Fig. 2. AmphiRAR and AmphiTR2/4 expression in normal amphioxus embryos. Anterior is towards the left. At 4.5 hours (mid-gastrula), the expression of the two genes considerably overlaps. However, by 9 hours (early neurula), their expression patterns begin to become complementary. AmphiRAR is expressed posteriorly in the neural plate (NP), weakly in the somites (S) and throughout the mesendoderm, with expression strongest posteriorly, whereas AmphiTR2/4 is most strongly expressed in the anterior neural plate and underlying mesendoderm. By 16 hours, complementarity is more pronounced. AmphiRAR is downregulated in posterior and anterior tissues. It is most strongly expressed in the middle third of the neural tube (NT), somites and endoderm but not in the cerebral vesicle (CV) or notochord (N). By contrast, AmphiTR2/4 is most strongly expressed in the cerebral vesicle, Hatschek’s anterior left diverticulum (HD) and the anterior and posterior endoderm. At 24 hours, endodermal expression of AmphiRAR is downregulated except in a small ventromedial area. Expression of AmphiTR2/4 remains high in the cerebral vesicle, Hatschek’s diverticulum, the chordoneural hinge (CNH) and anterior and posterior endoderm. By 30 hours, the primordia of the mouth (M) and first two gill slits (GS1; GS2) have formed. Expression of AmphiRAR is restricted to the middle third of the nerve cord and weakly in the middle third of the somites and endoderm while that of AmphiTR2/4 is strongest in the cerebral vesicle, Hatschek’s diverticulum, the tailbud, mouth and gill slits. NP, neural plate; SO, somite; CV, cerebral vesicle; HD, Hatschek’s left anterior diverticulum; NT, neural tube; CNH, cordoneural hinge; M, mouth; GS1 and GS2, gill slits.
