Fig. 2. The second phase of Dpp signaling is required for normal PNS formation. For clarity, in A-G the dorsal (d), lateral (l) and ventral (v) regions encompassing PNS clusters are colored blue, yellow and pink, respectively. (A-C) Whole-mount stage 16 embryos stained with the 22C10 antibody that reveals PNS neurons. (A) Wild type (WT) embryo. (B,C) Homozyogous shn¹ (B) and shn^k04412 (C) mutants have fewer neurons and present serious disorganization in the dorsal and lateral PNS clusters. shn¹ mutants fail to undergo dorsal closure. As a result of this failure, the gut is extruded through the top, the embryo is twisted and the ventral nerve cord is pulled inwards. Neurons belonging to the other side of the embryo are visible most ventrally. (D-G) Flat preparations showing one thoracic (T3) and four abdominal (A1-A4) segments (upper panels). Cuticle preparations of late stage embryos (lower panels) show no clear expansion of the ventral denticles in any genotype. Fewer neurons can be observed in shn^k04412 (E), Kr-Gal4;UAS-brk (F) and HS-ssog (G) embryos, especially in the dorsal and lateral clusters. In alleles of shn and overexpression of brk, the pentascolopodial organ (brackets and yellow in D-G) frequently has fewer neurons and is positioned close to the dorsal cluster. (H) Mean values, standard error (bars) and range (numbers to the right of the genotype) of the number of neurons per dorsal, lateral and ventral clusters in abdominal segments (A1-A7) in different genetic backgrounds. Abdominal segments A1-A7 on both sides of 10 embryos were quantified for each genotype. The inhibition of Dpp signaling results in the reduction in number of neurons, principally in the dorsal and lateral PNS clusters. WT, n=63; shn^k04412, n=116; shn^k00401, n=90; Kr-Gal4;UAS-brk, n=53; HS-ssog, n=182.

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