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Recent studies have shown that packings of cells, both eukaryotic cellulartissues and growing or swarming bacterial colonies, can often be understood asactive nematic fluids. A key property of volume-conserving active nematic modelsystems is chaotic self-mixing characterized by motile topological defects.However, for active nematics driven by growth rather than motility, less isunderstood about mixing and defect motion. Mixing could affect evolutionaryoutcomes in bacterial colonies by counteracting the tendency to spatiallysegregate into monoclonal sectors, which reduces the local genetic diversityand confines competition between subpopulations to the boundaries betweenneighboring sectors. To examine whether growth-driven active nematic physicscould influence this genetic demixing process, we conduct agent-basedsimulations of growing, dividing, and sterically repelling rod-like bacteria ofvarious aspect ratios, and we analyze colony morphology using tools from bothsoft matter physics and population genetics. We find that despite measurabledefect self-propulsion in growth-driven active nematics, the radial expansionflow prevents chaotic mixing. Even so, at biologically relevant cell aspectratios, self-mixing is more effective in growing active nematics of rod-likecells compared to growing isotropic colonies of round cells. This suggestspotential evolutionary consequences associated with active nematic dynamics.

Do active nematic self-mixing dynamics help growing bacterial colonies to maintain local genetic diversity?