EXPRESSION OF THE LABIAL GROUP HOX GENE HRHOX-1 AND ITS ALTERATION INDUCED BY RETINOIC ACID IN DEVELOPMENT OF THE ASCIDIAN HALOCYNTHIA-RORETZI

Ascidian embryogenesis shares several developmental features with vert ebrates. Thus, it is presumed that some molecular mechanisms that are critical for vertebrate development may also act in the early developm ent of ascidians, Here, we investigated expression of the ascidian lab ial group Hox gene HrHox-1 in the development of Halocynthia roretzi. Hi Hox-l showed a spatially restricted expression pattern along the an terior-posterior axis, which is remarkably similar to that of the vert ebrate gene, Hoxb-1, The expression of HrHox-1, however, was exclusive ly in tissues of ectoderm origin unlike its vertebrate counterpart. Ex posure of the embryos to 10(-6) M all-trans retinoic acid induced a la rval phenotype with elimination of the anteriormost structures, the pa pillae. In this phenotype, the level of HrHox-1 expression was enhance d and ectopic expression was observed at the anterior terminal epiderm is where the papillae are otherwise formed. These observations suggest that there are some conserved mechanisms in the spatial regulation of expression of labial group genes in embryogenesis of ascidians and ve rtebrates.