Fig. 4. Expression of AmphiTr2/4 in untreated (A-M) amphioxus embryos and in embryos treated with 1x10–6 RA (N,O) or BMS009 (P-U). For whole mounts, anterior is towards the left. Cross-sections (counterstained pink) viewed from posterior end. (A) Mid-gastrula. Expression in dorsal hypoblast and epiblast. (B) Thirteen hour neurula. Intense expression in anterior neural plate and endoderm. (C) Dorsal view of embryo in B. (D) Nineteen hour embryo. Transcripts present in the cerebral vesicle (cv) and the endoderm, most strongly in the posterior third and in Hatschek’s anterior left diverticulum (just below the cerebral vesicle). (E) Twenty-four hour neurula. Strong expression in cerebral vesicle and throughout the endoderm, in Hatschek’s anterior left diverticulum (arrow) and primordium of first gill slit (arrowhead). (F) Cross-section through x in E. Strong expression throughout the cerebral vesicle and in Hatschek’s anterior left diverticulum (arrow). (G) Cross-section through y in E. Weak expression throughout pharyngeal endoderm. (H) Twenty-six hour embryo, overstained. Expression in tailbud, cerebral vesicle and pharyngeal endoderm. (I) Higher magnification of an embryo at the same stage as in H. (arrow indicates Hatschek’s diverticulum; arrowhead indicates primordium of first gill slit. (J) Thirty-four hour larva. Expression upregulated in endoderm of first gill slit (arrowhead). (K) Cross-section through x in J. Strong expression in pharyngeal endoderm and ectoderm around the open mouth. (L) Cross-section through y in J. Strong expression in gill slit primordium. (M) Cross-section through z in J. Very weak expression in non-pharyngeal endoderm. (N) Twenty-four hour embryo, RA treated. AmphiTR2/4 expression anteriorized in the pharynx, but unaffected in cerebral vesicle. (O) Thirty-six hour larva, RA treated. Compare with control in J. Expression anteriorized in pharynx and largely downregulated in cerebral vesicle. (P) Twenty-six hour embryo, BMS009 treated, with slightly expanded pharynx. Expression of AmphiTR2/4 is strong in the endoderm (particularly in the anterior and posterior thirds) and posterior mesoderm and moderate in the cerebral vesicle. x, y, z indicate levels of cross sections in Q-S. (Q) Cross-section through level x in P. Expression is strong in endoderm, particularly in Hatschek’s diverticulum, and weaker in the cerebral vesicle. (R) Cross-section through level y in P. Expression is strong in gill slit primordium (arrowhead). (S) Cross-section through level z in P. (T) Forty-eight hour embryo (arrow indicates the mouth; arrowheads indicate the first pigment spot in the nerve cord; 1, 2, 3 indicate expanded non-penetrating gill slit primordia). (U) Cross-section through a 48 hour larva at the level of first pigment spot in the nerve cord. Expression is weak in the expanded pharyngeal endoderm. Scale bar: 50 µm in whole mounts; 25 µm in sections.
