Mechanisms and evolutionary origins of germ-soma specification in a multicellular green alga, Volvox carteri

生物圈的大部分是由微观的单细胞细菌和原生生物组成的。然而，在少数情况下，单细胞生物获得了聚集或在细胞分裂后保持联系并形成集体的能力，从而为大型复杂多细胞生命的显着进化奠定了基础，包括动物，植物和其他一些多细胞群体，如真菌和大型藻类。 该项目旨在了解一个重要但在很大程度上神秘的步骤，该步骤在向复杂多细胞过渡的过程中反复发生：不同生殖和非生殖细胞类型的进化。在大多数情况下，这种关键的转变是如此古老，以至于它的起源很难或不可能重建。 通过研究最近进化的多细胞物种绿色团藻的细胞类型特化，并将其与密切相关的单细胞亲戚衣原体进行比较，研究小组旨在以前所未有的遗传和基因组细节来了解进化途径和遗传机制，使单独的生殖和非生殖细胞类型出现。 这个项目不仅将解决进化中最神秘的转变之一的基本问题，而且对于理解协调的代谢重编程和特化如何可能被工程化为经济上有价值的绿色藻类以调节分配给细胞生长的资源与分配给增加的生物燃料或高-当细胞生长最小化时，这些产品的价值得到提高。该项目包括跨学科科学研究的研究生和博士后助理培训。本研究的长期目标是了解与多细胞组织和生殖体分工相关的起源和遗传机制。 团藻carteri '团藻'表现出一种流线型和实验上易于处理的生殖体分化形式。每个球状体个体仅包含两种细胞类型：约2000个无菌体细胞，提供运动性并产生细胞外基质，但注定会衰老和死亡;约16个称为分生孢子的大型生殖细胞，每个细胞都经历了细胞分裂和形态发生的定型模式，以产生新的球状体。 有人假设团藻的生殖体二分性是由来自单细胞祖先的时间调节和瞬时表达的差异基因表达程序的协同选择进化而来的;但这一想法尚未在全基因组范围内进行过测试。 在这个项目下，新的分子遗传学和基因组学资源将被用来阐明控制细胞团藻分化的基因表达网络，并破译它们的起源。 这些办法包括：1）比较分析从同步野生型团藻和从细胞分化突变体regA-和lagA-获得的细胞类型转录组，以阐明具有高时空分辨率的细胞类型特化网络; 2）使用染色质免疫沉淀和深度测序或类似方法鉴定RegA的直接靶标，RegA是核定位的转录因子和体细胞分化的主调节因子; 3）基因产物抑制生殖细胞体细胞分化的滞后突变体的表征和克隆。 这些方法将共同定义团藻细胞类型规范所涉及的监管网络和控制机制，并通过与近亲（如Chlamydia.This奖项反映了NSF的法定使命）的时间表达程序进行比较，严格测试其起源，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

The majority of the biosphere is populated by microscopic, single-celled bacteria and protists. Yet, in a few rare instances single-celled organisms acquired the ability to aggregate or to remain associated after cell division and form collectives, thus setting the stage for the remarkable evolution of large complex multicellular life including animals, plants and a few other multicellular groups like fungi and macroalgae. This project aims to understand an important but largely mysterious step that has repeatedly occurred in conjunction with the transition to complex multicellularity: the evolution of distinct reproductive and non-reproductive cell types. In most cases this key transition is so ancient that its origins are difficult or impossible to reconstruct. By investigating cell-type specialization in a much more recently evolved multicellular species, the green alga Volvox, and comparing it to a closely-related single-celled relative, Chlamydomonas, the research team aims to understand in unprecedented genetic and genomic detail the evolutionary pathway and genetic mechanisms that enabled separate reproductive and non-reproductive cell types to arise. This project will not only address fundamental questions underlying one of evolution's most enigmatic transitions, but also has practical implications for understanding how coordinated metabolic reprogramming and specialization might be engineered into economically valuable green algae in order to tune the balance between resources allocated to cell growth versus allocation towards increased production of biofuels or high-value products that are enhanced when cell growth is minimized. The project includes graduate student and postdoctoral associate training in interdisciplinary scientific research. Outreach to local middle school students will enable students to participate in scientific discovery.The long-term goal of this research is to understand the origins and genetic mechanisms associated with multicellular organization and germ-soma division of labor. Volvox carteri 'Volvox' exhibits a streamlined and experimentally tractable form of germ-soma differentiation. Each spheroidal individual contains just two cell types: ~2000 sterile somatic cells that furnish motility and produce extracellular matrix, but which are destined to senesce and die; and ~16 large reproductive cells called gonidia, each of which undergoes a stereotyped pattern of cell divisions and morphogenesis to produce a new spheroid. It has been hypothesized that the germ-soma dichotomy in Volvox evolved by cooption of temporally-regulated and transiently-expressed differential gene expression programs from a unicellular ancestor; but this idea has not been tested on a genome-wide scale. Under this project new molecular-genetic and genomics resources will be leveraged to elucidate gene expression networks that control cell Volvox differentiation and to decipher their origins. The approaches involve: 1) Comparative analyses of cell-type transcriptomes obtained from synchronized wild-type Volvox and from cell differentiation mutants regA- and lagA- to elucidate cell-type specification networks with high spatio-temporal resolution; 2) Identification of direct targets of RegA, a nuclear-localized transcription factor and master regulator of somatic differentiation, using chromatin immunoprecipitation and deep sequencing or similar approaches; 3) Characterization and cloning of lag- mutants whose gene products suppress somatic differentiation of germ cells. Together these approaches will define the regulatory networks and control mechanisms involved in Volvox cell-type specification and critically test their origins by comparison with temporal expression programs in close relatives such as Chlamydomonas.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.