  Chirality in shape and motility can evolve rapidly in microbes and cancer
cells. To determine how chirality affects cell fitness, we developed a model of
chiral growth in compact aggregates such as microbial colonies and solid
tumors. Our model recapitulates previous experimental findings and shows that
mutant cells can invade by increasing their chirality or switching their
handedness. The invasion results either in a takeover or stable coexistence
between the mutant and the ancestor depending on their relative chirality. For
large chiralities, the coexistence is accompanied by strong intermixing between
the cells, while spatial segregation occurs otherwise. We show that the
competition within the aggregate is mediated by bulges in regions where the
cells with different chiralities meet. The two-way coupling between aggregate
shape and natural selection is described by the chiral Kardar-Parisi-Zhang
equation coupled to the Burgers' equation with multiplicative noise. We solve
for the key features of this theory to explain the origin of selection on
chirality. Overall, our work suggests that chirality could be an important
ecological trait that mediates competition, invasion, and spatial structure in
cellular populations.
