RETINOIC ACID AFFECTS PATTERNING ALONG THE ANTERIOR-POSTERIOR AXIS OFTHE ASCIDIAN EMBRYO

Because retinoic acid (RA) is known to affect anterior-posterior patte rning in vertebrate embryos, it was questioned whether it shows simila r effects in a more primitive chordate, the ascidian Halocynthia roret zi. Ascidian embryos treated with RA exhibited truncated phenotypes in a dose-dependent manner similar to the anterior truncations seen in v ertebrate embryos. The most severely affected larvae possessed a round trunk without the papillae characteristic of the anterior terminal ep idermis. Retinoic acid also altered the expression of HrHox-1 and Hrot h in a dose-dependent manner. Expression of HrHox-1 increased, whereas expression of Hroth decreased with increasing levels of RA. In treate d embryos, HrHox-1 was first expressed pan-ectodermally, then degraded in all but specific regions of the embryo. By contrast, initiation of Hroth expression was not affected, but epidermal expression was lost while expression in the neural tube narrowed toward the anterior in ta il-bud embryos. These alterations in the expression of homeobox genes appear to correlate closely to the morphological defects elicited by R A treatment, suggesting broad conservation of developmental patterning mechanisms within the Phylum Chordata.