MINSUK, S.B.*; RAFF, R.A.; Konrad Lorenz Institute, Altenberg, Austria; Indiana U., Bloomington: Patterning the pentaradial adult sea urchin body plan
In a striking departure from their bilaterian ancestry, the bilaterally symmetric larvae of echinoderms give rise to radially symmetric adults. The adult body plan arises not from a gradual transformation of the larval one, but from the de novo formation of an oral-aboral (OA) axis in the adult rudiment, on the larval left side. Adult structures then differentiate around the OA axis in a pattern of five radial sectors, with concentric rings of appendages (primary podia, “adult” spines, “juvenile” spines). We know that signals between presumptive adult tissues (left coelomic mesoderm, vestibular ectoderm) trigger adult development, but nothing is known about the mechanisms underlying adult axial patterning. We have used NiCl2 treatment to dissect adult patterning in Heliocidaris erythrogramma. NiCl2 disrupts larval axial patterning, causing vestibular ectoderm to develop in a band encircling the larva, instead of remaining restricted to the left side; but left coelom develops normally. Gene expression in vestibular ectoderm overlying the coelom responds normally to coelomic signaling, and normal adult appendages develop. Appendage formation is centered on the larval left side, retaining aspects of normal OA patterning (podia in a center zone, adult spines in a ring surrounding the podia, juvenile spines outermost), but some adult spines are also distributed in a circumferential band around the larva. The appendage pattern is disorganized within each zone, showing no trace of radial sectors. These facts suggest that the OA axis and the radial sectoring are patterned by separate mechanisms. Radial sectoring requires normal vestibule and is probably ectoderm autonomous; OA axial patterning occurs even in disrupted vestibular ectoderm and is probably regulated by interacting coelomic and ectodermal signals.
