JAW AND BRANCHIAL ARCH MUTANTS IN ZEBRAFISH .2. ANTERIOR ARCHES AND CARTILAGE DIFFERENTIATION

In a large scale screen for mutants that affect the early development of the zebrafish, 109 mutants were found that cause defects in the for mation of the jaw and the more posterior pharyngeal arches, Here we pr esent the phenotypic description and results of the complementation an alysis of mutants belonging to two major classes: (1) mutants with def ects in the mandibular and hyoid arches and (2) mutants with defects i n cartilage differentiation and growth in all arches, Mutations in fou r of the genes identified during the screen show specific defects in t he first two arches and leave the more posterior pharyngeal arches lar gely unaffected (schmerle, sucker, hoover and sturgeon). In these muta nts ventral components of the mandibular and hyoid arches are reduced (Meckel's cartilage and ceratohyal cartilage) whereas dorsal structure s (palato-quadrate and hyosymplectic cartilages) are of normal size or enlarged, Thus, mutations in single genes cause defects in the format ion of first and second arch structures but also differentially affect development of the dorsal and ventral structures within one arch. In 27 mutants that define at least 8 genes, the differentiation of cartil age and growth is affected. In hammerhead mutants particularly the mes odermally derived cartilages are reduced, whereas jellyfish mutant lar vae are characterized by a severe reduction of all cartilaginous eleme nts, leaving only two pieces in the position of the ceratohyal cartila ges. In all other mutant larvae all skeletal elements are present, but consist of smaller and disorganized chondrocytes. These mutants also exhibit shortened heads and reduced pectoral fins. In homozygous knorr ig embryos, tumor-like outgrowths of chondrocytes occur along the edge s of all cartilaginous elements. The mutants presented here may be val uable tools for elucidating the genetic mechanisms that underlie the d evelopment of the mandibular and the hyoid arches, as well as the proc ess of cartilage differentiation.