Using directed evolution to study the origins of multicellular development.

摘要 关于新的多细胞发育程序是如何在进化中产生的，人们知之甚少，主要是因为 多细胞生物在过去很久以前就出现了，早期的步骤已经消失。私家侦探绕过了这一点 通过创建新的模型系统来约束，使用定向进化来直接研究多细胞的起源 和发展。拟议的研究将有助于解决以下领域的两个主要知识差距 发展生物学：1)发育是如何从头进化的？2)什么是长期进化？ 发展的后果？在几千代的定向进化之后，PI观察到 雪花酵母中自主细胞类型规范的进化，他早期多细胞的模式系统。 具体地说，雪花酵母通过自适应地形成多细胞群，在机械上更加坚韧 分化为两种细胞类型，在这两种类型中，位于簇内部深处的细胞将其萌发角度改变了90% 并使相邻的细胞分支相互交错。在该模型系统中(以及在最简单的多细胞系统中 生物体)，细胞的年龄提供有关其位置的关键信息，允许基因的时间变化 表达来驱动细胞分化的空间模式。初步数据显示，监护人 Hsp90蛋白被认为是年龄(和位置)相关的发育开关， 调节细胞类型之间的转换。拟议的研究将进一步审查这一演变过程 新的发育机制，并将使这些湿实验室实验与3D生物物理 模拟和进化模型，允许PI从这些实验中推导出一般原理 结果。这项拟议的研究还将检验随机细胞行为如何被用于对称性 破坏，使雪花酵母进化出更复杂的多细胞结构。最后，提出了 研究将考察发育的进化后果：即细胞分化如何 将血统巩固到多细胞状态。这项工作将研究细胞分化是如何使细胞 它们通过将退化的可能性限制为单细胞来实现进化的自主性。发展将因此 对有机体的进化动态产生双重深刻的影响：为增加 多细胞的复杂性，同时关闭了祖先的单细胞行为的机会。

ABSTRACT Little is known about how novel multicellular developmental programs arise in evolution, largely because multicellularity arose deep in the past and early steps have been lost to extinction. The PI has circumvented this constraint by creating a new model system, using directed evolution to directly study the origin of multicellularity and development. The proposed research will help resolve two major knowledge gaps in the field of developmental biology: 1) How does development evolve de novo? 2) What are the long-term evolutionary consequences of development? After thousands of generations of directed evolution, the PI has observed the evolution of autonomous cell type specification in snowflake yeast, his model system of early multicellularity. Specifically, snowflake yeast form multicellular groups that are far more mechanically tough by adaptively differentiating into two cell types, in which cells deep in the cluster interior change their budding angle by 90 degrees and interlock neighboring cellular branches. In this model system (and in most simple multicellular organisms), a cell’s age provides key information about its location, allowing temporal changes in gene expression to drive spatial patterns of cellular differentiation. Preliminary data suggests that the chaperone protein Hsp90 has been co-opted to act as an age (and thus location)-dependent developmental switch, regulating the transition between cell types. The proposed research will further examine the evolution of this novel developmental mechanism, and will contextualize these wet-lab experiments with both 3D biophysical simulations and evolutionary models, allowing the PI to derive general principles from these experimental results. The proposed research will also examine how stochastic cellular behaviors can be co-opted for symmetry breaking, allowing snowflake yeast to evolve more complex multicellular structures. Finally, the proposed research will examine the evolutionary consequences of development: namely, how cellular differentiation can entrench a lineage into a multicellular state. This work will examine how cellular differentiation strips cells of their evolutionary autonomy by limiting the potential for reversion to unicellularity. Development would thus have a doubly-profound impact on an organism’s evolutionary dynamics: opening new avenues for increased multicellular complexity while closing off opportunities for ancestral, unicellular behaviors.