Abstract
Unlike many single-celled organisms, the growth of fission yeast cells within a cell cycle is not exponential. It is rather characterized by three distinct phases (elongation, septation, and reshaping), each with a different growth rate. Experiments also showed that the distribution of cell size in a lineage can be bimodal, unlike the unimodal distributions measured for the bacterium Escherichia coli. Here we construct a detailed stochastic model of cell size dynamics in fission yeast. The theory leads to analytic expressions for the cell size and the birth size distributions, and explains the origin of bimodality seen in experiments. In particular, our theory shows that the left peak in the bimodal distribution is associated with cells in the elongation phase, while the right peak is due to cells in the septation and reshaping phases. We show that the size control strategy, the variability in the added size during a cell cycle, and the fraction of time spent in each of the three cell growth phases have a strong bearing on the shape of the cell size distribution. Furthermore, we infer all the parameters of our model by matching the theoretical cell size and birth size distributions to those from experimental single-cell time-course data for seven different growth conditions. Our method provides a much more accurate means of determining the size control strategy (timer, adder or sizer) than the standard method based on the slope of the best linear fit between the birth and division sizes. We also show that the variability in added size and the strength of size control in fission yeast depend weakly on the temperature but strongly on the culture medium. More importantly, we find that stronger size homeostasis and larger added size variability are required for fission yeast to adapt to unfavorable environmental conditions.
Highlights
The fission yeast Schizosaccharomyces pombe is a single-cell eukaryote whose shape is well approximated by a cylinder with hemispherical ends [1–3]
Later in mid G2 phase, the cell exhibits a transition in cell polarization, and growth is initiated at the new cell end, in a process called new end take-off (NETO) [1, 3]
The main aim of the present paper is to propose a detailed model of cell size dynamics in fission yeast that can characterize its non-exponential growth, cell division, and size homeostasis, as well as develop an analytical theory that can account for the bimodal shape of the cell size distribution
Summary
The fission yeast Schizosaccharomyces pombe is a single-cell eukaryote whose shape is well approximated by a cylinder with hemispherical ends [1–3]. The length of the rod-shaped cell increases during the G2 phase of the cell cycle, while its width (diameter) remains almost constant. The recent advent of microfluidic techniques allows the tracking of thousands of individual cells over hundreds of cell cycles which potentially enables a detailed investigation of cell growth and size control strategies [4]. It has been reported that cell size grows exponentially in many cell types such as bacteria, cyanobacteria, archaea, budding yeast, and mammalian cells [5–17]. Fission yeast undergoes a complex non-exponential growth pattern in each cell cycle, as illustrated by the timecourse data of cell size along a typical cell lineage (Fig 1A). At the beginning of the cell cycle, the rod-shaped cell starts to grow by extension at its old cell end (the end that existed before the last division). Cell length increases during the first *75% of the cell cycle. Cell elongation stops during the remaining *25% of the cell cycle, when mitosis and cytokinesis occur, and the cell subsequently divides into two almost identical progenies [1, 3]
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