Zebrafish Wnt9b synteny and expression during first and second arch, heart, and pectoral fin bud morphogenesis

Roles for Wnt9b in craniofacial development are indicated by the cleft lip mutant phenotype observed in the A/WySn mouse strain,(1) caused by a retrotransposon insertion mutation at the Wnt9b locus. Analyses of the zebrafish Wnt9b ortholog, wnt9b, were pursued to provide insight into early vertebrate craniofacial patterning events mediated by Wnt9b signaling. Zebrafish wnt9b cDNA clones were isolated and found to encode an open reading frame of 358 amino acids, with 68% amino acid identity to mouse Wnt9b and 70% amino acid identity to human WNT9B. Syntenic analyses demonstrated that wnt9b and wnt3 exist as a contiguous pair in amniote vertebrate species, and that these genes are separate in the zebrafish and Takifugu genomes. During the pharyngula period, a time of extensive growth and morphogenesis, zebrafish wnt9b exhibits discrete expression in dorsal and ventral first and second branchial arch tissues, the heart, and pectoral fin buds. These analyses suggest that in zebrafish, as in humans, wnt9b plays distinct roles in directing morphogenetic movements of developing branchial arch elements, and identify the zebrafish as a useful developmental model for the study of human craniofacial cleft lip and palate.