TEMPORAL COLINEARITY IN EXPRESSION OF ANTERIOR HOX GENES IN DEVELOPING CHICK-EMBRYOS

Temporal colinearity describes a correspondence between the spatial or dering of Hox genes within their clusters (in the direction 3' to 5') and the time of their first expression (earlier to later) during embry onic development (Izpisua-Belmonte ct al. [1991] EMBO J. 10:2279-2289) . It suggests that activation of each Hox gene might be controlled in some way by its position within the cluster. So far, in situ hybridiza tion experiments on vertebrate embryos have provided clear evidence of temporal colinearity only for ''posterior'' Hox genes (5' located, Ab dB related). We now describe a search in the chick embryo for evidence of temporal colinearity in the expression of some anterior Hox genes (Hoxb-1, b-3, b-4, b-6, and a-9). Clear evidence for temporal colinear ity was seen in neural tube tissue adjacent to the first few somites. Here, there were delays in the expression of Hoxb-3 following b-1, Hox b-4 following b-3, and Hoxb-6 following b-4. Temporal colinearity was also detected in anterior primitive streak tissue. Hox gene expression reached both the neural tube and the anterior streak following forwar d spreading from posteriormost parts of the primitive streak. Overall, therefore, temporal colinearity was seen as sequential waves of Hox g ene expression that proceeded forward (3' genes before 5' genes) along the developing chick embryo. Within posterior primitive streak tissue , there was only limited evidence for temporal colinearity. We discuss these results in terms of possible models for the establishment of Ho x gene expression patterns. (C) 1996 Wiley-Liss, Inc.