Dynamics of cell shape inheritance in fission yeast

Abstract

Every cell has a characteristic shape key to its fate and function. That shape is not only the product of genetic design and of the physical and biochemical environment, but it is also subject to inheritance. However, the nature and contribution of cell shape inheritance to morphogenetic control is mostly ignored. Here, we investigate morphogenetic inheritance in the cylindrically-shaped fission yeast Schizosaccharomyces pombe. Focusing on sixteen different 'curved' mutants--a class of mutants which often fail to grow axially straight--we quantitatively characterize their dynamics of cell shape inheritance throughout generations. We show that mutants of similar machineries display similar dynamics of cell shape inheritance, and exploit this feature to show that persistent axial cell growth in S. pombe is secured by multiple, separable molecular pathways. Finally, we find that one of those pathways corresponds to the swc2-swr1-vps71 SWR1/SRCAP chromatin remodelling complex, which acts additively to the known mal3-tip1-mto1-mto2 microtubule and tea1-tea2-tea4-pom1 polarity machineries.

Figure 1. Dynamics of cell shape inheritance of curved and straight cells from genotypically identical S. pombe populations.

A. Visual classification of curved (C) and straight (S) cells in the ‘curved’ mutant tea4Δ. The overall penetrance percentage was calculated by dividing the number of cells visually classified as curved (18) by the total number of cells (44). Bar, 10 µm. B. Quantitative classification of curved and straight cells. The radius of curvature of each cell was quantitatively estimated by calculating the inverted radius of a circle crossing the cell centre and two ends. Cells with an inverted radius of curvature greater than radius⁻¹ = 0.028 µm⁻¹ were considered curved. C. Time-lapse image sequence showing the lineage of a single tip1Δ cell over two rounds of cell division. Images were taken every 10 minutes and curvature was measured for each cell after birth and before septation. Unique name identifiers were given to each daughter and grand-daughter cell to indicate its origins, e.g. the daughters of 1 are 1.1 and 1.2. For cells selected as ‘curved’, a circle of radius equal to the cell’s radius of curvature is drawn around it. See also Electronic Movie S1. Bar, 10 µm. D. The six types of morphological cell division outcomes observed in a mixed population containing curved and straight cells. E. The frequencies of those six outcomes over the entire cell lineage were calculated for four curved mutants and the wild-type, based on the phenotype of progenitors and progeny before cell septation. The penetrance at septation for each strain is shown. F. Duration of cell cycle and cell length, at birth and before septation, of curved (blue) and straight (grey) cells belonging to each of the indicated strains.

Figure 2. Active modulation of cell shape throughout generations.

A. Percentage of septating curved cells resulting from divisions of curved (blue) and straight (grey) mothers, for each mutant and the wild-type. Strains exhibiting a ratio between the first and the second percentages higher than 1.3 are highlighted by a yellow box. B. Percentage of septating curved cells resulting from curved (blue) and straight (grey) grandmothers. Strains exhibiting a ratio between the first and the second percentages higher than 1.3 are highlighted by a yellow box. C. Image sequences showing the inheritance, during two cell cycle rounds, of the Qdot-tagged cell wall (see Materials and Methods) of a tea1Δ and a mal3Δ cell lineage. Images are Z-stack maximal intensity projections of the medial 2 µm of cells. In both cases, curved cell wall segments (arrows) are transmitted practically unaltered from mother cells to their progeny, influencing not only their initial but also their final morphology. Note that the bottom tea1Δ cell shown rotated slightly. Such rotations were exceptional and rotating cells were not included in the quantitative analysis. D. Percentage of granddaughter cells (G2) that retain more than a third (light grey) or than 45% (dark grey) of the cell wall of their grandmother, for two monopolar mutants (tea1Δ and tea4Δ) and the bipolarly growing mutant mal3Δ. E. A mal3Δ cell that divides asymmetrically (red asterisk) predisposing one of its daughters to grow curved from the newly generated cell end. F. A ‘curved’ mgr2Δ cell (red asterisk) with two seemingly straight segments joined at an angle of 22° that, after division, produces two straight descendants (orange asterisks). G. Straight-to-curved (StoC) transition of a growing swc2Δ cell. H. Curved-to-straight (CtoS) transition of an initially curved tea1Δ cell (red asterisk) that ‘corrects’ its shape dynamically by growing. All bars represent 10 µm.

Figure 3. Cell shape inheritance rules predict the existence of multiple pathways controlling axial cell growth.

A–C. Clustergrams of all ‘curved’ mutants based on different inheritance frequencies quantitated: cell shape inheritance comparing the phenotype of mothers before septation with that of daughters before septation (A; using the six frequencies described in Fig. 1D); shape changes during growth (B; 1–4 represent, respectively, the frequencies of straight cells that remain straight after growth, straight cells that become curved, curved cells that straighten and curved cells that continue being curved); and cell shape inheritance comparing the phenotype of mothers before septation with that of daughters directly after septation, when they are born (C; the frequencies 1–6 follow the same logics as in A). The clustergrams display in a colour scale (black: average, red/green: higher/lower than the average) the different frequencies for each and order them hierarchically in a dendrogram based on their level of similarity. D. Superdendrogram grouping the sixteen curved mutants based on their similarities in cell shape inheritance pattern, combining all features quantitated (including cell shape and growth pattern, inheritance pattern over two generations, cytokinesis defects and cell shape changes during growth, see text for details). Three distinctive groups - ‘mitochondrial/ribosomal’, ‘cell polarity/microtubule cytoskeleton’ (‘pol’) and ‘chromatin remodelling’ (‘chr’) - and the wild-type are obtained using a cut-off at a distance of 0.24 (arbitrary units). E. Comparison of the overall penetrances of single (‘chr’: swc2Δ, swr1Δ and vps71Δ; ‘pol’: tea1Δ, tea2Δ, tip1Δ and tea4Δ) and double curved mutants (‘chr×chr’, ‘pol×pol’ and ‘chr×pol’; generated by the combination of each of the aforementioned single mutants). Penetrances of single and double mutants of the same group were not significantly different (p = 0.348 for the ‘chr’ group; p = 0.209 for the ‘pol’ group). By contrast, penetrances of double mutants from different groups were significantly different from the former (p = 0.023 for ‘chr×chr’ against ‘pol×pol’; p = 0.001 for the other two). F. Schematic representation of the different machineries that control axial cell growth in S. pombe.