Figure 2: Delayed extinction can be described via an effective surface tension.
a) Schematic of a sector boundary (at steady state) in the null model exhibiting discontinuous front velocities (arrows). b) Tracks (blue/yellow arrows) of individual pioneer cells close to a clone boundary between a slower-growing (strain yJK22, yellow, s = −0.06) and a faster-growing sector (strain yMM9, false-color blue). White lines indicate the front position at shown times. Relative images positions are to scale. See movie S9 for complete time lapse series. c) Comparison of observed and expected clone-boundary and front behavior in a linear colony. Strains and colors as in b. Bottom and top panels are recorded at indicated times after inoculation and relative positions are to scale. Center: null model prediction. Blue/yellow arrows indicate the direction of motion of front cells perpendicular to the local front-line (white) as seen in panel b. Orange dashed lines and arrows delineate straight boundary trajectories as predicted by the null model. Dashed blue lines outline experimentally observed boundaries. d) Consecutive snapshots of a progressing wild-type front starting from cells deposited in a highly curved geometry. Arrows indicate different amounts of displacement of flat and curved regions. Time intervals are adjusted for constant displacement of flat regions (black arrows). e) Velocity v of cells at the front as a function of the local curvature κ (see movie S3 for cell tracking data). Fitting the linear relation v = v0(1 + Tκ) to the data results in v0 = 0.94μm/min and T = 27μm (dashed black line). f) Inset: Schematic of the surface tension model. v is the front velocity and vtype is the cell-type specific velocity of a flat front. The main graph shows the clone width w (average of N = 9 sectors shown in Supplementary Fig. S2) as a function of front position y with respect to yc = y(w = wc). The surface tension model is fitted to the experimental data, yielding a surface tension of T = 25μm. Gray shaded area indicates one standard deviation. The orange dotted line delineates the null model prediction. See panel g) for legend. g) Clone width-dependent effective fitness difference seff(w) calculated from the slope of w(y) (see Supplementary Sec. 1 and Fig. S9 for details).