Transverse flagellum contraction synchronized with longitudinal flagellum retraction in <i>Akashiwo sanguinea</i>, a dinokont dinoflagellate

Akashiwo sanguinea has a longitudinal and a transverse flagellum. The longitudinal flagellum shows unique retraction in addition to swimming undulation, whereas the transverse flagellum undulates within the girdle of the cell. However, it remains unclear how the transverse flagellum moves when the longitudinal flagellum retracts and whether it is anchored to the girdle along its entire length. To investigate these aspects, the movement of the transverse flagellum was observed using light microscopy. During undulation, the distal tip of the transverse flagellum was immobile and fixed at a position on the girdle. When the longitudinal flagellum retracted, the transverse flagellum stopped undulating and was pulled deeper into the girdle, with its distal tip pulling slightly towards the base, suggesting contraction. Occasionally, a partially detached transverse flagellum was observed, with the middle and/or distal parts detached and continuing to undulate whereas the other parts remained in the girdle. The contraction was more intense with the flagellum detached, suggesting that the transverse flagellum was fully attached to the girdle and contracted when the longitudinal flagellum retracted. A non-detergent-extracted model of the transverse flagellum demonstrated that contraction was induced by Ca2+ and not antagonized by Mg2+. Electron microscopy revealed that the flagellum contained an axoneme, a striated strand, and numerous external flagellar hairs. In conclusion, the transverse flagellum contracted when the longitudinal flagellum retracted. This contraction is caused by the shortening of the striated strand and is induced by Ca2+. In addition, the entire transverse flagellum was attached to the girdle.