Ras is an essential component for notochord formation during ascidian embryogenesis

In ascidian embryos, inductive interactions are necessary for the fate specification of notochord cells. Previous studies have shown that notochord induction occurs at the 32-cell stage and that basic fibroblast growth factor (bFGF) has notochord-inducing activity in ascidian embryos. In vertebrate, it is known that bFGF receptors have tyrosine kinase domain and the signaling pathway is mediated by the small-GTP binding protein, Ras. To study the role of Ras in ascidian embryos, we injected dominant negative Ras (RasN17) into fertilized eggs. RasN17 inhibited the formation of notochord, suggesting that the Ras signaling pathway is involved in signal transduction in the induction of notochord cells. When the presumptive-notochord (A6.2) blastomere was co-isolated with the inducer (A6.1) blastomere and then RasN17 was injected into the A6.2 blastomere, notochord differentiation was suppressed. The presumptive-notochord blastomeres injected with RasN17 were treated with bFGF. Many of them failed to develop notochord-specific features. Next, we examined the effect of injecting constitutively active Ras (RasV12) into the A6.2 blastomeres. However, microinjection of RasV12 into these cells did not bypass notochord induction. These results suggest that the Ras signaling pathway is essential for the formation of notochord and that another signaling pathway also must be activated simultaneously in notochord formation during ascidian embryogenesis.