Temporal and spatial relationships between segmentation and homeotic gene expression in Drosophila embryos: distributions of the fushi tarazu, engrailed, Sex combs reduced, Antennapedia, and Ultrabithorax proteins.

The specification of segment number and identity in the Drosophila embryo requires the activity of several classes of genes that may be grouped according to the array of pattern elements that they control. Double-label immunofluorescence was used to simultaneously localize the products of genes representative of the pair-rule segmentation class (fushi tarazu), the segment polarity class (engrailed), and the homeotic class (Sex combs reduced, Antennapedia, and Ultrabithorax) of pattern-regulating genes. The temporal order of appearance of each class of proteins and the precise spatial relationships between the products of the different genes are described with single-cell resolution. Boundaries of gene expression, particularly the parasegmental boundaries, are established by early-acting genes such as fushi tarazu and subsequently respected by the expression patterns of later appearing gene products such as engrailed and Ultrabithorax, suggesting regulatory relationships between certain pairs of genes. In addition, the dynamic transitions observed in spatial relationship among the Sex combs reduced, Antennapedia, and Ultrabithorax homeotic protein patterns during the early period of embryogenesis may reflect cross-regulatory interactions among these genes. Finally, some cells contain a single homeotic product, whereas other cells simultaneously contain several, suggesting that certain cells may be determined by the combinatorial action of homeotic genes.