Expansion dynamics alter mutualistic and antagonistic interactions in growing microbial colonies

From antibiotic resistance to cancer progression, evolutionary processes often involve competition among mutated subpopulations within dense cellular aggregates. This competition manifests in two distinct forms: a global race to expand outward and a local struggle for survival amidst neighboring cells. Recently, this dual competition has been modeled using a modified Kardar-Parisi-Zhang (KPZ) equation coupled with a generalized Fisher-Kolmogorov equation—the former capturing colony morphology and global expansion, and the latter describing local survival dynamics. In our work, we extend this framework by incorporating mutualistic and antagonistic interactions that operate at both local and global scales. We find that local mutualism and antagonism are respectively amplified or suppressed depending on whether mutants enhance or reduce each other's global expansion rates. Notably, under certain conditions, global mutualism leads to the formation of growing triangular bulges near the mutant interface, whereas global antagonism induces the development of expanding fissures on the colony surface.