Fig 1.
The model structure consists of two parts: ecology and development. Ecology: cells can express two phenotypes, non-sticky cells or sticky cells. The sticky and non-sticky cells occur in one of two niches: the liquid or the surface. Cells in liquid are well mixed. Cells on the surface are placed on hexagonal grid and can only stay on the surface when being sticky or directly surrounded by sticky cell. Cells can migrate from the liquid to the surface and vice versa. Migration to the surface is only possible when being sticky or when a cell is directly surrounded by sticky cell on the surface. Development: three differentiation strategies are examined in the model: (1) pure strategy, (2) probabilistic strategy, (3) decision-making strategy. In the pure strategy, cells only express one phenotype and can switch via mutations. In the probabilistic strategy, cells have a probability P to differentiate. In the decision-making strategy, cells can differentiate in response to the environment. Cells sense two environmental cues: the niche in which they occur (N) and the fraction of surrounding sticky cells (i.e. stickiness, S). Cells differentiate when the sum of regulatory input, weighted by connection weights (W1 and W2), exceeds the activation threshold (θ).
Fig 2.
Colonization of surface at end of evolution.
Representative surfaces at the end of evolution with sticky cells (red) and non-sticky cells (blue). Cells are attached to a hexagonal grid (black lines). Surfaces are shown for independent simulations at different relative cell division rates of sticky cells (R) and for the three differentiation strategies (see Fig 1).
Fig 3.
Population size and fraction of sticky cells on surface and in liquid.
(A) Population size on surface for three differentiation strategies (blue = pure strategy, red = probabilistic strategy, green = decision-making strategy) at different relative cell division rates of sticky cells (R). The carrying capacity of surface is 10.000 cells. (B) Fraction of sticky cells on surface and in liquid. Fraction of sticky cells on surface is only calculated for the simulations in which more than 100 cells attached to the surface. For low cell division rates of the sticky cell, only the decision-making cells attached to the surface. The black arrows in (A) and (B) point to simulations in which the sticky cells could not divide (R = 0). This condition is further examined in Fig 4 and Fig 5.
Fig 4.
Colony expansion and cell death.
(Left) Scheme showing the sequential stages of colony fission: (1) cell death, (2) cell differentiation, (3) cell division. Red and blue cells are sticky and non-sticky cells, respectively. (Right) Competition between three genotypes that differ in the death rate of sticky cells: 0% (red line), 5% (blue line) or 10% (green line) chance of cell death. Rate of cell death of non-sticky cells is the same for all genotypes (Pd = 10%). The average frequency ± SD (n = 10) is shown for each genotype over 10.000 time steps. The average population size (black line) is shown with respect to the total carrying capacity (including surface and liquid). At onset of competition, the population in the liquid is saturated (5.000 cells) and on the surface 100 cells of each genotype are randomly placed. Population growth is characterized by two phases: (A) population growth before the inflection point (i.e. colonization stage) and (B) population growth after the inflection point (i.e. climax stage). (Lower) Representative surfaces at different stages of competition. Colours correspond to the different genotypes (red = 0% death rate, blue = 5% death rate, green = 10% death rate). The cells with grey and black outline are non-sticky and sticky cells, respectively. Simulations were performed under low migration rate from the liquid to the surface (Pm = 0.01).
Fig 5.
Colony expansion and filament formation.
Competition between a filament-forming genotype (i.e. colonizing genotype) and a genotype that produces isolated sticky cells (i.e. climax genotype). (Left) Average frequency ± SD (n = 10) of genotypes (blue = colonizing genotype, green = climax genotype) during competition for 4.000 time steps. The average population size (black line) is shown with respect to the total carrying capacity (including surface and liquid). At onset of competition, the population in the liquid is saturated (5.000 cells) and on the surface 100 cells of each genotype are randomly placed. Population growth is characterized by two phases: (A) population growth before the inflection point (i.e. colonization stage) and (B) population growth after the inflection point (i.e. climax stage). (Right) Surfaces (A) and (B) show colony during colony expansion and at carrying capacity (for time sequence see S4 Fig). Colours correspond to the different genotypes (blue = colonizing genotype, green = climax genotype). The cells with grey and black outline are non-sticky and sticky cells, respectively. Simulations were performed under low migration rate from liquid to the surface (Pm = 0.01).
Fig 6.
Fraction of sticky cells and population size at end of evolution.
Level plots show the fraction of sticky cells and population size on the surface and in liquid for different parameter combinations of R (cell division rate of sticky cell) and Pm (migration rate). Each row of plots corresponds to one differentiation strategy. From left to right the level plots show (1) fraction of sticky cells on surface, (2) fraction of sticky cells in liquid, (3) population size on surface, (4) population size in liquid. Fraction of sticky cells ranges from 0 (blue) to 1 (red). The population size ranges from 0 (white) to the carrying capacity (green). The carrying capacity of the surface is 10.000 (100 x 100 positions on the hexagonal grid) and the carrying capacity of the liquid is 5.000 (K). All values are determined at the end of evolution (T = 400.000 time steps).
Fig 7.
Schematic overview of life cycles that evolve at different relative cell division rates of sticky cells (R) and migration rates (Pm). Four distinct life cycles evolved. Colonies reproduce either by producing propagules–that migrate via the liquid to new surface areas–or by fission. (Left) Parameter combinations at which different life cycles evolved (see also S5 Fig). (Right) Schematic representations of the different life cycles. Colony propagule shows behaviour of cells in the liquid, cells either remain non-sticky (blue) or differentiate to sticky cell (red). Grey squares represent surface: left squares shows the initial stage of surface colonization; the squares in the middle show the typical colony; the squares on the right show colony fission.