Toward deciphering the phylotranscriptomic hourglass in plant embryogenesis

胚胎发生是复杂的多细胞生命所必需的发育过程。胚胎发生的一种基本模式，即发育沙漏，可以追溯到19世纪初卡尔·恩斯特·冯·贝尔关于脊椎动物的开创性工作。发育沙漏描述了一种看似普遍的模式，即来自不同类群的动物胚胎在早期阶段看起来不同，在胚胎发育中期汇聚成类似的形式，然后在后期再次分化。这种形态模式最近得到了分子支持，因为它证明了中期胚胎发生在转录水平上也是最保守的。有趣的是，经典的形态沙漏模式从未在植物谱系中报道过，植物是第二个进化胚胎发生的王国。然而，通过结合系统发育和转录信息，我们最近能够证明在模式植物拟南芥的胚胎发生过程中存在系统转录沙漏。由于动植物的最后一个共同祖先是单细胞的，这强烈表明转录切割沙漏的收敛进化是控制进化年轻或快速进化的基因跨王国表达的基本发育轮廓。虽然我们推测这种机制可能是实现复杂多细胞生命的时空组织和分化所必需的，但我们还不清楚沙漏模式的分子起源。在这个建议中，我们的目的是调查系统转录分割沙漏模式是否也存在于其他植物物种中，或者它是否是模式植物A.thaliana所特有的。我们将通过生成RNA-SEQ数据集来解决这个问题，该数据集涵盖A.thaliana和另外两个物种的胚胎发生，这两个物种是近缘关系密切的十字花科植物Capsella Rubella和远亲关系较远的苔藓Physcomitrella patens。我们将把这些转录数据与各种进化、系统基因组和序列信息结合起来，以揭示形成这三个物种系统转录模式的因素。通过应用信息论方法，我们希望开始了解沙漏或植物胚胎发生中可能的其他模式的分子起源和进化。

Embryogenesis is an essential developmental processes that enables complex multicellular life. A fundamental pattern of embryogenesis, the developmental hourglass, dates back to pioneering work by Karl Ernst von Baer on vertebrates in the early 19th century. The developmental hourglass describes a seemingly universal pattern that animal embryos from various taxa appear different in early stages, converge to a similar form during mid-embryogenesis, and again diverge in later stages. This morphological pattern has recently received molecular support by demonstrating that mid-embryogenesis is also maximally conserved on the transcriptomic level.Interestingly, the classic morphological hourglass pattern had never been reported for the plant lineage, the second kingdom that evolved embryogenesis. However, by combining phylogenetic and transcriptional information we were recently able to demonstrate the existence of a phylotranscriptomic hourglass during embryogenesis of the model plant Arabidopsis thaliana. As the last common ancestor of plants and animals was unicellular, this strongly suggested convergent evolution of the transcriptomic hourglass as a fundamental developmental profile controlling the expression of evolutionarily young or rapidly evolving genes across kingdoms. While we speculate that such a mechanism may be required for enabling spatio-temporal organization and differentiation of complex multicellular life in general, we do not yet understand the molecular origin responsible for the hourglass pattern. In this proposal, we aim to investigate whether the phylotranscriptomic hourglass pattern also exists in other plant species or whether it is specific for the model plant A. thaliana. We will address this question by generating RNA-seq data sets that cover embryogenesis in A. thaliana and two additional species, the closely related Brassicaceae Capsella rubella and the distantly related moss Physcomitrella patens. We will combine these transcriptional data with various evolutionary, phylogenomic, and sequence information to unravel factors that shape phylotranscriptomic patterns in these three species. By applying information theoretic approaches, we expect to begin to understand the molecular origin and evolution of the hourglass or possible other patterns in plant embryogenesis.