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 C NOT1 gene in the chicken genome. The two chicken genes represent two d ifferent subgroups of the Not gene family, the first including CNOT1 a nd the Xenopus genes XNot1 and XNot2, and the second CNOT2 and the zeb rafish floating head gene. The overall expression pattern of CNOT2 in Hensen's node, notochord, neural plate, tailbud, and epiphysis resembl ed the CNOT1 pattern. However, several significant differences occurre d: CNOT2 expression was much stronger and more widespread in the prega strulation embryo, it showed an additional, transient domain on the an terior 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 o r by explanting them into tissue culture. CNOT gene expression from yo ung nodes was maintained in vivo, but required in vitro the addition o f retinoic acid. The generation of differentiated notochord structures could only be obtained, if either older node grafts were used in vitr o or young node grafts were transplanted close to the primary axis in vivo. We conclude that CNOT expression in the anterior streak is not e nough for notochord differentiation, but further influences are necess ary. A Not-related gene has previously been isolated from Drosophila m elanogaster 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 nervo us system of Drosophila and the early neuroectoderm in the chick and i ts implication for a phylogenetic relationship between neuroectoderm a nd the notochord is discussed. (C) 1996 Academic Press, Inc.