EXPRESSION OF AMPHIHOX-1 AND AMPHIPAX-1 IN AMPHIOXUS EMBRYOS TREATED WITH RETINOIC ACID - INSIGHTS INTO EVOLUTION AND PATTERNING OF THE CHORDATE NERVE CORD AND PHARYNX

Excess all-trans retinoic acid (RA) causes severe craniofacial malform ations in vertebrate embryos: pharyngeal arches are fused or absent, a nd a rostrad expansion of Hoxb-1 expression in the hindbrain shows tha t anterior rhombomeres are homeotically respecified to a more posterio r identity. As a corollary, neural crest migration into the pharyngeal arches is abnormal. We administered excess RA to developing amphioxus , the closest invertebrate relative of the vertebrates and thus a key organism for understanding evolution of the vertebrate body plan. In n ormal amphioxus, the nerve cord has only a slight anterior swelling, t he cerebral vesicle, and apparently lacks migratory neural crest. Neve rtheless, excess RA similarly affects amphioxus and vertebrates. The e xpression domain of AmphiHox-1 (homologous to mouse Hoxb-1) in the amp hioxus nerve cord is also extended anteriorly. For both the amphioxus and mouse genes, excess RA causes either (1) continuous expression thr oughout the preotic hindbrain (mouse) and from the level of somite 7 t o the anterior end of the nerve cord (amphioxus) or (2) discontinuous expression with a gap in rhombomere 3 (mouse) and a gap at the posteri or end of the cerebral vesicle (amphioxus). A comparison of these expr ession patterns suggests that amphioxus has a homolog of the vertebrat e hindbrain, both preotic and postotic. Although RA alters the express ion of AmphiHox-1 expression in the amphioxus nerve cord, it does not alter the expression of AmphiHox-1 in presomitic mesoderm or of alkali myosin light chain (AmphiMlc-alk) in somites, and the axial musculatu re and notochord develop normally. The most striking morphogenetic eff ect of RA on amphioxus larvae is the failure of mouth and gill slits t o form. In vertebrates effects of excess RA on pharyngeal development have been attributed solely to the abnormal migratory patterns of Hox- expressing cranial neural crest cells. This cannot be true for amphiox us because of the lack of migratory neural crest. Furthermore, express ion of Hox genes in pharyngeal tissues of amphioxus has not yet been d etected. However, the absence of gill slits in RA-treated amphioxus em bryos correlates with an RA-induced failure of AmphiPax-1 to become do wn-regulated in regions of pharyngeal endoderm that would normally fus e with the overlying ectoderm. In vertebrates, RA might similarly act via Pax-1/9, also expressed in pharyngeal endoderm, to impair pharynge al patterning.