Edsinger-Gonzales, E.*; van der Zee, M.; Dictus, W.J.A.G.; van den Biggelaar, J.A.M.: The development of radialized and twinned gastropod embryos and its implication for spiralian development

Establishment of the adult body plan in molluscs requires induction by the blastomere 3D during embryogenesis. To understand 3D’s developmental function, we interfered with its specification in the gastropod, Tectura scutum, and examined subsequent development. By incubating embryos in brefeldin-A, a fungal toxin that rapidly and reversibly dissolves the Golgi apparatus, we can temporarily shut down cell-cell communication. Varying either the concentration of brefeldin-A or the timing of treatment resulted in 100% radialized, twinned or normal larvae. In absence of 3D, radialized embryos did not establish bilateral cleavage symmetry. Despite this, development closely followed that of control larvae as they underwent gastrulatation and differentiated adult tissues. As veligers, gastropod larvae are bilaterally symmetric with the dorsal-ventral axis at a right angle to the animal-vegetal axis. In contrast, radialized larvae maintained the quadri-radial symmetry of the embryo, with dorsal-ventral structures occuring in a radial series along the animal-vegetal axis. Twinned embryos shared an animal apical tuft and a vegetal mouth while all other structures developed in duplicate and as mirror images across the animal-vegetal axis. We propose that 3D’s developmental function is limited to the establishment of bilateral symmetry. By breaking the continuity of quadri-radial symmetry against a backdrop of regional determinants along the animal-vegetal axis, 3D’s signal enables previously equivalent blastomeres to establish identities. The adult body plan then arises over the course of gastrulation by a process of progressive induction, the product of de novo interactions resulting from stereotypic, cell lineage-defined gastrulation movements.