Expression of twi (A, E, H, L), sim (B, F, I, M), sog (C, G, J, N, P, R, T, V) in wt embryos (A–C, R, T, V), Toll1-RNAi embryos (E–G, P), dl1-RNAi embryos (H–J) and Toll1-dpp-RNAi embryos (L–N) monitored by ISH. The view is ventral (A–C, J, T, V), or not determined as the expression is DV symmetric (E–G, H, I, L–N, P, R). Embryos are at the blastoderm stage (A–C, E–G, H–J, L–N: 26–32 hpf; P–V see figure labels). Green arrowheads mark the anterior border of sim expression. The scale bar (A) corresponds to 200 µm. For phenotype frequencies and confirmation of KD see Figure 5—figure supplement 1. (D, K, O, Q, S, U, W) Simulations of the reaction diffusion system described in Box 1 on a two-dimensional cylinder (Figure 10). Depicted is the ventral part of the cylinder. Blue: sog expression level (η). Gray: BMP concentration (b). (D) wt: sog expression is confined to a ventral stripe. (K) Upon loss of active NF-κB/Dorsal (d = 0) due to either KD of Toll1 or KD of dl1, early activation of sog (P) is insufficient to initiate patterning resulting in uniformly high BMP signaling. (O) Upon simultaneous loss of Dorsal (d = 0) and BMP (b = 0) sog activation is possible despite lack of NF-κB/Dorsal; however, activation is uniform. (Q) sog activation at early stages in the absence of Toll signaling (d = 0). This reflects ηo, NF-κB/Dorsal-independent sog activation (Box 1). (S, U, W) Developmental progression of sog activation (η) during blastoderm stages.