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向多细胞生物过渡的方式。在这个最基本的转变中，至少有两个基本问题。扩大规模的好处是什么？以及：细胞专业化背后的驱动力是什么？这些问题必须放在生命最基本特征的背景下看待：与环境不断交换营养物质和废物。考虑到最简单的单细胞生物和它们的多细胞后继者生活在水环境中，很明显，浮力、扩散和混合的物理特性应该在这些考虑中发挥重要作用。传统的生物学观点认为，流体运动的扩散比平流要快得多，这种观点适用于小型生物，如游动缓慢的单个细菌。对于较大的生物体来说，情况通常不是这样，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生物体大小的增加而增加，直到超过扩散的速度。利用流体力学的现代技术，我们最近确定了Volvocalean绿藻，一个光合生物的谱系，作为研究进化过渡到多细胞的模型，涵盖了扩散和搅拌之间的平衡的非常广泛的范围，从单细胞水平的扩散主导到由数千个细胞组成的菌落的搅拌主导。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成千上万的鞭毛协同作用驱动的，这意味着代谢动力学与被动扩散所限制的代谢动力学有着根本不同。我们最近的工作表明，这种流动在群体代谢中起着至关重要的作用，并可能赋予大型生物体进化优势。以上概述的发展使我们能够建立一个将运动性、混合性和多细胞性联系起来的工作假设。下一步是对这一假设的全面探索。使用藻绿藻类作为模型谱系，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与流体流动之间的联系。这将通过光学方法来完成，探测在这些藻类中发生的光合作用的数量，无论是在鞭毛跳动的存在还是不存在流体流动的情况下。这些流动将在微流体领域的方法创建的微观通道中产生。（ii）我们将开发一种方法来研究这些生物上多个鞭毛同步的方式。这将涉及使用高速成像来可视化游泳和趋光性过程中的鞭毛协调，作为分子马达的流体动力学同步的探针，这是集体流体流动的基础。一个关键问题是同步动力学对体细胞间间距的依赖，这一范围可以在其生命周期的不同点上使用不同的volvoline藻类成员来研究。（iii）进一步发展鞭毛驱动流动的数学模型及其对运动、代谢物交换和向多细胞生物进化过渡的尺度规律的影响。（四）为这些藻类通过调节鞭毛的跳动而转向光的动力学建立理论模型。

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.