EDSINGER GONZALES, Eric; VAN DEN BIGGELAAR, Jo; University of Utrecht: The spiralian developmental organizer and the evolution of animal design

Disparity in animal design is enigmatic, as the fossil record has failed to produce transitional forms linking the major metazoan clades. Development of the adult body plan in taxa as divergent as cnidarians, chordates and spiralians is similarly dependent upon patterning by a developmental organizer, but organizer homology remains unresolved, particularly in regards to spiralians. Here, we inhibited or else duplicated organizer specification in the gastropod mollusk Tectura scutum, using brefeldin-A to interfere with cell signaling. We found that organizer loss or duplication resulted in the specific loss or duplication of bilateral symmetry throughout development. Unexpectedly, radial differentiation of both dorsal and ventral structures occurred in radialized embryos lacking a dorsoventral axis. Organizer-dependent, experimental transformations of the adult body plan mimic the disparity observed between three major metazoan clades: the Cnidaria, Bilateria and Ctenophora. Our results suggest that evolutionary changes in the spatial localization of an ancestral developmental organizer might underlie the early evolution of body plan disparity and could account for the absence of transitional forms in the Pre-Cambrian fossil record.