Genetic interactions underlying otic placode induction and formation

The formation of the otic placode is a complex process requiring multiple inductive signals. in zebrafish, fgf3 and fgf8, dlx3b and dlx4b, and foxi1 have been identified as the earliest-acting genes in this process. fgf3 and fgf8 are required as inductive signals, whereas dix3b, dix4b, and foxil appear to act directly within otic primordia. We have investigated potential interactions among these genes. Depletion of either dlx3b and dlx4b or foxil leads to a delay of pax2a expression in the otic primordia and reduction of the otic vesicle. Depletion of both foxil and dlx3b results in a complete ablation of otic placode formation. A strong synergistic interaction is also observed among foxil, fgf3, and fgf8, and a weaker interaction among dlx3b, fgf3, and fgf8. Misexpression of foxil can induce expression of pax8, an early marker for the otic primordia, in embryos treated with an inhibitor of fibroblast growth factor (FGF) signaling. Conversely, morpholino knockdown of foxil blocks ectopic pax8 expression and otic vesicle formation induced by misexpression of fgf3 and/or fgf8. The observed genetic interactions suggest a model in which foxil and dlx3b/dlx4b act in independent pathways together with distinct phases of FGF signaling to promote otic placode induction and development. (C) 2004 Wiley-Liss, Inc.