果将提供对关键发育调节剂的作用如何随着藻类和陆地植物谱系的分化而改变的理解，产生对基本发育过程的机械理解，并提供一个比较框架，在该框架中测试多细胞陆地植物孢子体如何进化的新假设。2）Sakakibara K等人（2013）Science 339，1067 3）Tanahashi T等人（2005）Development 132，1727 4）Okana Y等人（2009）PNAS 106，16321 5）Mosquna A等人（2009）Development 136，2433.

Land plants evolved from aquatic green algae around 470 million years ago. The transition from water to land was facilitated by the evolution of a number of traits that permitted growth and survival in the relatively harsh terrestrial environment. One of these acquired traits was the embryo, a feature so fundamental that all land plants are collectively known as 'embryophytes'. Both molecular and morphological analyses reveal that the algal group that is most closely related to the land plants is the charophytes. Charophycean green algae exhibit a range of body plans, ranging from single cells to highly branched multicellular structures, but these are all found in the haploid (gametophyte) generation of the lifecycle. The diploid (sporophyte) phase of the charophyte lifecycle is invariant and unicellular - zygotes are formed after fertilization between two haploid mating types and then the first zygotic cell division is 'meiotic' - halving the chromosome number in the two daughter cells to form haploid spores that germinate to form new gametophytes. By contrast, the zygote in all land plants undergoes multiple 'mitotic' cell divisions that retain the chromosome number in each daughter cell and create a multicellular diploid embryo that develops into a sporophyte body. The sporophyte initially goes through a vegetative phase that can persist for days (e.g. in mosses) or years (e.g. in trees) before converting to reproductive mode and undergoing meiosis to produce haploid gametophytes. The evolution of the embryo thus required the adoption of a mechanism to promote mitosis and/or delay meiosis in the zygote, intercalating a vegetative phase between zygote formation and the production of new gametophytes.This proposal aims to investigate the mechanisms that regulate embryo formation in two of the earliest divergent land plant lineages - the liverworts and the mosses. Five regulators of major transitions in the moss lifecycle have previously been identified by mutant analysis [1-5]. Through transgenic analysis, we aim to characterize the ancestral role of these regulators in charophyte algae, and determine the extent to which their role in embryo development is conserved between mosses and liverworts. In addition, we aim to identify novel regulators of embryo patterning, through the analysis of a dataset that distinguishes genes expressed in unicellular zygotes of charophyte algae from those expressed in early stage embryos of mosses and/or liverworts. The function of five genes that are expressed in embryos but not in single-celled zygotes will be tested by transgenic analysis in mosses and liverworts. In combination, the results will provide an understanding of how the role of key developmental regulators was altered as algal and land plant lineages diverged, generate a mechanistic understanding of a fundamental developmental process, and provide a comparative framework in which to test new hypotheses of how multicellular land plant sporophytes evolved.1) Sakakibara K et al (2008) Evol Dev 10, 555 2) Sakakibara K et al (2013) Science 339, 1067 3) Tanahashi T et al (2005) Development 132, 1727 4) Okana Y et al (2009) PNAS 106, 16321 5) Mosquna A et al (2009) Development 136, 2433.