Inverse relationship of Ca-dependent flagellar response between animal sperm and prasinophyte algae.

Symmetry/asymmetry conversion of eukaryotic flagellar waveform is caused by the changes in intracellular Ca. Animal sperm flagella show symmetric or asymmetric waveform at lower or higher concentration of intracellular Ca, respectively. In Chlamydomonas, high Ca induces conversion of flagellar waveform from asymmetric to symmetry, resulting in the backward movement. This mirror image relationship between animal sperm and Chlamydomonas could be explained by the distinct calcium sensors used to regulate the outer arm dyneins (Inaba 2015). Here we analyze the flagellar Ca-response of the prasinophyte Pterosperma cristatum, which shows backward movement by undulating four flagella, the appearance similar to animal sperm. The moving path of Pterosperma shows relatively straight in artificial seawater (ASW) or ASW in the presence of a Ca ionophore A23187, whereas it becomes circular in a low Ca solution. Analysis of flagellar waveform reveals symmetric or asymmetric waveform propagation in ASW or a low Ca solution, respectively. These patterns of flagellar responses are completely opposite to those in sperm flagella of the sea urchin Anthocidaris crassispina, supporting the idea previously proposed that the difference in flagellar response to Ca attributes to the evolutional innovation of calcium sensors of outer arm dynein in opisthokont or bikont lineage.