SUMMARYFGF and Wnt pathways constitute two fundamental signaling cascades, which appear to crosstalk in cooperative or antagonistic fashions in several developmental processes. In vertebrates, both cascades are involved in pigment cell development, but the possible interplay between FGF and Wnt remains to be elucidated. In this study, we have investigated the role of FGF and Wnt signaling in development of the pigment cells in the sensory organs of C. intestinalis. This species possesses the basic features of an ancestral chordate, thus sharing conserved molecular developmental mechanisms with vertebrates. Chemical and targeted perturbation approaches revealed that a FGF signal, spreading in time from early gastrulation to neural tube closure, is responsible for pigment cell precursor induction. This signal is transmitted via the MAPK pathway, which activates the Ci-Ets1/2 transcription factor. Targeted perturbation of Ci-TCF, a downstream factor of the canonical Wnt pathway, indicated its contribution to pigment cell differentiation Furthermore, analyses of the Ci-Tcf regulatory region revealed the involvement of the FGF effector, Ci-Ets1/2, in Ci-Tcf transcriptional regulation in pigment cell precursors. Our results indicate that both FGF and the canonical Wnt pathways are involved in C. intestinalis pigment cell induction and differentiation. Moreover, we present a case of direct transcriptional regulation exerted by the FGF signaling cascade, via the MAPK-ERK-Ets1/2, on the Wnt downstream gene CiTcf. Several examples of FGF/Wnt signaling crosstalk have been described in different developmental processes; however, to our knowledge, FGF-Wnt cross-interaction at the transcriptional level has never been previously reported. These findings further contribute to clarifying the multitude of FGF-Wnt pathway interactions.

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