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Fig. 3. Stages of notochord development affected by mutations. The transition from
dorsal organiser to chordamesoderm is affected by mutations in several genes.
At the top of the dorsal specification cascade are ichabod and
ß-catenin proteins (Kelly et al.,
2000). In their absence, ventral types of mesoderm can form but
not chordamesoderm. The bozozok and floating head loci each
encode homeodomain-containing proteins that are necessary for chordamesoderm
specification. In the complete absence of oep, as in the
maternal-zygotic (Mz) mutant, mesendoderm is not specified, including the
chordamesoderm. The laminins (bashful, grumpy and sleepy),
the coatomers (sneezy, happy and dopey), brachyury (no
tail) and one unknown locus, doc, are all required for proper
notochord differentiation. Arrowheads indicate (A) chordamesoderm progenitor
region of the shield, (B) the chordamesoderm and (C) the differentiated
notochord.