We use a realistic simulation framework to model large cell groups, and observe that cell lines can spontaneously segregate from each other in space as the group expands. Natural selection typically favors individuals that maximize their own reproduction, so how is it that many diverse organisms, from bacteria to humans, have evolved to help others at a cost to themselves? Research has shown that cooperation can most readily evolve when cooperative individuals preferentially help each other, but this leaves open another critical question: How do cooperators achieve selective interaction with one another? We focus on this question in the context of unicellular organisms, such as bacteria, which exhibit simple forms of cooperation that play roles in nutrient acquisition and pathogenesis. We argue that cooperative and exploitative cell lineages will spontaneously segregate in space under a wide range of conditions and, therefore, that cellular cooperation may evolve more readily than naively expected.Ĭooperation is a fundamental and widespread phenomenon in nature, yet explaining the evolution of cooperation is difficult. Realistic changes in these parameters qualitatively alter the emergent structure of cell groups, and thereby determine whether cells with cooperative phenotypes can locally and globally outcompete exploitative cells. Our simulations reveal that the spatial distribution of genetic lineages within these groups is linked to a small number of physical and biological parameters, including cell growth rate, nutrient availability, and nutrient diffusivity. Here we use a multi-agent computational model to study the origin of spatial structure within growing cell groups. However, if cooperative cells are segregated in space and preferentially interact with each other, they may prevail. It is difficult for cooperative cells to succeed in competition if they become mixed with non-cooperative cells, which can exploit the public good without themselves paying a cost. Population spatial structure may be critical for this problem because it influences the extent of interaction between cooperative and non-cooperative individuals. Simple cooperative phenotypes appear to abound in the microbial world, but explaining their evolution is challenging because they are often subject to exploitation by rapidly growing, non-cooperative cell lines. However, via strength in numbers and the expression of cooperative phenotypes, such cells can enormously impact their environments. On its own, a single cell cannot exert more than a microscopic influence on its immediate surroundings. As a result, when it commits a large amount of resources to enzyme secretion, the cooperative cell line can fair so poorly in local competition that it fails to invade on a global scale. This departure from our broader conclusion occurs because thin active layer conditions, while promoting cell lineage segregation and generally favoring cooperation, also increase the strength of competition within cell groups. It should be noted, however, that when cooperative cells invest heavily into enzyme secretion, they may fare better in global competition under thick active layer conditions that promote lineage mixing (For enzyme production rate = 2, the blue trace is above the invasion criterion in D, but below the invasion criterion in E and F). In this important sense, our results are robust. When cooperative cells can invade, however, they can also prevent the exploitative cell type from re-invading (red traces are below unity). (E–F) Under thin active layer conditions, there is a narrower range of enzyme production rates at which cooperative cells can invade (blue traces).

The exploitative cell type can also strongly invade (red trace). (D) Cooperative cells can often invade under thick active layer conditions (blue trace), even though such conditions prevent them from prevailing in local competition. Each filled circle denotes mean invasiveness, and bars denote standard deviations. Panels D–F summarize the global invasion analysis. (B–C) Under thin active layer conditions, cooperative cells can prevail over exploitative cells, though under a much narrower range of enzyme production rates than for lower cost/benefit ratio of cooperation (Compare with Figure 5B–C, Main Text). (A) Under thick active layer conditions, cooperative cells always lose in local competition. As for Figure 5 in the main text, each open black circle represents the reproductive fitness of the cooperative strain after a single simulation. Panels A–C summarize the local competition simulations. Figure S3: The local competition and global invasion analyses were repeated with a higher cost/benefit ratio for cooperative enzyme secretion.

Maxbulk mailer infection