Fig. 2.

Morphogen target genes are activated in a threshold-dependent manner. (A) Dl threshold-dependent gene activation along the DV axis of Drosophila embryos. The graph shows how three different Dl thresholds (t₁, t₂ and t₃) establish three gene expression domains along the DV axis. Schematic of an embryo cross-section showing three gene expression domains (types I-III) dictated by threshold-dependent responses to graded nuclear Dl levels (red). (B) A set of Bcd target genes is controlled by Bcd in a threshold-dependent manner. In wild-type embryos with two copies of bcd, Bcd exists in a graded manner along the AP axis [2x BCD (wt), black line] and also supports hb expression (pink) at the anterior of embryos. In a bcd mutant, hb expression is lost. When the Bcd gradient is flattened by assay in specific mutant backgrounds, resulting in intermediate [6x BCD (flat), mid-gray line] or low [2x BCD (flat), light-gray line] levels of Bcd throughout the embryo, hb expression expands to the posterior pole region. Another Bcd-target gene, otd (blue), also is expressed at the anterior in wild-type embryos, but 6x BCD embryos exhibit only posterior expansion of otd, not the ubiquitous expression seen for hb (Ochoa-Espinosa et al., 2009). EL, egg length.