The zebrafish bozozok locus encodes Dharma, a homeodomain protein essential for induction of gastrula organizer and dorsoanterior embryonic structures

The dorsal gastrula organizer plays a fundamental role in establishment of the vertebrate axis. We demonstrate that the zebrafish bozozok (boz) locus is required at the blastula stages for formation of the embryonic shield, t he equivalent of the gastrula organizer and expression of multiple organize r-specific genes. Furthermore, boz is essential for specification of dorsoa nterior embryonic structures, including notochord, prechordal mesendoderm, floor plate and forebrain, We report that boz mutations disrupt the homeobo x gene dharma. Overexpression of boz in the extraembryonic yolk syncytial l ayer of boz mutant embryos is sufficient for normal development of the over lying blastoderm, revealing an involvement of extraembryonic structures in anterior patterning in fish similarly to murine embryos. Epistatic analyses indicate that boz acts downstream of beta-catenin and upstream to TGF-beta signaling or in a parallel pathway. These studies provide genetic evidence for an essential function of a homeodomain protein in beta-catenin-mediate d induction of the dorsal gastrula organizer and place boz at the top of a hierarchy of zygotic genes specifying the dorsal midline of a vertebrate em bryo.