Abstract
Green algae of theVolvocinelineage, spanning from unicellularChlamydomonasto vastly largerVolvox, are models for the study of the evolution of multicellularity, flagellar dynamics, and developmental processes. Phototactic steering in these organisms occurs without a central nervous system, driven solely by the response of individual cells. All such algae spin about a body-fixed axis as they swim; directional photosensors on each cell thus receive periodic signals when that axis is not aligned with the light. The flagella ofChlamydomonasandVolvoxboth exhibit an adaptive response to such signals in a manner that allows for accurate phototaxis, but in the former the two flagella have distinct responses, while the thousands of flagella on the surface of sphericalVolvoxcolonies have essentially identical behaviour. The planar 16-cell speciesGonium pectoralethus presents a conundrum, for its central 4 cells have aChlamydomonas-like beat that provide propulsion normal to the plane, while its 12 peripheral cells generate rotation around the normal through aVolvox-like beat. Here, we combine experiment, theory, and computations to reveal howGonium, perhaps the simplest differentiated colonial organism, achieves phototaxis. High-resolution cell tracking, particle image velocimetry of flagellar driven flows, and high-speed imaging of flagella on micropipette-held colonies show how, in the context of a recently introduced model forChlamydomonasphototaxis, an adaptive response of the peripheral cells alone leads to photo-reorientation of the entire colony. The analysis also highlights the importance of local variations in flagellar beat dynamics within a given colony, which can lead to enhanced reorientation dynamics.
Publisher
Cold Spring Harbor Laboratory
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