Biology subjects

Ueki, N.

Publications and source records attributed to Ueki, N..

3 recordsLinked to original sources

bioRxiv2023

Multicellularity and increasing Reynolds number impact on the evolutionary shift in photo-induced ciliary response in Volvocales

Volvocales include species with different cell numbers and sizes, reflecting a history of gradual size increase evolution. Unicellular species live in low Reynolds-number (Re) environments where viscous forces dominate, whereas multicellular species live in higher Re environments with nonnegligible inertial forces. Despite significant changes in the physical environment, during the evolution of multicellularity they maintained photobehaviors (i.e., photoshock and phototactic responses), which allows them to survive under changing light conditions. In this study, we classified photo-induced ciliary responses in Volvocales into four patterns: temporal waveform conversion, no obvious response, pause in ciliary beating, temporal changes in ciliary beating directions. We found that which species exhibit which pattern depends on Re associated with the individual size of each species rather than phylogenetic relationships. These results suggest that species with increased cell numbers acquired their responses adapted to higher Re fluid environments. Significance StatementVolvocales green algae include species with various cell numbers and are excellent organisms for studying the evolution of multicellularity. They exhibit photobehaviors by changing the pattern of ciliary beating, which could be categorized into four patterns. We found that the difference in patterns among the organisms is due to the Reynolds number, the ratio of viscous and inertia forces, rather than their phylogenetic relationships. This study indicates that the fluid environment was an important factor in natural selection for behavioral changes in microalgae during evolution. The results link evolution and physics while contributing to the design of micromachines.

evolutionary biology↗

bioRxiv2020

Basis for the phototaxis sign reversal in the green alga Chlamydomonas reinhardtii studied by high-speed observation

For organisms that respond to environmental stimuli using taxes, reversal of the tactic sign should be tightly regulated for survival. The biciliate green alga Chlamydomonas reinhardtii is an excellent model for studying reversal between positive and negative phototaxis. C. reinhardtii cells change swimming direction by modulating the balance of beating forces between their two cilia after photoreception at the eyespot; however, it remains unknown how they reverse phototactic sign. In this study, we observed cells undergoing phototactic turns with a high-speed camera and found that two key factors determine the phototactic sign: which of the two cilia beats more strongly for phototactic turning and when the strong beating starts. The timing of the strong ciliary beating is suggested to be regulated by ROS-regulated switching between the light-on and light-off responses at the eyespot, which leads to the switching between positive and negative phototaxis. This idea is supported by a mathematical model that introduces the timing of the strong ciliary beating after photoreception.

cell biology↗

bioRxiv2020

Conservation and divergence of DNA replication control in Chlamydomonas reinhardtii

We recently isolated temperature-sensitive cell cycle mutants in Chlamydomonas reinhardtii for which the causative mutations were located in genes annotated for potential involvement in DNA replication. Chlamydomonas has a very long G1 period during which cells grow up to ~10-fold without division, followed by rapid cycles of DNA replication and mitosis ( multiple fission). All of the candidate DNA replication mutants tested were defective in completion of the first round of DNA replication, and also failed to produce mitotic spindles. For a subset of the mutants, we rescued temperature-sensitive lethality with tagged transgenes and used the resulting strains to analyze abundance and localization control of the tagged protein. All of the DNA replication proteins tested were essentially undetectable until late G1, accumulated during the period of multiple fission and then were degraded as cells completed their terminal divisions. MCM4 and MCM6 were localized to the nucleus during the division cycle except for transient cytoplasmic localization during mitosis. CDC45 showed strict protein location to the nucleus and co-localized to spindles during mitosis. In contrast, CDC6 was detected in the nucleus only transiently during early divisions within the overall multiple fission cycle. Cdc6 protein levels were very low, but increased upon treatment with MG132, a proteasome inhibitor. We also tested if these DNA replication proteins are regulated by cyclin dependent kinase (CDK). There are two main CDKs in Chlamydomonas, CDKA1 and CDKB1. We found that CDC6 protein level was severely reduced in a cdka1 mutant, but not in a cdkb1 mutant. MG132 did not detectably increase CDC6 levels in the cdka1 mutant, suggesting that CDKA1 upregulates CDC6 at the transcription level. Since MCM4, MCM6 and CDC6 were all essentially undetectable during the long G1 before DNA replication cycles began, we speculate that loading of origins with the MCM helicase may not occur until the end of the long G1, unlike in the budding yeast system. These results provide a microbial framework for approaching replication control in the plant kingdom.

plant biology↗