In vertebrate inner ears,sensory patches,which containing hair cells and supporting cells,are essential for hearing and balance functions.How the stereotypically organized sensory patches are formed remains to be understood.In this study,we isolated a zebrafish mutant in which the jag1b gene is disrupted by an EGFP insertion.Loss of Jag1b causes cell death in the developing posterior crista and results in down regulation of fgf10a in the posterior prosensory cells.Inhibition of FGFR activity in wild type embryos also causes loss of the posterior crista,suggesting that Fgf10a mediates the Jag1b activity.In contrast,in anterior prosensory domain,Jag1b regulates separation of a single morphogenetic field into anterior and lateral cristae byflatting cells destined to form nonsensory epitheliums between the two cristae.MAPK activation in the nonsensory epithelium precursors is required for the separation.In the Jag1bmutant,MAPK activation and cell flatting are extended to anterior crista primordia,causing loss of anterior crista.More importantly,inhibition of MAPK activity,which blocks the differentiation of nonsensory epithelial cells,generated a fused large crista and extra hair cells.Thus Jag1b uses two distinct mechanisms to form three sensory cristae in zebrafish.