Figure 1: The sfw mutant displays heterotaxia due to loss of Spaw activity.

(a) Schematic depicting cardiac asymmetries observed during zebrafish development. At 28 hpf, the heart is a linear tube positioned under the left eye, which normally undergoes dextral looping by 48 hpf. (b) Analysis of heart position at 28 hpf by in situ analysis of myl7 expression, dorsal views, anterior to top. (c) Bright-field images of wild-type (wt) and sfw mutant embryos at 48 hpf. (d) Quantification of heart-looping direction at 48 hpf in wt and sfw mutant embryos. Heart is highlighted by myl7 expression. (e) Quantification of digestive system laterality at 48 hpf in wt embryos and sfw mutants, using foxA3 in situ hybridization to visualize the digestive tract. (f) Quantification of habenulae laterality in wt and sfw mutant embryos at 4 dpf by lov in situ hybridization. (d–f) Error bars indicate s.e.m. Significance determined by Student’s t-test indicated by asterisks: *P<0.05, **P<0.01, ***P<0.005. (d,e) On the basis of 25 biological replicates (total embryo number=796), (f) based on 3 biological replicates (total embryo number 126). L, liver; P, pancreas; asterisks denote position of lov expression in the brain. (g) The sfw mutation was mapped to linkage group 5. x/96 denotes number of recombinants in 96 meioses. Genomic position is noted in Mb. (h) Amino acid alignment of Nodal-related genes in zebrafish, Xenopus, mouse and human. The mutated cysteine residue is highlighted by the red box and is conserved in all Nodals. Scale bars, 100 μM (b,c).