MINSUK, S.B.*; RAFF, R.A.: Inductive Signals Initiating Adult Body Plan Development in Echinoderms

The echinoderm juvenile develops within the larval body, with its own set of radially symmetric adult axes entirely separate from the larva’s bilateral ones. Clearly, echinoderm ontogeny has undergone major evolutionary changes since the bilaterally symmetric ancestor. But little is known about the mechanisms controlling juvenile development. We studied juvenile rudiment formation in the direct-developing sea urchin Heliocidaris erythrogramma, taking advantage of its large eggs and fast development. The rudiment forms as in indirect-developing sea urchins, by an interaction between the vestibular ectoderm and the left coelom (which arises from the archenteron), giving rise to the pentameral water vascular system, podia, and nervous system. We investigated inductive signaling between these two layers microsurgically. After early archenteron removal, gastrula ectoderm developed normal larval structures including vestibule, but no juvenile structures. Archenteron that was removed and then re-inserted developed into gut and coeloms, and rescued rudiment development. In the presence of gut and left coelom together, juvenile structures developed, but in the presence of gut alone, they did not. Therefore vestibule development is autonomous to the ectoderm, but subsequent rudiment development requires signals from the left coelom. This is confirmed by gene expression patterns. Normally, ectodermal HeET-1 expression ceases in the newly formed vestibule. In the absence of coelom, it regulates normally, reflecting autonomous vestibule development. HeARS expression normally ceases in the vestibule floor (apposed to left coelom) but persists in the vestibule roof. In the absence of coelom, it persists throughout the vestibule, indicating a downstream response to coelomic signaling.