Characterizing zebrafish maternal effect genes involved in cytokinesis and axis formation

The zebrafish has emerged as a powerful system to study a variety of biological processes. Studying maternal effect mutants in zebrafish provides a great tool to dissect the molecular mechanisms of maternal control in vertebrate development. Here we present the characterization of positional cloning of two zebrafish maternal effect mutations, one in the gene nebel that affects cytokinesis, and another one in the gene hecate that affects dorsal axis formation.;nebel is a recessive, strictly maternal gene involved in cytokinesis. nebel mutant embryos show a dramatic reduction in the intensity of the slow calcium waves that accompany furrow formation. nebel mutant embryos show cytokinesis defects in cleavage stages as well as later defects in dorsal gene expression and convergence extension. Positional cloning indicates that the mutation in nebel is associated with the substitution of a highly conserved amino acid in the gene L-aspartate dehydrogenase. Studies of this gene in bacteria indicate that it may be involved in the synthesis of key players in calcium signaling.;hecate mutation results in ventralized embryos that have defects in the formation of dorsoanterior structures. hecate mutant embryos have reduced expression of genes specific to the dorsal organizer. Positional cloning reveals that hecate encodes glutamate receptor interacting protein 2 (Grip2) and is essential for dorsal determination. Zebrafish grip2 mRNA localizes to one side of the vegetal pole of the yolk during the zygote and cleavage stages before midblastula transition. In hecate mutant embryos, grip2 mRNA expression is strongly reduced. Further studies of this gene will provide great insight to our understanding of zebrafish early dorsal determination.