Detergent-extracted Volvox model exhibits an anterior-posterior gradient in flagellar Ca²⁺ sensitivity

Volvox rousseletii is a multicellular spheroidal green alga containing ∼5,000 cells, each equipped with two flagella (cilia). This organism shows striking photobehavior without any known intercellular communication. To help understand how the behavior of flagella is regulated, we developed a method to extract the whole organism with detergent and reactivate its flagellar motility. Upon addition of ATP, demembranated flagella (axonemes) in the spheroids actively beat and the spheroids swam as if they were alive. Under Ca²⁺-free conditions, the axonemes assumed planar and asymmetrical waveforms and beat toward the posterior pole, as do live spheroids in the absence of light stimulation. In the presence of 10^-6 M Ca²⁺, however, most axonemes beat three-dimensionally toward the anterior pole, similar to flagella in photostimulated live spheroids. This Ca²⁺-dependent change in flagellar beating direction was more conspicuous near the anterior pole of the spheroid, but was not observed near the posterior pole. This anterior-posterior gradient of flagellar Ca²⁺ sensitivity may explain the mechanism of V. rousseletii photobehavior.