Spatial Bistability Generates hunchback Expression Sharpness in the Drosophila Embryo
Figure 1
Control of Hb sharpness and position.
(A,B) An embryo in mid-nuclear cleavage cycle 14A, with immunostaining to Bicoid (Bcd) (A) and Hunchback (Hb) (B) proteins. Anterior pole on left, dorsal side on top. (C) Fluorescence profiles versus anteroposterior position for A (green) and B (blue). Hb position, 47.1% EL; sharpness, 82.7°. (D) Hb profiles for homozygotes of hb14F, an allele coding a non-DNA binding Hb protein [52], showing dramatically reduced sharpness (63.9°). Heavy blue line: hunchback self-regulatory (HSR) model (Figure 3) prediction for absence of self-regulation. (E) hb14F heterozygotes and wild-type together, showing similar position (44.3% EL) and sharpness (81.3°). See Figure S2 for non-normalized data. (F) Hb profiles from bcdE1/+ embryos (Bcd mRNA half-dosage). Heavy blue line: simulation for this background, by reducing Bcd synthesis in the HSR model. (G,H) Hb profiles from embryos expressing one copy of bcdK57R, an allele affecting Bcd cooperativity [54], gives two outcomes: a small anterior shift ((G); 3.0% change from bcdE1/+); and a strong anterior shift ((H); 36.7% change from bcdE1/+). Heavy blue lines: HSR model simulations for weakly and strongly reduced Bcd cooperative binding ((G) and (H), respectively). Maximum intensities are normalized to one, to allow comparison of profile sharpness from different experiments. All embryos are between 26 and 39 minutes into nuclear cleavage cycle 14A, as determined by membrane invagination and the relative position from surface to cortex (see Materials and Methods). In (D) there are two T7 embryos, showing normal posterior pattern (not used for sharpness or position measurements). Individual embryo images are shown in Figure S1.