Evolutionary emergence of the rac3b / rfng / sgca regulatory cluster refined mechanisms for hindbrain boundaries formation.

Developmental programs often rely on parallel morphogenetic mechanisms that guarantee precise tissue architecture. While redundancy constitutes an obvious selective advantage, little is known on how novel morphogenetic mechanisms emerge during evolution. In zebrafish, rhombomeric boundaries behave as an elastic barrier, preventing cell intermingling between adjacent compartments. Here, we identify the fundamental role of the small-GTPase Rac3b in actomyosin cable assembly at hindbrain boundaries. We show that the novel rac3b / rfng / sgca regulatory cluster, which is specifically expressed at the boundaries, emerged in the Ostariophysi superorder by chromosomal rearrangement that generated new cis -regulatory interactions. By combining 4C-seq, ATAC-seq, transgenesis, and CRISPR-induced deletions, we characterized this regulatory domain, identifying hindbrain boundary-specific cis -regulatory elements. Our results suggest that the capacity of boundaries to act as an elastic mesh for segregating rhombomeric cells evolved by cooption of critical genes to a novel regulatory block, refining the mechanisms for hindbrain segmentation.