Figure 6. Asymmetric fitness effects of cooperators and cheaters on partners drive self-organization.

Time progression of self-organization in a simulated community as observed in top-views (top) and vertical cross-sections (bottom). Heterotypic cooperative partners (green) supply large benefits to both cooperators (red) and cheaters (blue). Since the benefit is spatially localized, only cooperators and cheaters that are close to partners will grow. Given that cells dividing toward partners will on average have more access to benefits than those dividing away from partners, both cooperators and cheaters pile over partners. Cooperators reciprocate by supplying a large, but different, localized benefit to the partner, while cheaters do not. Thus, partners grow and pile over cooperators but not cheaters. Consequently, further growth of cooperators is facilitated, while cheaters become isolated and disfavored. In this simulation, cheaters have an 8% fitness advantage over cooperators. To mimic the top-view from microscopy, the top-view in this simulation represents the top-most layer of cells instead of the total intensity integrated over z at each pixel.

DOI: http://dx.doi.org/10.7554/eLife.00960.018

Figure 6—figure supplement 1. Localization of cooperative benefits is required for self-organization.

The partner association indexes and the $R_{\to A}^{\leftarrow L}$:$C^{\leftarrow L}$ ratios of simulated cooperating and cheating spatial communities were compared at different levels of localization of benefits. ‘Co&Ch’ communities had parameters similar to those in Figure 3—figure supplement 2B (purple circles) and therefore had normal levels of spatial localization of benefits. In ‘Exc. release’ communities (brown squares), cooperators and partners released excessive amounts of adenine and lysine, respectively (200-fold higher compared to the original communities). Since the neighboring cells could not consume the released metabolites fast enough, the benefits no longer remained localized to the vicinity of the releasing cell. In ‘Inst. distr.’ cases (black diamonds), the diffusion coefficient in the community was assumed to be very large, such that any released metabolite was instantly distributed among all cells. When benefits were delocalized either because of excessive metabolite release or rapid distribution of metabolite throughout the community, self-organization was diminished (A) and cooperators were disfavored compared to cheaters (B). In all cases, cheaters had an 8% intrinsic fitness advantage over cooperators, and communities were not disturbed (i.e., cells were not repositioned). Error bars show the standard deviation of ratios in six independent communities. Sim: simulation.

DOI: http://dx.doi.org/10.7554/eLife.00960.019