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Fig. 1. The threshold response of C15 to the Mad-P gradient
depends on both Smads and Zen. Dorsal views of late stage 5/early stage 6
embryos with anterior to the left. The widest area of expression is delineated
with a white line labelled to show the width in number of cells. (A)
Wild-type embryo hybridized with C15 probes. Transcripts are present
in a five- to ten-cell-wide dorsal strip of cells in the main body region and
a faint spot in the presumptive head region. Cells in this region are
magnified in the inset, which lies perpendicular to the embryo. (B)
Wild-type embryo stained with an antiphospho-Smad1 antibody, which also
recognizes Mad-P. Strong staining is observed in a five- to
six-cell-wide stripe with less staining in three to four cells to either side
of the stripe (see magnified inset). In terminal regions, the Mad-P
stripe can be up to 16 cells wide. (C) Wild-type embryo hybridized with
Race probes. Transcripts are present in a four- to six-cell-wide
stripe. (D) Wild-type embryo hybridized with tup probes. The
dorsal stripe is ten to 14 cells wide. (E-H) Mutant embryos
hybridized with C15 probes. C15 is absent in
dpphr4 homozygotes (E) and zenw36
homozygotes (F). C15 is in a broad domain in
sogYS06 hemizygous embryos (G). The C15 pattern
broadens to about 26 cells at its widest in a maternal
GAL4>UAS-zen embryo (H).