Fig. 1.

S. cerevisiae colony-growth experiments hint at physical interactions between strains. (A) Combined fluorescent and bright-field image of a colony, 4 d after the inoculation of a mixed droplet of strains yAG1 (blue) and yAG2 (yellow) at relative frequencies of 99% and 1%, grown at 30 °C. The white dashed circle marks the inoculum size. Because yAG2 divides faster at 30 °C, some single-strain yellow sectors expand their opening angle $\varphi$ as the colony grows radially, while others go extinct due to stochastic effects in the early stages of growth. By expanding faster, the faster-growing strain displaces the other strain at both sides of yellow sectors (A1) and can enhance the expansion speed of the slower-growing strain when it is trapped between two yellow sectors (blue filaments in A2) and is carried along. (B and C) Even nonproliferating cells (magenta) can be pulled outward by a proliferating (cyan) strain. The images show combined fluorescent and bright-field images of the same colony at early and later times. In this experiment, the non–heat-sensitive strain yAG19 (cyan, initial frequency 10%) and the heat-sensitive strain yAG20 (magenta, initial frequency 90%) were grown at 28 °C for 2 d and subsequently imaged with a stereoscope incubated at 37 °C, a temperature at which yAG20 cells cannot divide. B shows the first snapshot at which the heat-sensitive strain stopped expanding (SI Appendix, Fig. S1D). C shows the colony after a further 8.2 h. (C1) Close-up of the region marked with a white square in C, highlighting the displacement of the nongrowing, heat-sensitive strain next to expanding sectors of the growing strain and an expanding magenta filament trapped between two cyan sectors. Because the heat-sensitive strain did not divide between B and C, its displacement is due to the physical interaction between the two strains.