Expression of rasgef1b in zebrafish.

The rasgef genes encode a subgroup of highly conserved Ras guanine nucleotide exchange factors. While EST projects revealed the presence of rasgef genes in organisms that range from nematodes to humans, their functions remain to be elucidated. In zebrafish two rasgef genes, rasgef and rasgef1b, have been identified and high throughput analysis revealed tissue specific embryonic expression for rasgef1b. Here, we show that three rasgef1b-transcripts are generated from two transcriptional start sites and by alternative splicing. Detailed expression analyses show that rasgef1b is expressed in a subset of adaxial cells, in the anterior part of somites, in the rostral part of the mid-hindbrain boundary and in the rhombomere boundaries. In the larva, rasgef1b is further expressed in the pallium and the inner nuclear layer of the retina. We also find that rasgef1b is expressed maternally and that the ubiquitous distribution of maternal transcripts disappears shortly after mid-blastula transition. At early epiboly stages, rasgef1b expression is restricted to the margin with low levels of expression on the ventral and high levels of expression on the dorsal side. Finally, we show that early zygotic expression is regulated by Nodal and FGF signals and that these signals have different activities in regulating the level and distribution of early zygotic rasgef1b mRNA expression.