ECTOPIC EXPRESSION OF HOXA-1 IN THE ZEBRAFISH ALTERS THE FATE OF THE MANDIBULAR ARCH NEURAL CREST AND PHENOCOPIES A RETINOIC ACID-INDUCED PHENOTYPE

Considerable evidence has demonstrated that retinoic acid influences t he formation of the primary body axis in vertebrates and that this may occur through the regulation of Hox gene expression. In this study,we show that the phenotype induced by exogenous retinoic acid in the zeb rafish can also be generated by the overexpression of. Hoxa-1 followin g injection of synthetic RNA into the fertilised egg. The isolation, s equence and expression pattern of the zebrafish Hoxa-1 gene is describ ed. We show that exogenously applied retinoic acid causes the ectopic accumulation of Hoxa-1 message during gastrulation in the hypoblast in the head region. Overexpression of Hoxa-1 following injection of RNA causes abnormal growth of the anterior hindbrain, duplication of Mauth ner neurons in rhombomere (r) 2 and fate changes of r2 mesenchymal and neurogenic neural crest. These results are discussed in terms of the role of Hoxa-1 in controlling anterior hindbrain patterning and the re lationship between expression of Hoxa-1 and retinoic acid.