Fig. 7. Diagram of the effects RA and the RA-antagonist BMS009 on pharyngeal morphology and expression of AmphiRAR and AmphiTR2/4 at the late neurula/early larva stage. (Left) In normal embryos, AmphiRAR and AmphiTR2/4 are expressed in approximately complementary patterns: AmphiTR2/4 expression being high in the cerebral vesicle, pharynx and tailbud where AmphiRAR expression is low. (Right) Application of RA shortens the pharynx and shifts it anteriorly (mouth and gill slits never form), while the RA antagonist BMS009 expands the pharynx posteriorly (subsequently an enlarged mouth forms; gill slit primordia shift posteriorly; extra ones are initiated, but none penetrates). AmphiRAR expression is upregulated and shifted anteriorly by RA and completely downregulated by BMS009. By contrast, although the level of AmphiTR2/4 expression is only slightly affected by RA and BMS009, RA shifts pharyngeal expression anteriorly, while BMS-009 expands it posteriorly. Together, these results suggest that levels of RA signaling, mediated in part by competitive inhibition of AmphiRAR by AmphiTR2/4, regulate anterior/posterior patterning in the nerve cord and endoderm.
