Effects of Cellular Chirality on Competition and Cooperation in Microbial Colonies
Is it better to be left- or right-handed? The answer depends on whether the goal is making a handshake or winning a boxing match. The need for coordination favors the handedness of the majority, but being different could also provide an advantage. The same rules could apply to microbial colonies and cancer tumors. Like humans, cells often have handedness or chirality that reflects the lack of mirror symmetry in their shapes or movement patterns. Moreover, chirality can easily evolve, and a change in chirality is often accompanied by an increase in fitness.
To answer how chirality provides a direct fitness advantage, we developed a minimal reaction-diffusion model of chiral growth in compact microbial colonies. For strains with equal chirality, our model reproduces logarithmic twisting of boundaries between the strains, as observed in the experiments. For strains with different chiralities, our model predicts either the exclusion of the less chiral strain or stable coexistence. Selection for specific chirality is mediated by bulges along the colony edge that appear in regions where the strains with different chirality meet. We developed an analytical framework to study this two-way coupling between selection and colony shape, which is ubiquitous in spatially structured populations, yet remains poorly understood. For chiral strains, the theory of population dynamics is described by the chiral KPZ equation coupled to the Burger’s equation with multiplicative noise. We obtain exact solutions for the key features of this theory including the shape of the bulges and their effect on competition between the strains. Overall, our work suggests that chirality could be an important ecological trait that mediates competition, invasion, and spatial structure in cellular populations.