Strongly oversized fission yeast cells lack any size control and tend to grow linearly rather than bilinearly

Abstract

During the mitotic cycle, the rod‐shaped fission yeast cells grow only at their tips. The newly born cells grow first unipolarly at their old end, but later in the cycle, the ‘new end take‐off’ event occurs, resulting in bipolar growth. Photographs were taken of several steady‐state and induction synchronous cultures of different cell cycle mutants of fission yeast, generally larger than wild type. Length measurements of many individual cells were performed from birth to division. For all the measured growth patterns, three different functions (linear, bilinear and exponential) were fitted, and the most adequate one was chosen by using specific statistical criteria, considering the altering parameter numbers. Although the growth patterns were heterogeneous in all the cultures studied, we could find some tendencies. In cultures with sufficiently wide size distribution, cells large enough at birth tend to grow linearly, whereas the other cells generally tend to grow bilinearly. We have found that among bilinearly growing cells, the larger they are at birth, the rate change point during their bilinear pattern occurs earlier in the cycle. This shifting near to the beginning of the cycle might finally cause a linear pattern, if the cells are even larger. In all of the steady‐state cultures studied, a size control mechanism operates to maintain homeostasis. By contrast, strongly oversized cells of induction synchronous cultures lack any sizer, and their cycle rather behaves like an adder. We could determine the critical cell size for both the G1 and G2 size controls, where these mechanisms become cryptic. TAKE AWAY Most individual fission yeast cells in steady‐state cultures grow bilinearly. In strongly oversized fission yeast cells, linear growth dominates over bilinear. Above birth length thresholds, both the G1 and G2 size controls become cryptic.

Cell Length Growth in the Fission Yeast Cell Cycle: Is It (Bi)linear or (Bi)exponential?

“…Every individual visible cell was measured in every frame from birth to division. As the photos had a relatively high resolution, the length growth patterns were not smoothed in contrast to several previous studies [5,6,[20][21][22]. For every individual cell, the elongation period was determined by eye, omitting once the data of the constant length period, and sometimes also some data from the beginning of the cycle (if growth abnormalities occurred).…”

“…The cultures are steady-state ones, i.e., the cells are studied in a mid-exponential phase without any perturbation. The films taken for this study have much better spatial and time resolutions than our formerly analysed films [5,[19][20][21][22]36]. In all these former studies, the length growth pattern data were smoothed before the analyses; however, the better resolution makes it possible to ignore smoothing in the present research.…”

“…Fission yeast has been an attractive model organism for several decades in studies of eukaryotic cellular growth [1][2][3][4][5][6][7]. Revealing the rules of cellular growth is crucial in understanding how size control mechanisms maintain size homeostasis in steady-state cell cultures, and this point has also been extensively studied in fission yeast [5,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].…”

“…To date, three different models have mainly been proposed to describe the cellular growth process during the fission yeast cell cycle [3,4,6,7,22,[36][37][38]. First of all, the simplest linear model (with two parameters) assumes a constant growth rate during the elongation period, which is supported by tip growth [31].…”

“…In this case, a non-linear regression is suitable to position the RCP (rather than the researcher oneself), making the fitting more accurate. For 'historical' reasons, this RCP positioned by regression is designated as RCP2 later in this paper [5,20,22]. More precisely, this second type model is a linearized biexponential one (LinBiExp); however, it is often referred as 'bilinear' for simplicity [3].…”