Figure 1. Nutrient limitation alters morphology and diversity of expanding colonies.

(a) Colony expansion is due to growth and motility. Cells divide at a rate proportional to the nutrient concentration and migrate without any spatial bias; the migration rate depends on the biomass concentration and possibly on nutrient concentration. (b) Our model can reproduce many of the typically observed colony morphologies including circular disks and colonies with finger-like protrusions or branches. Note that similar morphologies could have very different rates of genetic drift. This is evident from the number, crispness, and motility of boundaries between two different genotypes labeled with red and green colors. Starting from well-mixed initial conditions (yellow centers), the monochromatic domains (sectors) emerge because one of the genotypes becomes extinct locally due to the vagaries of reproduction and migration at the front.