Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138.
Whitehead Institute for Biomedical Research, Cambridge, MA 02142.
Proc Natl Acad Sci U S A. 2020 Apr 14;117(15):8649-8656. doi: 10.1073/pnas.1922873117. Epub 2020 Mar 31.
For more than 225 million y, all seed plants were woody trees, shrubs, or vines. Shortly after the origin of angiosperms ∼140 million y ago (MYA), the Nymphaeales (water lilies) became one of the first lineages to deviate from their ancestral, woody habit by losing the vascular cambium, the meristematic population of cells that produces secondary xylem (wood) and phloem. Many of the genes and gene families that regulate differentiation of secondary tissues also regulate the differentiation of primary xylem and phloem, which are produced by apical meristems and retained in nearly all seed plants. Here, we sequenced and assembled a draft genome of the water lily , an emerging system for the study of early flowering plant evolution, and compared it to genomes from other cambium-bearing and cambium-less lineages (e.g., monocots and ). This revealed lineage-specific patterns of gene loss and divergence. is characterized by a significant contraction of the HD-ZIP III transcription factors, specifically loss of , which influences cambial activity in other angiosperms. We also found the and monocot copies of cambium-associated CLE signaling peptides display unique substitutions at otherwise highly conserved amino acids. displays no obvious divergence in cambium-associated genes. The divergent genomic signatures of convergent loss of vascular cambium reveals that even pleiotropic genes can exhibit unique divergence patterns in association with independent events of trait loss. Our results shed light on the evolution of herbaceousness-one of the key biological innovations associated with the earliest phases of angiosperm evolution.
超过 2.25 亿年来，所有种子植物都是木本树木、灌木或藤本植物。大约在 1.4 亿年前被子植物起源后不久，Nymphaeales（睡莲）成为最早偏离其祖先木本习性的谱系之一，失去了维管形成层，即产生次生木质部（木材）和韧皮部的分生细胞群体。许多调节次生组织分化的基因和基因家族也调节初生木质部和韧皮部的分化，初生木质部和韧皮部由顶端分生组织产生，并保留在几乎所有种子植物中。在这里，我们对睡莲进行了测序和组装，睡莲是研究早期开花植物进化的新兴系统，并将其与具有和不具有形成层的谱系（例如单子叶植物和双子叶植物）的基因组进行了比较。这揭示了谱系特异性的基因丢失和分化模式。是 HD-ZIP III 转录因子的显著收缩的特征，特别是的丢失，这影响了其他被子植物的形成层活动。我们还发现，形成层相关 CLE 信号肽的 和单子叶植物拷贝在其他高度保守的氨基酸处显示出独特的取代。在形成层相关基因中没有明显的分化。趋同的维管束形成层丢失的发散基因组特征表明，即使是多效基因也可以与独立的性状丢失事件相关，表现出独特的分化模式。我们的研究结果为草本特性的进化提供了线索，这是与被子植物进化早期相关的关键生物学创新之一。
