Fig. 3. Expression of AmphiRAR (A-N) in amphioxus embryos in the absence (A-K) and presence (L-N) of 1x10–6 M RA and of AmphiPax1 in the presence of 1.5x10–6 M BMS009 (O-Q). Whole mounts and frontal sections (counterstained pink) with anterior towards the left. Cross sections viewed from posterior end. (A) No expression in the blastula. (B) Gastrula with ubiquitous expression. (C) Early neurula (15 hours). Expression downregulated in the cerebral vesicle (arrow), anterior endoderm and non-neural ectoderm. (D) Frontal section through x-x in C. Transcripts abundant in posterior mesoderm, somites and neural plate posterior to the cerebral vesicle. (E) Frontal section through y-y in C. Transcripts most abundant in the posterior three quarters of the endoderm. (F) Eighteen hour neurula. Expression is downregulated in the anterior third of the nerve cord and upregulated in the middle third. (G) Cross section through x in F. Expression throughout the nerve cord and very weakly in the somites adjacent the notochord. (H) Cross section through y in F. Expression strong in the nerve cord, somites and endoderm. (I) Twenty hour neurula. Expression downregulated in the pharyngeal endoderm. (J) Twenty-four hour embryo. Expression strong in the nerve cord posterior to the cerebral vesicle and a small region of endoderm, but largely downregulated elsewhere. (K) Two day larva. Expression most pronounced in middle of nerve cord. No expression in posterior quarter of the embryo or in forming gill slits. (L) Twenty-two hour embryo (RA treated). Expression generally upregulated extending into the dorsal part of the cerebral vesicle (arrow). (M) Twenty-six hour embryo (RA treated). Expression in the cerebral vesicle and upregulated in the pharynx. (N) Forty hour larva (RA treated). Gill slits and mouth absent. Expression anteriorized and upregulated in the pharynx. (O) Expression of AmphiPax1/9 in the pharynx in a normal 26 hour embryo (arrow indicates first pigment spot in nerve cord; arrowhead indicates posterior limit of pharynx marked by the posterior limit of Pax1/9 expression). (P) AmphiPax1/9 expression is expanded posteriorly in a 24 hour embryo treated with BMS009 (arrow and arrowhead as in O). (Q) AmphiPax1/9 expression remains posteriorly expanded in a 36 hour larva treated with BMS009 (arrow and arrowhead as in O). Scale bars: 50 µm for whole mounts; 25 µm for sections. n, notochord.
