RETINOIC ACID DISRUPTS ANTERIOR ECTODERMAL AND ENDODERMAL DEVELOPMENTIN ASCIDIAN LARVAE AND POSTLARVAE

In vertebrates, excess all-trans retinoic acid (RA) applied during axi s formation leads to the apparent truncation of anterior structures. I n this study we sought to determine the type of defects caused by ecto pic RA on the development of the ascidian Herdmania curvata. We demons trate that H. curvata embryos cultured in the presence of RA develop i nto larvae whose trunks are shortened and superficially resemble those of early metamorphosing postlarvae. Despite RA-treated larvae lacking papillar structures they respond normally to natural cues that induce metamorphosis, indicating that chemosensory functionality previously mapped to the most anterior region of normal larvae is unaffected by R A. Excess RA applied during postlarval development leads to a graded l oss of the juvenile pharynx, apparently by respecifying anterior endod erm to a more posterior fate. This structure is considered homologous to the gill slits of amphioxus. which are also lost upon RA treatment. This suggests that RA may have had a role in the development of the p harynx of the ancestral chordate and that this function has been maint ained in ascidians and cephalochordates and lost in vertebrates.