Mutations affecting skeletal muscle myofibril structure in the zebrafish.

We describe embryonic lethal mutations in the zebrafish, Brachydanio rerio, which affect organization of skeletal muscle myofibrils. The mutations, fub-1(b45) and fub-1(b126), were independently isolated from progeny of gamma-irradiated females. Each segregates as a single recessive gene: b45 is located about 23 map units from its centromere. The b126 mutation has a similar but slightly larger apparent gene-centromere distance and a less severe phenotype. The two mutations fail to complement, suggesting that they are allelic. Homozygous b45 mutant embryos are paralyzed, and their axial skeletal muscle cells are unstriated, containing severely disorganized myofibrillar components. Gel-electrophoretic comparisons of b45 mutant and wild-type muscle proteins failed to reveal absent or altered major myofibrillar proteins. Embryos genetically mosaic for b45 were also phenotypically mosaic, suggesting that the defect is cell-autonomous. We suggest that these mutations identify a gene required for proper organization of skeletal muscle myofibrils, and that the more severe mutation may represent a null allele.