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Accueil du site > Animations Scientifiques > Séminaires 2009 > Raymond Goldstein — Flagellar Synchronization, Eukaryotic Random Walks, and the Fidelity of Multicellular Phototaxis

Raymond Goldstein — Flagellar Synchronization, Eukaryotic Random Walks, and the Fidelity of Multicellular Phototaxis

Speaker :

Raymond Goldstein, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK

When :

Wednesday 16 December at 14h30

Where :

C023 (RDC LR6 côté CECAM)

Title :

Flagellar Synchronization, Eukaryotic Random Walks, and the Fidelity of Multicellular Phototaxis

Abstract :

It has long been conjectured that hydrodynamic interactions between beating eukaryotic flagella underlie their ubiquitous forms of synchronization, yet there has been no experimental test of this connection. The biflagellated alga Chlamydomonas is a simple model for such studies, as its two flagella are representative of those most commonly found in eukaryotes. Using micromanipulation and high-speed imaging we show that the flagella of a C. reinhardtii cell present periods of synchronization interrupted by phase slips. The dynamics of slips and the statistics of phase-locked intervals are consistent with a low-dimensional stochastic model of hydrodynamically coupled oscillators, with a noise amplitude set by the intrinsic fluctuations of single flagellar beats. Moreover, in the dark, a single cell stochastically switches between periods of synchrony with slips and periods of asynchrony ("drifts") in which the two flagella beat at significantly different frequencies. We show by means of extensive three-dimensional tracking of swimming trajectories that drifts are associated with sharp turns, resulting in a eukaryotic version of the run-and-tumble locomotion found in bacteria. Finally, the dynamics of phototaxis in simple eukaryotes is considered from the perspective of evolutionary transitions to multicellularity. The unicellular alga Chlamydomonas swims in helical trajectories whose geometric properties are modulated by signals received from a photosensor that sweeps the surroundings as the cell rotates. Its large spherical multicellular descendent Volvox is composed of thousands of Chlamydomonas-like cells, and spins about a body-fixed axis as it swims. Using micromanipulation and particle-imaging velocimetry of flagella-driven flows, we show that the frequency response of Volvox carteri to periodic light signals is tuned to match the natural rotational frequency of the colony. A hydrodynamic model of phototactic steering shows that colony rotation is necessary to achieve accurate phototaxis.

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