XTcf-3 is a maternally expressed Xenopus homolog of the mammalian HMG box factors Tcf-1 and Lef-1. The N-terminus of XTcf-3 binds to beta-catenin. Microinjection of XTcf-3 mRNA in embryos results in nuclear translocation of beta-catenin. The beta-catenin-XTcf-3 complex activates transcription in a transient reporter gene assay, while XTcf-3 by itself is silent. N-terminal deletion of XTcf-3 (delta N) abrogates the interaction with beta-catenin, as well as the consequent transcription activation. This dominant-negative delta N mutant suppresses the induction of axis duplication by microinjected beta-catenin. It also suppresses endogenous axis specification upon injection into the dorsal blastomeres of a 4-cell-stage embryo. We propose that signaling by beta-catenin involves complex formation with XTcf-3, followed by nuclear translocation and activation of specific XTcf-3 target genes.

SUMMARY The widespread availability of programmable site-specific nucleases now enables targeted gene disruption in the zebrafish. In this study, we applied site-specific nucleases to generate zebrafish lines bearing individual mutations in more than twenty genes. We found that mutations in only a small proportion of genes caused defects in embryogenesis. Moreover, mutants for ten different genes failed to recapitulate published Morpholino-induced phenotypes (morphants). The absence of phenotypes in mutant embryos was not likely due to maternal effects or failure to eliminate gene function. Consistently, a comparison of published morphant defects with the Sanger Zebrafish Mutation Project revealed that approximately eighty percent of morphant phenotypes were not observed in mutant embryos, similar to our mutant collection. Based on these results, we suggest that mutant phenotypes become the standard metric to define gene function in zebrafish, after which Morpholinos that recapitulate respective phenotypes could be reliably applied for ancillary analyses.

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