The proper orientation of mechanosensory hair cells along the lateral-line organ of a fish or amphibian is essential for the animal's ability to sense directional water movements. Within the sensory epithelium, hair cells are polarized in a stereotyped manner, but the mechanisms that control their alignment relative to the body axes are unknown. We have found, however, that neuromasts can be oriented either parallel or perpendicular to the anteroposterior body axis. By characterizing the strauss mutant zebrafish line and by tracking labeled cells, we have demonstrated that neuromasts of these two orientations originate from, respectively, the first and second primordia. Furthermore, altering the migratory pathway of a primordium reorients a neuromast's axis of planar polarity. We propose that the global orientation of hair cells relative to the body axes is established through an interaction between directional movement by primordial cells and the timing of neuromast maturation.

Claudins, the major transmembrane proteins of tight junctions, are members of the tetraspanin superfamily of proteins that mediate cellular adhesion and migration. Their functional importance is demonstrated by mutations in claudin genes that eliminate tight junctions in myelin and the testis, abolish Mg 2؉ resorption in the kidney, and cause autosomal recessive deafness. Here we report that two paralogs among 15 claudin genes in the zebrafish, Danio rerio, are expressed in the otic and lateral-line placodes at their earliest stages of development. Related claudins in amphibians and mammals are expressed in a similar manner in vertebrate primordia such as sensory placodes, branchial arches, and limb buds. We also show that the claudin gene family may have expanded along the chordate stem lineage from urochordates to gnathostomes, in parallel with the elaboration of vertebrate characters. We propose that tight junctions not only form barriers in mature epithelia, but also participate in vertebrate morphogenesis.

To identify genes important for hair-cell function, we conducted a mutagenic screen in zebrafish. Larvae from one mutant line, ru848, were unresponsive to acoustic stimuli and unable to balance. The mutation results in a 90% reduction in hair-cell number and partial retinal degeneration by 5 days postfertilization. We localized the recessive ru848 mutation by positional cloning to the zebrafish homolog of the human Choroideremia gene, which encodes Rab escort protein 1. This protein is essential for the normal prenylation of Rabs. Mutations in the human gene induce choroideremia, a disease marked by slow-onset degeneration of rod photoreceptors and retinal pigment epithelial cells. The degenerative phenotype resulting from a null mutation in the zebrafish gene indicates that hair cells and retinal cells require Rab escort protein 1 for survival. The internal ear, one of the most complex organs of a vertebrate organism, converts mechanical stimuli into neuronal responses. The task of mechanotransduction is accomplished by the ear's sensory receptor, the hair cell. The lateralline organs of fishes and aquatic amphibians also use this receptor to detect water movements. Despite our detailed knowledge of the hair cell's mechanical and electrical properties (1), our progress in understanding its development and operation has been impeded by our lack of knowledge of the relevant molecular components.One strategy used to identify genes important for hair-cell function is the investigation of their loss-of-function phenotypes, deafness and disequilibrium. The principal advantage of such a genetic approach is that detailed characterization of the mutant phenotype provides explicit clues about the role of the gene involved. Large-scale mutagenesis screens for mechanosensory mutants in Drosophila melanogaster and Caenorhabditis elegans have identified molecules critical for vibration and touch sensitivity (2-4). The absence of hair cells in invertebrates, however, makes screens specific to hair-cell defects impossible in these organisms.To identify important proteins in the hair cell, we have created point mutations randomly throughout the zebrafish genome and screened for impairment of ear and lateral-line-organ development and function. Several of the identified mutations have been subjected to further phenotypic analysis and genetic mapping and cloning. We report here the characterization of a mutation that identifies an important process in hair cells and serves as a model of degenerative disease in the human retina. Materials and Methods ru848Identification and Breeding. Zebrafish were maintained in aquaria (Marine Biotech, Beverly, MA) according to published protocols (5). Embryos were raised in a 28.5°C incubator in facility water containing 1 g͞ml methylene blue and staged as described (6). Male fish of the TL strain were mutagenized with ethylnitrosourea and bred for three generations to create homozygous F 3 larvae (7) that were screened for hearing defects by observing their response to a sharp tap on the side o...