Flagellar dynamics in the Volvocales and the emergence of coordination.

细胞的特性及其相互作用是一个漫长而复杂的适应环境的进化过程的结果。由于这种选择性压力很大程度上有物理根源，因此出现了一个问题，即物理施加的限制如何影响生命形式的发展。在细胞水平上，鞭毛是一个10-20微米长的毛状细胞器，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在低雷诺数下运动的特殊要求，由此产生的游泳行为可能是为了最大限度地减少与捕食者的相遇速度而选择的。在多细胞生物体中，大量鞭毛的运动可以通过创造宏观流动来直接影响细胞外环境，这些流动负责包括呼吸系统中的液体运输在内的任务，并打破胚胎的左右对称性。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。一个新出现的假设是，协调是流体动力相互作用的结果，但对这种可能性的直接实验测试很少。该提案的重点是绿藻目Volvocales中鞭毛的动态和协调问题。这一组包括各种密切相关的物种，从单细胞双鞭毛衣藻，真核生物鞭毛生物学研究的首选模式生物，到大型多细胞球形殖民地，如Volvox carteri，外层有数千个体细胞，负责运动。尽管它们有复杂的系统发育历史，但这一组中的物种具有基本相同的鞭毛装置，这使得可以直接比较它们的跳动动力学。我们的初步观察表明，尽管没有任何直接的细胞间连接，但卡特里弧菌不同体细胞的搏动动力学是显著协调的，并产生了从菌落前面到后面的波状拍动模式。这一建议旨在研究如何从单个细胞的行为中产生如此大规模的协调，特别侧重于表征存在外部机械应力时鞭毛动力学的变化，以及导致这些变化的细胞内信号。卡特氏弧菌具有分离良好的体细胞，可以从菌落中分离出来并进行单独研究，而莱茵哈迪尔弧菌是真核生物鞭毛生物学研究的中心，是解决这一问题的理想选择。对Volvocales鞭毛动力学的研究为揭示真核鞭毛动力学的一些最基本的方面提供了一个很好的机会。

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.