TY - JOUR
T1 - The shape effect of flagella is more important than bottom-heaviness on passive gravitactic orientation in Chlamydomonas reinhardtii
AU - Kage, Azusa
AU - Omori, Toshihiro
AU - Kikuchi, Kenji
AU - Ishikawa, Takuji
N1 - Funding Information:
This study was funded by KAKENHI grants from the Japan Society for the Promotion of Science (25000008 to T.I. and T.O.; 17K15154 to A.K.), and a Sumitomo Foundation Grant for Basic Research Projects (160670 to A.K.).
Publisher Copyright:
© 2020. Published by The Company of Biologists Ltd.
PY - 2020
Y1 - 2020
N2 - The way the unicellular, biflagellated, green alga Chlamydomonas orients upward has long been discussed in terms of both mechanics and physiology. In this study, we focus on the mechanics, i.e. the 'passive' mechanisms, of gravitaxis. To rotate the body upwards, cellular asymmetry is critical. Chlamydomonas can be depicted as a nearly spherical cell body with two anterior, symmetric flagella. The present study looks at the question of whether the existence of the flagella significantly affects torque generation in upward reorientation. The 'density asymmetry model' assumes that the cell is spherical and bottom-heavy and that the shape and weight of the flagella are negligible, while the 'shape asymmetry model' considers the shape of the flagella. Both our experimental and simulation results revealed a considerable contribution from shape asymmetry to the upward orientation of Chlamydomonas reinhardtii, which was several times larger than that of density asymmetry. From the experimental results, we also quantified the extent of bottom-heaviness, i.e. the distance between the centers of gravity and the figure when the cell body is assumed to be spherical. Our estimation was approximately 30 nm, only one-third of previous assumptions. These findings indicate the importance of the viscous drag of the flagella to the upward orientation, and thus negative gravitaxis, in Chlamydomonas.
AB - The way the unicellular, biflagellated, green alga Chlamydomonas orients upward has long been discussed in terms of both mechanics and physiology. In this study, we focus on the mechanics, i.e. the 'passive' mechanisms, of gravitaxis. To rotate the body upwards, cellular asymmetry is critical. Chlamydomonas can be depicted as a nearly spherical cell body with two anterior, symmetric flagella. The present study looks at the question of whether the existence of the flagella significantly affects torque generation in upward reorientation. The 'density asymmetry model' assumes that the cell is spherical and bottom-heavy and that the shape and weight of the flagella are negligible, while the 'shape asymmetry model' considers the shape of the flagella. Both our experimental and simulation results revealed a considerable contribution from shape asymmetry to the upward orientation of Chlamydomonas reinhardtii, which was several times larger than that of density asymmetry. From the experimental results, we also quantified the extent of bottom-heaviness, i.e. the distance between the centers of gravity and the figure when the cell body is assumed to be spherical. Our estimation was approximately 30 nm, only one-third of previous assumptions. These findings indicate the importance of the viscous drag of the flagella to the upward orientation, and thus negative gravitaxis, in Chlamydomonas.
KW - Eukaryotic flagella
KW - Gravitaxis
KW - Microalga
KW - Microswimmer
KW - cell asymmetry
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U2 - 10.1242/jeb.205989
DO - 10.1242/jeb.205989
M3 - Article
C2 - 31988163
AN - SCOPUS:85081147599
VL - 223
SP - 1C
JO - Journal of Experimental Biology
JF - Journal of Experimental Biology
SN - 0022-0949
IS - 5
M1 - jeb205989
ER -