Differential Activation of the Clustered Homeobox GenesCNOT2andCNOT1during Notogenesis in the Chick

AbstractCNOT2,a newly identified homeobox gene, is physically linked to theCNOT1gene in the chicken genome. The two chicken genes represent two different subgroups of theNotgene family, the first includingCNOT1and theXenopusgenesXNot1andXNot2,and the secondCNOT2and the zebrafishfloating headgene. The overall expression pattern ofCNOT2in Hensen's node, notochord, neural plate, tailbud, and epiphysis resembled theCNOT1pattern. However, several significant differences occurred:CNOT2expression was much stronger and more widespread in the pregastrulation embryo, it showed an additional, transient domain on the anterior intestinal portal, and lacked expression on the early anterior neural folds and the anterodistal limb bud. We studied CNOT expression by transplanting parts of the primitive streak into growing embryos or by explanting them into tissue culture.CNOTgene expression from young nodes was maintainedin vivo,but requiredin vitrothe addition of retinoic acid. The generation of differentiated notochord structures could only be obtained, if either older node grafts were usedin vitroor young node grafts were transplanted close to the primary axisin vivo.We conclude thatCNOTexpression in the anterior streak is not enough for notochord differentiation, but further influences are necessary. ANot-related gene has previously been isolated fromDrosophila melanogasterand its expression was detected in the posterior brain and the neuroblasts (Dessain and McGinnis, 1993.Adv. Dev. Biochem.2, 1–55). The correspondence betweenNotgene-expressing cells in the nervous system ofDrosophilaand the early neuroectoderm in the chick and its implication for a phylogenetic relationship between neuroectoderm and the notochord is discussed