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Fig. 8. An interactive hierarchy of zinc-finger transcription factors establishes
trunk and gnathal/head segment types. In the trunk segments, TSH represses
disco expression and directs segments along the trunk developmental
pathway. SALM defines the boundary between the head and trunk (broken line) by
repressing tsh in the gnathal segments. disco is expressed
in the gnathal segments activating the gnathal development pathway. In this
manner, the distribution of TSH and DISCO regionalizes the embryo. When
combined with the HOM-C proteins, specific segment identities arise. Note that
the HOM-C protein SCR is expressed in both the gnathal and trunk domains and
yields a different identity depending upon which co-factor is present. DFD can
establish either a maxillary or mandibular identity depending upon the
presence of the Cap-n-collar (CNC) protein
(Mohler et al., 1995). mn,
mandibular; mx, maxillary; lb, labial; t2, second thoracic.