MINSUK, S.B.*; RAFF, R.A.; Dept. of Biology, Indiana University, Bloomington: A New Angle on Axial Patterning: Co-option of a Dorsal-Ventral Patterning Mechanism to Control Left-Right Differentiation in a Direct-Developing Sea Urchin

Axial patterning is a central question in developmental biology. In sea urchins it is not known how the dorsoventral (DV) and left-right (LR) larval axes are initially specified. Once specified, pattern formation along all three axes unfolds progressively, leading to commitment of the major regions of the embryo by the gastrula stage. In indirect-developing sea urchins, nickel treatment causes radialization by the expansion of ventral tissue (oral ectoderm). We treated embryos of the direct developer Heliocidaris erythrogramma and its indirect-developing sister species, H. tuberculata, with nickel. Nickel ventralizes H. tuberculata exactly like other indirect developers, establishing that basic patterning mechanisms are conserved in this genus. Nickel also radializes H. erythrogramma, and the process shares several features of timing and morphology with that in other sea urchins. Explant and tissue recombination experiments demonstrate that the effect of nickel is autonomous to the ectoderm, another feature in common with indirect developers. However, H. erythrogramma is distinctly “leftized” rather than ventralized, resulting in an expansion of left-side tissue (vestibular ectoderm) along the LR axis. This geometric anomaly in the midst of pervasive functional similarity indicates that nickel-sensitive processes in H. erythrogramma axial patterning, homologous to those in indirect developers, have been redeployed, and hence co-opted, from their ancestral role in DV axis specification to a new role in LR axis specification. This effect provides clues about the nature of the axial specification processes and about the interaction between DV and LR patterning in sea urchins.