A forward genetic screen of N-ethyl-N-nitrosourea mutagenized Xenopus tropicalis has identified an inner ear mutant named eclipse (ecl). Mutants developed enlarged otic vesicles and various defects of otoconia development; they also showed abnormal circular and inverted swimming patterns. Positional cloning identified specificity protein 8 (sp8), which was previously found to regulate limb and brain development. Two different loss-of-function approaches using transcription activator-like effector nucleases and morpholino oligonucleotides confirmed that the ecl mutant phenotype is caused by down-regulation of sp8. Depletion of sp8 resulted in otic dysmorphogenesis, such as uncompartmentalized and enlarged otic vesicles, epithelial dilation with abnormal sensory end organs. When overexpressed, sp8 was sufficient to induce ectopic otic vesicles possessing sensory hair cells, neurofilament innervation in a thickened sensory epithelium, and otoconia, all of which are found in the endogenous otic vesicle. We propose that sp8 is an important factor for initiation and elaboration of inner ear development.T he vertebrate inner ear is a sensory organ responsible for balance and sound detection. Dysfunction of the inner ear is among the most common congenital disorders, affecting at least 1 in 500 births (1) and ∼40% of sensorineural deafness is associated with inner ear malformations (2). However, the study of hearing and balance impairment in humans is limited by the inability to follow inner ear development. Vertebrates share similarities in the sequence of developmental events that form the inner ear: the formation of an otic placode from an ectodermal thickening, morphogenesis to form the otocyst, and regional patterning of the otic vesicle (OV), resulting in the 3D membranous labyrinth (3, 4). Multipotent sensory progenitor cells are induced in the ectoderm surrounding the anterior neural plate, a domain termed the preplacodal region (PPR), and six1 has been characterized as marking this panplacodal domain. Signals from hindbrain and the regional expression of different transcription factors differentiate the PPR into the otic placode. The OV is partitioned by asymmetrical expression of various developmental regulators to pattern subdomains of the developing inner ear.The abilities of Xenopus to elucidate the cellular and molecular aspects of developmental processes position it as a valuable model organism (5-7). Earlier studies of lineage analysis and spatiotemporal expression of transcription factors during inner ear development led to construction of an inner ear fate map in Xenopus and this fate map allows us to interpret gene expression patterns within the context of the anatomy (8, 9). In recent years, genetic and genomic approaches have been developed in Xenopus tropicalis (10-13). To advance our understanding of inner ear development, we have screened N-ethyl-N-nitrosourea (ENU) mutagenized X. tropicalis colonies and recovered an inner ear mutant named eclipse (ecl). The ecl mutant perturbs specificity protei...