Stem cell competition.

Cell competition is a conserved, fitness-based process in which "winner" cells eliminate or displace less-fit "loser" neighbors, thereby shaping development, tissue homeostasis, and disease. This review synthesizes how principles of competition operate in stem cell compartments, focusing on the adult Drosophila testis as a genetically tractable model where neutral competition provides a baseline for stem cell turnover, and defined genetic perturbations bias competition to favor specific clones. I compare intra- and inter-lineage competition among somatic cyst stem cells, germline stem cells, and dedifferentiating germ cells, highlighting mechanisms that link signaling, proliferation, adhesion, and niche remodeling to competitive fitness. I then discuss mammalian paradigms such as clonal hematopoiesis, intestinal "supercompetitors," and spermatogonial stem cell dynamics in open niches, framing stem cell competition as a driver of aging, clonal evolution, and cancer risk.