Tuesday April 01 2025 at 10am GMT+2, 5pm (Japan time)

Tadashi Maruyama¹

Two Distinct Complex Flagella, Each Exhibiting Two Types of Motion, in the Dinoflagellate Akashiwo sanguinea

¹. Japan Agency for Marine-Earth Science and Technology

Dinokont dinoflagellates are well known for possessing two structurally complex but distinct flagella: the transverse and longitudinal flagella. Both contribute to locomotion, with the longitudinal flagellum primarily facilitating translational swimming, while the transverse flagellum plays a greater role in rotation and helical swimming. During movement, the longitudinal flagellum undulates posteriorly in a planar fashion, whereas the transverse flagellum undulates within the girdle groove encircling the cell.
More than 40 years ago, I reported a unique retractile motion of the longitudinal flagellum in Tripos (formerly Ceratium) (1). Recently, we investigated the motion and structure of these two flagella in Akashiwo sanguinea and identified this retractile motion in its longitudinal flagellum, marking the second known case in dinoflagellates. Additionally, we found that the transverse flagellum also contracts (2, 3). The longitudinal flagellum exhibited a distinct retractile motion in response to physical stimulation, folding from tip to base. This retraction is driven by the contraction of a specialized fibrous structure, the Retraction-Fiber (R-fiber), and is regulated by Ca²⁺, with Mg²⁺ acting as an antagonist.
The transverse flagellum, like those of other dinokont dinoflagellates, undulates within the girdle groove but is difficult to observe under a light microscope. To facilitate observation, cell movement was suppressed by confining cells in a thin chamber on a glass slide. When the longitudinal flagellum retracted, the transverse flagellum also stopped undulating and contracted. This contraction occurred uniformly along its entire length and appeared to be driven by the striated strand within the flagellum. While Ca²⁺ induced contraction in the transverse flagellum, Mg²⁺ did not antagonize this response. Observations of transverse flagellum movement and occasional detachment from the girdle revealed that it remains attached along its entire length from base to tip. Although the exact attachment mechanism is unknown, it is likely mediated by fine flagellar hairs.
In conclusion, Akashiwo sanguinea possesses a unique longitudinal flagellum equipped with the R-fiber, enabling retractile motion, and a transverse flagellum that contracts via the striated strand in synchrony with longitudinal flagellum retraction. Both flagella exhibit two types of motility: undulation and contraction. However, the contractile mechanisms differ, as only the longitudinal flagellum's retraction is antagonized by Mg²⁺, whereas the transverse flagellum's contraction is not. These findings suggest that the R-fiber and the striated strand operate through distinct contractile mechanisms.

References:

1. Maruyama (1981) Motion of the longitudinal flagellum in Ceratium tripos (Dinoflagellida): A retractile flagellar motion. J. Protozool. 28: 328-336.
2. Maruyama et al. (2023) Retractile motion of the longitudinal flagellum in a dinoflagellate, Akashiwo sanguinea. Cytologia 88: 321-329.
3. Maruyama et al. (2024) Transverse flagellum contraction synchronized with longitudinal flagellum retraction in Akashiwo sanguinea, a dinokont dinoflagellate. Cytologia 89: 307-320.