GENES ESTABLISHING DORSOVENTRAL PATTERN-FORMATION IN THE ZEBRAFISH EMBRYO - THE VENTRAL SPECIFYING GENES

We identified 6 genes that are essential for specifying ventral region s of the early zebrafish embryo, Mutations in these genes cause an exp ansion of structures normally derived from dorsal-lateral regions of t he blastula at the expense of ventrally derived structures, A series o f phenotypes of varied strengths is observed with different alleles of these mutants, The weakest phenotype is a reduction in the ventral ta il fin, observed as a dominant phenotype of swirl, piggytail, and somi tabun and a recessive phenotype of min fin, lost-a-fin and some piggyt ail alleles, With increasing phenotypic strength, the blood and pronep hric anlagen are also reduced or absent, while the paraxial mesoderm a nd anterior neuroectoderm is progressively expanded, In the strong phe notypes, displayed by homozygous embryos of snailhouse, swirl and somi tabun, the somites circle around the embryo and the midbrain region is expanded laterally, Several mutations in this group of genes are semi dominant as well as recessive indicating a strong dosage sensitivity o f the processes involved, Mutations in the piggytail gene display an u nusual dominance that depends on both a maternal and zygotic heterozyg ous genotype, while somitabun is a fully penetrant dominant maternal-e ffect mutation, The similar and overlapping phenotypes of mutants of t he 6 genes identified suggest that they function in a common pathway, which begins in oogenesis, but also depends on factors provided after the onset of zygotic transcription, presumably during blastula stages, This pathway provides ventral positional information, counteracting t he dorsalizing instructions of the organizer, which is localized in th e dorsal shield.