AsianScientist (Feb. 28, 2018) – Scientists at the Tokyo Institute of Technology (Tokyo Tech) in Japan have obtained a clearer understanding of how multicellular spheroidal alga move about. Their findings have implications on how flagellar motility increased in complexity as single-celled organisms evolved into multicellular forms. They published their findings in the Proceedings of the National Academy of Sciences.
As a photosynthetic alga, the spheroid Volvox rousseletii must move in a light-sensitive way to survive. It achieves this by beating numerous flagella toward its posterior end for swimming forward and turning via changing the direction of flagellar motion from back to front if it perceives light. Exactly how this movement is regulated remains unclear, and existing techniques for studying the mechanism underlying flagellar motility are more suitable for single-celled organisms.
In this study, a team of scientists led by Dr. Noriko Ueki and Dr. Ken-ichi Wakabayashi at Tokyo Tech used detergent to remove the cell membranes of V. rousseletii to study the biochemical reactions and motions of the alga’s flagella. The scientists called the demembranated V. rousseletii ‘Zombie Volvox.’ The researchers took advantage of the fact that the motility of Zombie Volvox can be induced by the addition of an ATP buffer. The effects of other substances on motility can also be studied using this system.
Because calcium ions are a common modulator of flagella beating, the scientists tested its influence on Zombie Volvox. In the presence of both ATP and calcium, these zombie organisms proceeded to move as if they were alive and responsive to sudden changes in light intensity—their flagella beat toward the anterior direction, and swimming speeds decreased.
In contrast, when only ATP was available, the flagella beat toward the posterior end to enable faster swimming speeds, as is the case for live V. rousseletii when there is no sudden change in light intensity.
Additionally, the effects of calcium addition were stronger on flagella at the anterior region of the spheroid compared with flagella on the posterior end, indicating that the anterior part of V. rousseletii is more sensitive to calcium ions than the posterior part. The sensitivity gradient is critical for appropriate movement in response to light.
The researchers demonstrated that when light is provided from the side, the changes in the beating pattern of all flagella will cause the organism to spin instead of orienting toward or away from the light. It is possible that the gradient also allows more fine-tuned, efficient light-responsive motility than what is possible in unicellular photosynthetic organisms.
The article can be found at: Ueki & Wakabayashi (2018) Detergent-extracted Volvox Model Exhibits an Anterior–posterior Gradient in Flagellar Ca2+ Sensitivity.
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Source: Tokyo Institute of Technology; Photo: Frank Fox/Wikimedia/CC BY-SA 3.0.
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