Flagellar dynamics in the Volvocales and the emergence of coordination.

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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.