The ventralizing activity of Radar, a maternally expressed bone morphogenetic protein, reveals complex bone morphogenetic protein interactions controlling dorso-ventral patterning in zebrafish

In Xenopus and zebrafish, BMP2, 4 and 7 have been implicated, after the ons et of zygotic expression, in inducing and maintaining ventro-lateral cell f ate during early development. We provide evidence here that a maternally ex pressed bone morphogenetic protein (BMP), Radar, may control early ventral specification in zebrafish. We show that Radar ventralizes zebrafish embryo s and induces the early expression of bmp2b and bmp4. The analysis of Radar overexpression in both swirl/bmp2b mutants and embryos expressing truncate d BMP receptors shows that Radar-induced ventralization is dependent on fun ctional BMP2/4 pathways, and may initially rely on an Alk6-related signalin g pathway. Finally, we show that while radar-injected swirl embryos still e xhibit a strongly dorsalized phenotype, the overexpression of Radar into sw irl/bmp2b mutant embryos restores ventral marker expression, including bmp4 expression. Our results suggest that a complex regulation of different BMP pathways controls dorso-ventral (DV) patterning from early cleavage stages until somitogenesis. (C) 2000 Elsevier Science Ireland Ltd. All rights res erved.