Survival of the chiral: Chiral cells invade by altering spatial structure

Short Abstract: Is it better to be leftor 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 (chirality) that reflects the lack of mirror symmetry in their shapes or movement patterns. We find that cells gain a substantial fitness advantage by either increasing the magnitude of their chirality or switching to the opposite handedness. Selection for specific chirality is mediated by the formation of bulges along the colony edge in regions where cells with different chiralities meet. Long Abstract: The origin of chirality has intrigued many generations of scientists, who explained it by physical laws, chance, or the need for homochirality. The role of natural selection, however, has been largely overlooked. Yet, chirality in shape, motility, and interactions with the environment can readily evolve in unicellular organisms such as bacteria and cancer cells. Moreover, the change in chirality is often accompanied by an increase in fitness. To answer whether chirality can provide a direct fitness advantage, we developed a minimal reaction-diffusion model of chiral growth in compact microbial colonies. For strains with equal chiralities, our model quantitatively reproduces logarithmic twisting of boundaries between the strains and other spatial patterns 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 chiralities 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. 1 ar X iv :1 71 0. 09 45 9v 1 [ qbi o. PE ] 2 5 O ct 2 01 7