Muscle Stem Cells Undergo Extensive Clonal Drift during Tissue Growth via Meox1-Mediated Induction of G2 Cell-Cycle Arrest.

Organ growth requires a careful balance between stem cell self-renewal and lineage commitment to ensure proper tissue expansion. The cellular and molecular mechanisms that mediate this balance are unresolved in most organs, including skeletal muscle. Here we identify a long-lived stem cell pool that mediates growth of the zebrafish myotome. This population exhibits extensive clonal drift, shifting from random deployment of stem cells during development to reliance on a small number of dominant clones to fuel the vast majority of muscle growth. This clonal drift requires Meox1, a homeobox protein that directly inhibits the cell-cycle checkpoint gene ccnb1. Meox1 initiates G cell-cycle arrest within muscle stem cells, and disrupting this G arrest causes premature lineage commitment and the resulting defects in muscle growth. These findings reveal that distinct regulatory mechanisms orchestrate stem cell dynamics during organ growth, beyond the G/G cell-cycle inhibition traditionally associated with maintaining tissue-resident stem cells.