Ventro-posterior morphogenesis in zebrafish (Danio rerio): mutational analysis, genetic characterization and genomic mapping

In a large scale genetic screen for zygotic effect, embryonic mutations in zebrafish, 25 mutations that affect specification of cell fates and/or cellular rearrangements during gastrulation were recovered. These mutations define at least 15 loci and 14 complementation groups. Phenotypic analysis of the ten novel loci revealed three groups of mutations. One group consists of mutations that lead to deficiencies in dorsal fates. Mutations from the second group affect formation of ventro-posterior embryonic structures. Mutations in the third group affect primarily cellular rearrangements during gastrulation. The second class, of eight mutations, affecting ventro-posterior morphogenesis lead to a specific set of tail defects: loss of ventral fin fold, multiplicated posterior fin arrays and tail truncations. Six of these mutations comprise the captain hook (cpt) complementation group. The remaining two represent distinct loci, ogon and lost-a fin. Physical mapping of $cptsp{m52}$ and m169, a non-complementing mutation, reveals that these two mutations are at different loci and thus exhibit non-allelic non-complementation. Fine mapping of $cptsp{m52}$ and $dinosp{m84},$ a mutation recovered in Germany, confirms that they are alleles at the same locus. Consistent with malformations in ventral and posterior fates in cpt mutant embryos, expression of the ventro-posterior marker eve-1 shows an expansion from the ventral margin into the dorso-lateral margin at shield stage in some genetic backgrounds. The expression of molecular markers at the 20 somite stage in the posterior ventral regions of the $cptsp{m52}$ embryo is abnormal, including: a ventral expansion of myoD expression with fusion of somites at the ventral midline posterior to the anus; ectopic ventral expression of caudal; reduction in the ventral posterior expression of BMP-4. These alterations indicate a role for $cptsp{m52}$ in influencing the lateral boundary of dorso-lateral and ventro-lateral tissues during gastrulation and tail morphogenesis. Time-lapse analysis shows that cell migrations are abnormal at the tailbud stage, with cells migrating from dorsal regions of the extending tailbud into ventral regions. Heterogenotypic transplantations point to a cell non-autonomous defect in $cptsp{m52}$ cell migration.