Concentration-dependent patterning by an ectopic expression domain of the Drosophila gap gene knirps

The asymmetric distribution of the gap gene knirps (kni) in discrete expression domains is critical for striped patterns of pair-rule gene expression in the Drosophila embryo. To test whether these domains function as sources of morphogenetic activity, the stripe 2 enhancer of the pair-rule gene even-skipped (eve) was used to express kni in an ectopic position. Manipulating the stripe 2-kni expression constructs and examining transgenic lines with different insertion sites led to the establishment of a series of independent lines that displayed consistently different levels and developmental profiles of expression. Individual lines showed specific disruptions in pair-rule patterning that were correlated with the level and timing of ectopic expression. These results suggest that the ectopic domain acts as a source for morphogenetic activity that specifies regions in the embryo where pair-rule genes can be activated or repressed. Evidence is presented that the level and timing of expression, as well as protein diffusion, are important for determining the specific responses of target genes.