Somites are metameric structure transiently formed beside the neural tube during vertebrate development and give rise to various tissues including vertebral bone, skeletal muscle and dermis in the adult body. During somitogenesis, somites are sequentially and periodically generated from the anterior end of presomatic mesoderm (PSM). The periodical generation of somite is known to be explained by “Clock and Wavefront” model, in which the spatio-temporal information provided by the cyclic activation of “segmentation clock” and posterior regression of “wave front” are integrated and converted to the morphological pattern of somite at the anterior PSM. In vertebrate somitogenesis, the wave of the activation of segmentation clock propagates from the posterior to anterior PSM and ceases at the anterior end of PSM, in which future segmentation boundary is formed. Previously the termination of segmentation clock was considered to be caused by the gradual decrease of oscillation frequency of segmentation clock, because wave length of segmentation clock became narrow before the termination of segmentation clock. However, recent time-lapse analysis suggested that termination of segmentation clock regulated in the manner independent to the decrease of oscillation frequency. Thus the molecular mechanisms to coordinate the formation of segmentation boundary and the termination of segmentation clock still remains to be elucidated.
In this study, we found Ripply, a groucho interacting protein, negatively regulated expression of the clock genes, her1 and her7, resulting in termination of the oscillatory expression of her1 and her7 in the anterior PSM. Conversely, Her1 and Her7 negatively regulate expression of ripply1 and ripply2 in the anterior PSM, suggesting that mutual repression between ripply and her is important for segmentation clock termination. Since ripply1 and ripply2 are required for the formation of somite boundaries, the mutual repression is important for coupling of the timing of the boundaries formation and the clock termination in zebrafish somitogenesis.