FGF IS A PROSPECTIVE COMPETENCE FACTOR FOR EARLY ACTIVIN-TYPE SIGNALSIN XENOPUS MESODERM INDUCTION

Normal pattern formation during embryonic development requires the reg ulation of cellular competence to respond to inductive signals. In the Xenopus blastula, vegetal cells release mesoderm-inducing factors but themselves become endoderm, suggesting that vegetal cells may be prev ented from expressing mesodermal genes in response to the signals that they secrete, We show here that addition of low levels of basic fibro blast growth th factor (bFGF) induces the ectopic expression of the me sodermal markers Xbra, MyoD and muscle actin in vegetal explants, even though vegetal cells express low levels of the FGF receptor, Activin, a potent mesoderm-inducing agent in explanted ectoderm (animal explan ts), does not induce ectopic expression of these markers in vegetal ex plants. However, activin-type signaling is present in vegetal cells, s ince the vegetal expression of Mix.1 and goosecoid is inhibited by the truncated activin receptor, These results, together with the observat ion that FGF is required for mesoderm induction by activin, support ou r proposal that a maternal FGF acts at the equator as a competence fac tor, permitting equatorial cells to express mesoderm in response to an activin-type signal, The overlap of FGF and activin-type signaling is proposed to restrict mesoderm to the equatorial region.