Differential activation of the clustered homeobox genes CNOT2 and CNOT1 during notogenesis in the chick.

CNOT2, a newly identified homeobox gene, is physically linked to the CNOT1 gene in the chicken genome. The two chicken genes represent two different subgroups of the Not gene family, the first including CNOT1 and the Xenopus genes XNot1 and XNot2, and the second CNOT2 and the zebrafish floating head gene. The overall expression pattern of CNOT2 in Hensen's node, notochord, neural plate, tailbud, and epiphysis resembled the CNOT1 pattern. However, several significant differences occurred: CNOT2 expression 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. CNOT gene expression from young nodes was maintained in vivo, but required in vitro the addition of retinoic acid. The generation of differentiated notochord structures could only be obtained, if either older node grafts were used in vitro or young node grafts were transplanted close to the primary axis in vivo. We conclude that CNOT expression in the anterior streak is not enough for notochord differentiation, but further influences are necessary. A Not-related gene has previously been isolated from Drosophila melanogaster and its expression was detected in the posterior brain and the neuroblasts (Dessain and McGinnis, 1993. Adv. Dev. Biochem. 2, 1-55). The correspondence between Not gene-expressing cells in the nervous system of Drosophila and the early neuroectoderm in the chick and its implication for a phylogenetic relationship between neuroectoderm and the notochord is discussed.