(a) In Sox10:Cre; acta2:loxP-BFP-Stop-loxP-dsRed fish at 5 dpf, the developing pseudobranch (white arrow) and gill buds (yellow arrow) consist of Cre-converted dsRed+ neural crest-derived mesenchyme (magenta) and unconverted BFP+ epithelia (gray). (b) At 6 dpf, kdrl:mCherry labeling of vasculature reveals a branch of the first aortic arch in the position of the pseudobranch (white arrow), and branches of the posterior aortic arches in the positions of the gills (yellow arrow). (c) Endoderm is labeled in red by adding 4OH-tamoxifen to sox17:CreERT2; ubb:loxP-Stop-loxP-mCherry fish at 6.5 hr post-fertilization to induce Cre recombination that removes the Stop cassette and allows mCherry expression (mCherry channel alone shown in inset). Co-localization shows fgf10b:nEOS expression (green) in endodermal pouches. (d), Endoderm labeling by addition of 4OH-tamoxifen to sox17:CreERT2; ubb:loxP-Stop-loxP-mCherry fish at 6.5 hr post-fertilization results in contribution to cdh1:mlanYFP+ pseudobranch epithelium at 5 dpf. (e,f) In fgf10:nEOS embryos, photoconversion of first pouch endoderm (and some more ventral mandibular cells) at 1.5 dpf labels pseudobranch (white arrow) but not gill epithelia (yellow arrow) at 5 dpf in (e), and conversion of third pouch cells labels the first gill filament epithelium (yellow arrow, boxed region magnified to right and shown in merged and red-only channels) in (f). Scale bars, 50 µM.