Retinoic acid (RA) regulation of Hox6 and Hox14 . Expression patterns of B. lanceolatum Hox6 and Hox14 and their alterations in embryos treated with RA (upper panels) or the RA antagonist BMS009 (lower panels). (A′) Hox6 was expressed only in a very specific region of the neural plate at the mid-neurula stage and not in the mesoderm (dorsal view in inset). Its anterior limit was enlarged rostrally in RA-treated embryos (A) compared with controls (A′); the posterior limit remained fixed (limits marked by black arrows). The Hox6 domain disappeared in BMS009-treated embryos (A′′). (B′, C′) Hox14 was expressed in the posterior half of the notochord and gut, with a diffuse anterior limit (black arrow),in the cerebral vesicle (white arrowhead) and the left side of the pharyngeal endoderm (black arrowhead). A dorsal view of the head region (inset of B′), shows expression in the left part of the pharyngeal endoderm. (B, C) In RA-treated embryos, Hox14 expression in the notochord and gut has extended rostrally in both pre-mouth stage and 2-day-old larvae compared with DMSO-treated embryos (B′, C′), taking the pigment spot (ps) as a reference point. In embryos treated with BMS009, the anterior border of expression was caudally shifted strongly in notochordal and gut domains (B′′, C′′). Expression in the cerebral vesicle was strongly reduced by RA or RA antagonist treatments (B and C; B′′ and C′′,). The pharyngeal domain remained after both treatments (B, C and B′′, C′′), even though the severe phenotype in the case of the RA-treated pre-mouth larva showed strongly reduced expression in the pharynx (B). (D) Schemes of the genomic regions surrounding the loci of Hox6 and Hox5 (top), and Hox14 (bottom) depicting the RAREs of types DR1 (red), DR2 (yellow), and DR5 (blue) that are conserved in both B. floridae and B. lanceolatum.