Schematic representation of the spatial and functional links between otp1 expression and Nodal, SHH and FGF signaling cascades in the 24 hpf hypothalamic forebrain. A, schematic lateral view of the zebrafish forebrain with anterior to the left and dorsal up. otp1, fgf8 and shh expression domains are painted with different colors (purple, otp1; blue, shh; red, fgf8). B, presumptive Nodal, HH, and FGF signaling cascades genetic interactions regulating otp1 expression in the two hypothalamic areas indicated by solid arrows. otp1 expression in PO is modulated negatively by FGF8 and positively by SHH; the effect of SHH on otp1 expression is diminished by Dzip1, a negative regulator of SHH signaling, and Nodal activates otp1 expression through SHH. In PT, otp1 expression is independent from HH and FGF signaling cascades, but it is activated by Nodal in a HH-independent way. Nodal-dependent otp1 expression in PT is required for the proper differentiation of a subset of CA neurons. cf, cephalic flexure; d, diencephalon; or, optic recess; os, optic stalks; po, preoptic area; pt, posterior tuberculum; t, telencephalon.