Position-dependent roles of somatic cells in phototaxis of <i>Volvox</i>

A spherical green alga, <i>Volvox</i>, achieves phototaxis via a simple on/off switch of flagellar beating in response to changes in light intensity, without the need for complex signal transduction between cells. Moreover, the alga can change its susceptibility to light in order to adapt to its environment. To identify the mechanisms of susceptibility regulation, experiments were conducted at three different levels: population, individual, and cellular. The light intensity dependence of the average velocity at the population level and that of the change in flow speed obtained at the individual level were consistent, indicating that susceptibility regulation occurred in each <i>Volvox</i> colony. Furthermore, by measuring the probability of stopping flagellar beating when the light intensity was changed, susceptibility regulation was found to result from the properties of somatic cells as differential and adaptive photosensors. These photosensing properties deteriorated from the anterior to the posterior regions of the colony. Considering the mechanical motion of a <i>Volvox</i> colony, the position-dependent ability of somatic cells indicates that the anterior cells play the role of a rudder, whereas the posterior cells play the role of a rower. The position-dependent properties of somatic cells imply an early stage of cell differentiation that allows for an efficient response to changes in the circumstances.