Edited by Krishna K. Niyogi, Howard Hughes Medical Institute, University of California, Berkeley, CA, and approved December 8, 2017 (received for review September 1, 2017)
The multicellular green alga Volvox rousseletii displays phototaxis by changing its flagellar beating pattern in response to photoreception. However, the molecular mechanism underlying flagellar regulation is unknown. This study describes a method to demembranate whole spheroids using a nonionic detergent, with the addition of ATP reactivating flagellar motility. These reactivated spheroids swam like live spheroids. Flagellar beating direction was altered in a Ca2+-dependent manner, with a greater change in the anterior hemisphere than in the posterior hemisphere. These findings indicate that V. rousseletii has an anterior–posterior gradient of flagellar sensitivity to Ca2+, which likely plays a key role in V. rousseletii phototaxis.
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.
Author contributions: N.U. and K.W. designed research, performed research, analyzed data, and wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1715489115/-/DCSupplemental.
Published under the PNAS license.
