A forward genetic screen identifies ugly duckling(udu)/gon4l as a regulator of axis extension in zebrafish embryos. a Schematic of a synthetic screen to identify enhancers of the short axis phenotype in knym818/m818 zebrafish mutants. b–e Phenotypes at 24 hpf: wild type (WT) (b), knym818/m818 (c), vu66/vu66 (d), knym818/m818; vu66/vu66 compound mutants (e). Images are representative of phenotypes observed at Mendelian ratios in multiple independent clutches. f Diagram of mapping vu66 mutation to Chromosome 16. Bold numbers below specify the number of recombination events between vu66 and the indicated loci. g Diagram of the Gon4l protein encoded by the udu locus. Arrowheads indicate residues mutated in vu66 and other described udu alleles. h–k Live WT (h, j) and maternal zygotic (MZ)udu (i, k) embryos at yolk plug closure (YPC) (h–i) and 20 somite stage (j–k). 100 percent of MZudu mutants from more than 15 germline-replaced females exhibited the pictured phenotypes. l–m Whole mount in situ hybridization (WISH) for dlx3 (purple) and hgg1 (red) in WT (l) and MZudu−/− (m) embryos at two-somite stage. n–o WISH for myoD in WT (n) and MZudu−/− (o) embryos at eight-somite stage. Anterior is to the left in b–e, j–k; anterior is up in h–i, l–o. Fractions indicate the number of embryos with the pictured phenotype over the number of embryos examined. Scale bar is 500 μm in b–e and 300 μm in h–o
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