Fig. 4. Time course of dorsal mesendoderm development in sqt^-/-; cyc^+/+, sqt^-/-; cyc^+/- and sqt^-/-; cyc^-/- mutants. gsc expression in wild-type (A), sqt^-/-; cyc^+/+ (B), sqt^-/-; cyc^+/- (C) and sqt^-/-; cyc^-/- (D) embryos. For each stage, wild-type, sqt^-/-; cyc^+/+, sqt^-/-; cyc^+/- are siblings; the sqt^-/-; cyc^-/-, mutants were derived from a separate cross that was processed in parallel. The images for panels C6 and D6 come from a separate cross of sqt^+/-; cyc^+/- adults. gsc expression initiates in sqt^-/-; cyc^+/+, sqt^-/-; cyc^+/- and sqt^-/-; cyc^-/- embryos (A1,B1,C1,D1), but is rapidly lost (B2,C2,D2). In sqt^-/-; cyc^+/+ embryos, gsc is expressed only in a few cells at 5 h, but steadily increases throughout gastrulation such that it is often indistinguishable from wild-type at bud stage (B2-B6). Dorsal views are shown, except for animal views at 5 and 10 h. The genotypes of all embryos shown were determined by PCR after photography, except for panels C6 and D6, which were determined by morphology. In the sqt^+/-; cyc^+/+ intercross depicted in A6, B6, only 3/51 embryos displayed reduced gsc expression; the rest of the embryos, including the remaining sqt mutants, have a wild-type pattern as shown. Notably, we found no reduction of gsc expression in cyc^m294 homozygotes at 8 h (data not shown). This contrasts with earlier work on cyc^b16 (Thisse et al., 1994), which is now known to be a deficiency that removes other genes in the cyclops region of linkage group 12 (Talbot et al., 1998).