Gradients and forward spreading of vertebrate Hox gene expression detectedby using a Hox/lacZ transgene

Elucidation of the kinetics with which vertebrate Hox expression patterns d evelop may help us to choose between various models already proposed to exp lain this process, The chick Hoxa-7/lacZ transgene, expressed in mouse embr yos, changes over time in the distribution of its activity along the develo ping posterior to anterior axis. During an establishment (E) phase (lasting at least up to 10 days) expression is graded from highest levels posterior ly, to low levels anteriorly. Within the graded domain, the overall level o f expression spreads forward with time along both neurectoderm and paraxial mesoderm. Spreading in expression is not due to movement of cells, but to increases in both the proportion of lacZ expressing cells and the intensity of expression per cell. By 10.8 days, embryos have reached a late (L) phas e in which an anterior up-regulation in expression, together with a posteri or down-regulation, cause the graded nature of the expression to be lost. E and L phases are also seen for Hox gene expression detected by in situ hyb ridization. The switch from E to L occurs at progressively later times as w e move 3' to 5' along the Hox cluster. The results are in keeping with mode ls in which Hox genes become differentially expressed according to a graded concentration of an inducer. Binding motifs for the caudal (cdx) proteins, already proposed as such inducers, are conserved in mouse and chick Hoxa-7 enhancer elements. (C) 2001 Wiley-Liss, Inc.