Physical aspects of evolutionary transitions to multicellularity

在复杂生命形式的进化中，一个非常深刻和有趣的问题是从单细胞生物到多细胞生物的转变发生的方式。在这个最基本的转变中，至少有两个基本问题。增加规模的好处是什么？细胞特化背后的驱动力是什么？这些问题必须从生命最基本的特征来看待：与环境不断交换营养物和废物。考虑到最简单的单细胞生物及其多细胞后继者都生活在水环境中，浮力、扩散和混合的物理学在这些考虑中显然应该发挥重要作用。在传统的生物学观点中，扩散比流体运动的平流快得多，这种观点适用于像单个细菌这样游动缓慢的小生物。对于较大的生物体来说，情况通常并非如此，它们通过大量鞭毛的作用在自身周围产生流体流动。这些流动的速度随着有机体大小的增加而增加，以至于它们超过了扩散。利用现代技术从流体力学，我们最近已经建立了Volvocalean绿色藻类，一个谱系的光合生物，作为一个模型的进化过渡到多细胞的研究，涵盖了非常广泛的范围之间的平衡扩散和搅拌，从扩散为主的单细胞水平，搅拌为主的殖民地组成的数千个细胞。这种谱系提供了解构运动和代谢规律的可能性，这可能有助于解释导致多细胞生物的进化驱动力。这些流动是由这些菌落表面上数千个鞭毛的协调作用驱动的，这意味着代谢动力学与被动扩散所限制的代谢动力学根本不同。我们最近的研究表明，这种流动在殖民地的新陈代谢中起着至关重要的作用，并赋予较大的生物体以进化优势。上面概述的发展使我们能够建立一个工作假设，将运动，混合和多细胞性联系起来。下一步是对这一假设的全面探索。使用volvocine绿色藻类作为模型谱系，我们有四个主要目标。(i)我们将实施一种实验方法，通过这种方法可以测试代谢活动和液体流动之间的联系。这将通过光学方法来实现，该方法探测在这些藻类中发生的光合作用的量，无论是在存在还是不存在来自鞭毛跳动的流体流的情况下。这些流动将在用“微流体学”领域中的方法创建的微观通道中创建。（ii）我们将开发一种方法来研究多个鞭毛在这些器官上同步的方式。这将涉及使用高速成像可视化鞭毛协调在游泳和趋光性，作为探针的流体动力学同步的分子马达的基础上的集体流体流动。一个关键的问题是依赖于体细胞间的间距，其中一系列的同步动力学可以使用不同的成员volvocine藻类在其生命周期的各个点进行研究。(iii)进一步发展鞭毛驱动流的数学模型及其对运动、代谢物交换以及向多细胞进化过渡的标度律的影响。(iv)发展理论模型的动力学，这些藻类转向光通过调制他们的鞭毛跳动。

A very profound and interesting issue in the evolution of complex life forms is the manner by which the transition from unicellular organisms to multicellular ones occurred. There at least two fundamental issues in this most basic transition. What are the advantages of increasing size? and: What are the driving forces behind cell specialization? These questions must be viewed in the context of the most basic feature of life: the continuous exchange with the environment of nutrients and wastes. Given that the simplest unicellular organisms and their multicellular successors inhabit an aqueous environment, it is clear that the physics of buoyancy, diffusion, and mixing should play an important role in these considerations. In the conventional biological view appopriate to small organisms such as individual bacteria which swim slowly, diffusion is much faster than advection by fluid motions. Such is generally not the case for larger organisms, which create fluid flows around themselves by the action of multitudes of flagella. These flows increase in speed with increasing organism size, to the point that they outpace diffusion. Using modern techniques from fluid mechanics, we have recently established that the Volvocalean green algae, a lineage of photosynthetic organisms that serves as a model for research on evolutionary transitions to multicellularity, covers a very broad range of the balance between diffusion and stiring, from diffusion-dominated at the single cell level, to stirring-dominated for colonies composed of thousands of cells. This lineage affords the possibility of deconstructing laws in motility and metabolism that may help explain the evolutionary driving forces which led to multicellularity. These flows are driven by the coordinated action of thousands of flagella on the surface of these colonies, and imply metabolic dynamics fundamentally different than those limited by passive diffusion. Our recent work suggests that such flows can play a crucial role in the colony metabolism, and would have conferred an evolutionary advantage to larger organisms. The developments outlined above have allowed us to establish a working hypothesis which links motility, mixing, and multicellularity. The next step is the full exploration of this hypothesis. Using the volvocine green algae as a model lineage, we have four main goals. (i) We will implement an experimental method by which the link between metabolic activity and fluid flow can be tested. This will be accomplished with optical methods that probe the amount of photosynthesis occurring in these algae, both in the presence and absence of fluid flow from flagellar beating. These flows will be created in microscopic channels created with methods in the field of 'microfluidics.' (ii) We will develop a method to study the manner in which multiple flagella becomes synchronized on these organsisms. This will involve the use of high-speed imaging to visualize flagellar coordination during swimming and phototaxis, as a probe of the hydrodynamic synchronization of molecular motors which underlies the collective fluid flows. A key issue is the dependence of synchronization dynamics on inter-somatic cell spacing, a range of which can be studied using diverse members of the volvocine algae at various points in their life cycles. (iii) Further develop mathematical models of flagella-driven flows and their implications for scaling laws in locomotion, metabolite exchange, and thus evolutionary transitions to multicellularity. (iv) Develop theoretical models for the dynamics by which these algae steer toward the light by modulating their flagellar beating.