During normal forebrain development in vertebrates, rostral neural tissue must be protected from Wilt signals via the actions of locally expressed Writ antagonistic factors. in zebrafish zygotic oep (Zoep) mutants, forebrain structure is severely disrupted with reduced expression of the Writ antagonists secreted frizzled related protein I and dickkopf1. To analyze the temporal effects of Writ antagonism on forebrain development, we generated transgenic zebrafish that overexpressed the dominant negative form of frizzled8a (DNfz8a) in wild-type and Zoep mutants Under the control of a heat-inducible promoter. This model allowed for assessment of the dynamics of Writ antagonistic signaling during forebrain development. Our results demonstrated that overexpression of DNfz8a in Zoep embryos between 7 and 16 hpf increased putative forebrain region demarcated by arif and distal-less2 expressions. These results suggest that normal forebrain development requires Continual Wilt antagonism from the early gastrula to the mid-somitogenesis stage. (C) 2009 Elsevier Inc. All rights reserved.