Appearance of (A) wild-type (WT) and (D) spt- live embryos at 24 hours postfertilization (h.p.f.; anterior, left). In both panels, arrow marks the first visible somite. Chevron-shaped somites are visible in the trunk and tail of WT embryos, but are absent from the trunk of spt- embryos. In spt- embryos, presumptive trunk mesodermal progenitors migrate abnormally and form a mass of cells at the tail tip (Ho and Kane, 1990). (B,E) Dorsal views of 24 h.p.f. embryos stained with an antibody to mero-myosin (anterior, uppermost). Bilateral rows of expression are visible either side of the notochord in the WT embryo (B) but staining in the spt- embryo (E) is significantly reduced and is patchy. (C,F) Embryos at 50% epiboly obtained from intercrosses of sptb333/+ adults hybridized with spt probe (blue stain). 75% of the embryos (C) express spt, whereas 25% of the embryos from such a cross, which presumably represent the mutant embryos (F), had no detectable spt expression, even after a prolonged colour reaction. (G) Bulk-segregant analysis of sptb333 genomic DNA by PCR. As a control, floating head (flh) was amplified from pooled DNA isolated from phenotypically WT and spt- embryos, whereas the spt candidate gene could not be amplified from the mutant DNA. (H) Schematic maps of WT and truncated Spt proteins; the putative regulatory domain is inferred from studies of the Brachyury protein (Kispert et al., 1995a). Numbers above the coding regions refer to amino acid position. See Methods for details of the molecular lesions.
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