The role of BMP signaling in outgrowth and patterning of the Xenopus tail bud

Tail bud formation in Xenopus depends on interaction between a dorsal domai n (dorsal roof) expressing lunatic fringe and Notch, and a ventral domain ( posterior wall) expressing the Notch ligand Delta. Ectopic expression of an activated form of Notch, Notch ICD, by means of an animal cap graft into t he posterior neural plate, results in the formation of an ectopic tail-like structure containing a neural tube and fin. However, somites are never for med in these tails. Here, we show that BMP signaling is activated in the po sterior wall of the tail bud and is involved in the formation of tail somit es from this region. Grafts into the posterior neural plate, in which BMP s ignaling is activated, will form tail-like outgrowths. Unlike the Notch ICD tails, the BMP tails contain well-organized somites as well as neural tube and fin, with the graft contributing to both somites and neural tube. Thro ugh a variety of epistasis-type experiments, we show that the most likely m odel involves a requirement for BMP signaling upstream of Notch activation, resulting in formation of the secondary neural tube, as well as a Notch-in dependent pathway leading to the formation of tail somites from the posteri or wall. (C) 2001 Academic Press.