Hoxd-4 expression during pharyngeal arch development in flounder (Paralichthys olivaceus) embryos and effects of retinoic acid on expression

Teleost fish develop seven pharyngeal arches (mandible, hyoid and five gill arches) which give rise to the jaw and gills, and skeletal cell populations which originate from the cranial neural crest. The anterior border of expression of the Deformed (Dfd) group is located in the hindbrain and pharyngeal region. To investigate pharyngeal skeletal formation in the teleost fish, we cloned the cDNA coding Hoxd-4 from a cDNA library for flounder (Paralichthys olivaceus) embryos, and analyzed gene expression pattern during embryogenesis and the effects of retinoic acid (RA) on this gene expression. Between the 21-somite and prim-5 stages, Hoxd-4 was expressed in the central nervous system from rhombomere 7 to the spinal cord, and in the pharyngeal region posterior from gill arch 2. Its expression then became restricted to cartilage precursor cells of gill arches 2-5. When embryos in the early shield stage were exposed to RA at concentrations above 10^-7 M, the anterior border of Hoxd-4 expression shifted anteriorly in a dose-dependent manner, both in the central nervous system and pharyngeal region. We propose that, during gill skeleton formation, Hoxd-4 functions in the specification of regional identity between gill arches 1 and 2, and that their identity is affected by treatment with RA.