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A single light-responsive sizer can control multiple-fission cycles in Chlamydomonas

F. S. Heldt,J. Tyson,F. Cross,B. Novák

Published 2019 in bioRxiv

ABSTRACT

Proliferating cells need to coordinate cell division and growth to maintain size homeostasis. Any systematic deviation from a balance between growth and division results in progressive changes of cell size over subsequent generations. While most eukaryotic cells execute binary division after a mass doubling, the photosynthetic green alga Chlamydomonas can grow more than eight-fold during daytime before undergoing rapid cycles of DNA replication, mitosis and cell division at night, which produce up to 16 daughter cells. Here, we propose a mechanistic model for multiple fission and size control in Chlamydomonas. The model comprises a light-sensitive and size-dependent biochemical toggle switch that acts as a sizer and guards transitions into and exit from a phase of cell-division cycle oscillations. We show that this simple ‘sizer-oscillator’ arrangement reproduces the experimentally observed features of multiple-fission cycles and the response of Chlamydomonas cells to different light-dark regimes. Our model also makes testable predictions about the dynamical properties of the biochemical network that controls these features and about the network’s makeup. Collectively, these results provide a new perspective on the concept of a ‘commitment point’ during the growth of Chlamydomonas cells and hint at an intriguing continuity of cell-size control in different eukaryotic lineages. Graphical abstract G1-sizer and S/M-oscillator can give rise to multiple-fission cycles in Chlamydomonas Light-responsive bistable switch may guard transition between G1 and S/M-cycles Illumination increases S/M-entry threshold, causing multiple-fission cycles Dark shift lowers S/M-entry threshold, allowing small cells to commit to fewer divisions

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