INTRODUCTION The neural crest, a transitory population of cells that is characteristic of vertebrate embryos, forms at the border of the neural plate, posteriorly to the diencephalon. After induction, neural crest cells undergo an epithelial-to-mesenchymal transition and migrate into several locations to give rise to a large variety of derivatives (for a review, see Le Douarin and Kalcheim, 1999). Experimental manipulations in chick, fish and amphibian embryos have shown that both the ectoderm and the neural plate can give rise to neural crest cells when they are juxtaposed (Moury and Jacobson, 1989; Moury and Jacobson, 1990; Selleck and Bronner-Fraser, 1995; Mancilla and Mayor, 1996; Woo and Fraser, 1998). However, in vivo, the neural crest forms adjacent to three different tissues, the non neural ectoderm, the neural plate and the underlying paraxial mesoderm, all of which thus constitute potential sources of neural crest inducers (Schroeder, 1970). Although many studies have focused on neural crest induction by the ectoderm in the chick embryo (Dickinson et al., 1995; Basch et al., 2000; Knecht and Bronner-Fraser, 2002), a pioneering study by Raven and Kloos (Raven and Kloos, 1945) showed that the paraxial mesoderm can induce neural crest formation in the ectoderm of amphibians. More recent studies also show that recombining the paraxial mesoderm with naive ectoderm in Xenopus laevis embryos results in potent neural crest induction in the ectodermal part of the explant and that excising the paraxial mesoderm results in lack of neural crest formation in vivo (Mancilla and Mayor, 1996; Bonstein et al., 1998; Marchant et al., 1998). In chick embryos, some data also indicate that the melanocytes, which are neural crest derivatives, are induced after neural plate-paraxial mesoderm recombination (Selleck and Bronner-Fraser, 1995). Although tested separately in these experimental assays, the possibility that the inducing activities from the ectoderm and the mesoderm might act in concert during normal development remains to be explored. In the amphibian embryo, the current analysis of the molecular basis of ectoderm-neural tissue interactions results in a two-step model of neural crest induction detailed below (reviewed by Aybar and Mayor, 2002; Knecht and Bronner- Fraser, 2002). Slug was generally used in these studies as a 3111 Development 130, 3111-3124 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00531 At the border of the neural plate, the induction of the neural crest can be achieved by interactions with the epidermis, or with the underlying mesoderm. Wnt signals are required for the inducing activity of the epidermis in chick and amphibian embryos. Here, we analyze the molecular mechanisms of neural crest induction by the mesoderm in Xenopus embryos. Using a recombination assay, we show that prospective paraxial mesoderm induces a panel of neural crest markers (Slug, FoxD3, Zic5 and Sox9), whereas the future axial mesoderm only induces a subset of these genes. This induction is blocked by a dominant negative (dn) form of FGFR1. However, neither dnFGFR4a nor inhibition of Wnt signaling prevents neural crest induction in this system. Among the FGFs, FGF8 is strongly expressed by the paraxial mesoderm. FGF8 is sufficient to induce the neural crest markers FoxD3, Sox9 and Zic5 transiently in the animal cap assay. In vivo, FGF8 injections also expand the Slug expression domain. This suggests that FGF8 can initiate neural crest formation and cooperates with other DLMZ-derived factors to maintain and complete neural crest induction. In contrast to Wnts, eFGF or bFGF, FGF8 elicits neural crest induction in the absence of mesoderm induction and without a requirement for BMP antagonists. In vivo, it is difficult to dissociate the roles of FGF and WNT factors in mesoderm induction and neural patterning. We show that, in most cases, effects on neural crest formation were parallel to altered mesoderm or neural development. However, neural and neural crest patterning can be dissociated experimentally using different dominant-negative manipulations: while Nfz8 blocks both posterior neural plate formation and neural crest formation, dnFGFR4a blocks neural patterning without blocking neural crest formation. These results suggest that different signal transduction mechanisms may be used in neural crest induction, and anteroposterior neural patterning. Key words: FGF, WNT, FGF8, Paraxial mesoderm, Xenopus embryo, Neural crest, Neural patterning SUMMARY Neural crest induction by paraxial mesoderm in Xenopus embryos requires FGF signals Anne-Hélène Monsoro-Burq*, Russell B. Fletcher and Richard M. Harland Department of Molecular and Cellular Biology, University of California at Berkeley, CA 94720, USA *Author for correspondence (e-mail: monsoro@uclink.berkeley.edu) Accepted 30 March 2003