Gravitaxis in motile micro-organisms: the role of fore–aft body asymmetry

Abstract

Scale model experiments on axially symmetric bodies exhibiting fore–aft asymmetry are described. Body shapes are specified by a three parameter equation: two of the parameters (a and b) describe the length and breadth of the body and the third (c) the degree of asymmetry. Objects of this shape orientate as they sediment downwards under gravity until the narrower end lies uppermost, after which they fall vertically downward with no further change in orientation. For the range of parameters investigated the sedimentation velocities, both when vertical and horizontal, are governed principally by a and b, while the rate of orientation is determined by c. The sedimentation characteristics of bodies which cannot be described exactly by the equation can be predicted approximately using best-fit values for a, b and c. These results are applied to consider the role of front–rear asymmetry in ciliated free-swimming micro-organisms. The shape asymmetry is probably sufficient to account for the observed orientation rates in the ciliated protozoan Paramecium. It is suggested that these results may be used to deduce the sedimentation behaviour of ciliates from microscope images of individual cells. In small flagellates such as Chlamydomonas the orientating effects of the protruding flagella are much larger than the effects of cell body asymmetry. The extreme sensitivity of the orientation rate to slight changes in body shape and flagellar beat patterns may explain why experiments to distinguish between various orientational mechanisms involved in gravitaxis have in the past produced equivocal results.