Figure 1.
(a) A micrograph of the edge of a two-dimensional budding yeast S. cerevisiae cell colony grown on petri dish, with the initial population containing mostly yellow converted cells, with a few unconverted blue. In this linear range expansion (characterized by a conversion rate, μ, and a selective advantage, s, of blue cells over yellow cells), some of the blue cells at the colony periphery form a spatial sector with an opening angle, θ, marked with overlaid black lines. With each cell division, the blue cells enjoying a selective advantage, s, convert to yellow ones at a rate μ, which creates yellow patches within the blue sector. The growth, or timelike, direction is indicated. (b) A phase diagram indicating where theory predicts the eventual extinction of the blue strain as a function of its selective advantage, s, and conversion rate μ for a linear range expansion. In the yellow “inactive” phase, a genetic sector formed by a blue cell always dies out, leading to a fully converted population. In the blue “active” phase, there is a nonzero probability of forming a surviving cluster, leading to a population with conversion occurring stochastically. The transition line for a well-mixed population is also shown for comparison (dashed line). In the well-mixed case, the active phase forms a much larger region in the -plane. The insets show examples of simulated sectors. Note the resemblance between the sector in the active phase and the experimental sector in (a). To see this figure in color, go online.