Fig. 4

Trunk hoxca initiate transcription during gastrulation in a manner that is collinear with the gene′s position in the cluster. A–L: Expression analysis of hoxc4a (A–C), hoxc6a (D–F), hoxc8a (G–I), and hoxc10a (J–L) in wild-type embryos at 60% epiboly (left column), 75% epiboly (middle column) and 95% epiboly (right-most columns). Because hox expression (revealed using NBT/BCIP) is very weak at these early stages, embryos were counterstained for myod using fast red and photographed using epifluoresence. Fast red trapped in the yolk was used to backlit the specimen and reveal the site of hox transcription as a black shadow (NBT/BCIP precipitate quenches fluorescence). C′,F′,I′,L′: Control embryos at 95%–100% epiboly stained as in C, F, I, and L, and photographed under bright field illumination. At this stage, the increased signal-to-noise ratio of the in situ stain allows direct comparison of expression between embryos photographed under different illumination conditions. Embryos are shown slightly tilted from the vegetal pole, dorsal to the top. Expression of myod in adaxial cells is used as a dorsal reference point to determine hox expression domain at the midline (white bars). Embryo in L2 was not stained for myod. Scale bars = 200 microns.