Saving zebrafish mutants.

At present, the zebrafish Danio rerio is the only vertebrate species for which a large-scale mutagenesis effort to identify developmental genes has been reported. Several laboratories are now intensely pursuing the molecular characterization of the genes affected by these mutations. One important criterion for the identity of the mutated gene is the rescue of the mutant phenotype by a wild-type (wt) copy of the gene. Until recently, most rescue attempts were carried out by injecting wt messenger RNA (mRNA) into fertilized eggs. A report by Yan and collaborators shows the partial rescue of floatinghead mutants by injection of genomic fragments cloned in either bacterial artificial chromosomes or bacteriophage lambda vectors. Combined with other ongoing efforts to characterize the zebrafish genome, this approach of mutant rescue opens interesting avenues for a systematic functional analysis of vertebrate genes.