The Spirogyra genome: signatures of shared and divergent division and differentiation

Zygnematophytes emerged as the unexpected closest algal relatives of land plants despite their simple body plans, raising questions about the morphogenetic toolkit present in the last common ancestor of land plants and algae. Genomic analyses have revealed that zygnematophytes are cellular giants, sharing homologous frameworks for several phytohormones, secondary metabolites, and key morphogenetic and transcriptional regulatory processes. Zygnematophytes fall into five orders, each of which has charted its own evolutionary path. Here, we have sequenced a contiguous genome of Spirogyra pratensis, the eponymous representative of Spirogyrales and a classical model system for evolutionary cell biology in the green lineage. Building on this genome, we transcriptionally profiled the tractable life cycle of Spirogyra and its responses to a bifactorial gradient of light and temperature. Our data highlight the activation of quiescence and homeostatic programs. Yet what stands out most in Spirogyra is its spiral chloroplast—undulating intracellularly and abscising during mixed phragmoplast formation and furrowing. Leveraging the genome in tandem with co-expression network analyses, we describe the molecular underpinnings of the unique cytokinetic processes that govern both cell and plastid division. We find that Spirogyra deploys a molecular program characteristic of Phragmoplastophyta, yet lacks the deeply conserved plastid division machinery found in other archaeplastid plastids接合藻类（Zygnematophytes）尽管躯体结构简单，却被意外证实为与陆生植物亲缘关系最近的藻类近亲，这一发现引发了学界对陆生植物与藻类的最近共同祖先所携带的形态发生工具箱的诸多疑问。基因组分析显示，接合藻类属于巨型细胞类群，其拥有多类植物激素、次生代谢物以及关键形态发生与转录调控过程的同源调控框架。接合藻类下辖5个目，每个类群均走出了独立的演化路径。本研究完成了草地水绵（Spirogyra pratensis）的连续基因组测序，该物种是水绵目（Spirogyrales）的模式代表物种，同时也是绿色植物谱系中进化细胞生物学研究的经典模型系统。基于该基因组数据，我们对草地水绵可实验操作的生命周期及其对光与温度双因子梯度的响应开展了转录组分析。研究数据凸显了静止程序与稳态程序的激活过程。而草地水绵最引人注目的特征当属其螺旋状叶绿体——该叶绿体在细胞内呈波浪状分布，且在混合成膜体形成与沟裂过程中发生缢裂。借助该基因组与共表达网络分析相结合的研究手段，我们阐明了调控细胞与质体分裂的独特胞质分裂过程的分子基础。研究发现，草地水绵采用了成膜体植物类群（Phragmoplastophyta）特有的分子程序，却缺失了其他古叶绿体生物（Archaeplastida）的质体中普遍保守的质体分裂机制。