Flagellar dynamics in the Volvocales and the emergence of coordination.

细胞的特性及其相互作用是适应环境的漫长而复杂的进化过程的结果。由于这种选择性压力有很大一部分是物理学上的，因此出现了这样一个问题：物理学所施加的限制如何影响生命形式的发展。在细胞水平上，鞭毛是一种10-20微米长的毛发状细胞器，在真核生物中高度保守，可以说是细胞与环境直接相互作用的最佳例子之一。事实上，单细胞真核生物鞭毛的形状和跳动模式已经进化到满足低雷诺数下运动的特殊要求，由此产生的游泳行为可能已经被选择以最大限度地减少与捕食者的遭遇率。在多细胞生物中，大群鞭毛的运动可以通过产生宏观流动对细胞外环境产生直接影响，这些宏观流动负责包括呼吸系统中的流体运输和打破胚胎左右对称性的任务。这些流动也可能在多细胞的发展中发挥了作用。不同鞭毛之间的协调对于成功地执行这些功能通常是至关重要的。一个新兴的假设是，协调的结果从流体动力学的相互作用，但很少有直接的实验测试这种可能性。该提案的重点是鞭毛动力学和协调的问题，在团藻目，一个命令的绿色藻类。这一组包括各种密切相关的物种，从单细胞双鞭毛的莱茵衣藻，真核生物鞭毛生物学研究的首选模式生物，到大型多细胞球形菌落，如团藻，外层有数千个体细胞，负责运动。尽管其复杂的系统发育史，在这组物种有基本相同的鞭毛装置，这使得它们的跳动动力学的直接比较。我们的初步观察表明，尽管没有任何直接的细胞间连接，在V. carteri不同的体细胞的跳动动力学是非常协调的，并产生了一个波浪状的跳动模式，从前面到后面的殖民地。该提案旨在研究这种大规模的协调如何从单个细胞的行为中出现，特别关注外部机械应力存在下鞭毛动力学的特征变化，以及引起这些变化的细胞内信号。卡氏弧菌（V. carteri）和C. Reinhardtii是真核鞭毛生物学研究的中心，非常适合解决这个问题。在团藻目鞭毛动力学的研究是一个很好的机会，解开一些最基本的真核生物鞭毛的动力学方面。

The properties of cells, and their interactions, are the result of a long and intricate evolutionary process of adaptation to the environment. As much of this selective pressure has a physical origin, the question arises of how the constraints imposed by physics influence the development of life forms. At the cellular level, the flagellum, a 10-20 micrometres long hair-like organelle highly conserved across eukaryotic species, is arguably one of the best examples of direct interaction between a cell and its environment. Indeed, the shape and beating pattern of flagella in unicellular eukaryotes have evolved to satisfy the peculiar requirements of locomotion at low Reynolds number, and the resulting swimming behaviour may have been selected to minimize the rate of encounter with predators. In multicellular organisms, the motion of large groups of flagella can have a direct impact on the extracellular environment by creating macroscopic flows which are responsible for tasks including fluid transport in the respiratory system, and breaking embryonic left-right symmetry. These flows may have also played a role in the development of multicellularity. Coordination among different flagella is often of paramount importance to perform these functions successfully. An emerging hypothesis is that coordination results from hydrodynamic interactions, yet there is very little direct experimental test of this possibility. The proposal focuses on the issue of flagellar dynamics and coordination in the Volvocales, an order of green algae. This group includes a variety of closely related species, from the unicellular biflagellate Chlamydomonas reinhardtii, the preferred model organism for biological studies of the eukaryotic flagellum, up to large multicellular spheroidal colonies, like Volvox carteri, with an external layer of thousands of somatic cells, responsible for locomotion. Despite their complex phylogenetic history, species in this group have fundamentally identical flagellar apparatuses, which allows a direct comparison of their beating dynamics. Our preliminary observations reveal that, despite the absence of any direct intercellular connections, the beating dynamics of different somatic cells in V. carteri is remarkably coordinated, and gives rise to a wave-like beating pattern that travels from the front to the back of the colony. This proposal aims at studying how such large scale coordination emerges from the behaviour of a single cell, with a particular focus on characterizing changes in flagellar dynamics in presence of an external mechanical stress, and the intracellular signals that cause these changes. V. carteri, with well separated somatic cells which can be isolated from the colony and studied individually, and C. reinhardtii, which is at the centre of biological research on eukaryotic flagella, are ideally suited to address this problem. The study of flagellar dynamics in the Volvocales represents a great opportunity to unravel some of the most fundamental aspects of the dynamics of the eukaryotic flagellum.