DICKINSON, A.J.G.*; CROLL, R.P.: Early Development and Cell Fate Specification of the Larval Nervous System and Musculature of the Prosobranch Mollusc Ilyanassa obsoleta

Molluscs have served as important models for understanding early developmental cleavage programs. While the cell lineages of the prosobranch Ilyanassa obsoleta have been thoroughly investigated, the origins and mechanisms in which individual neurons and muscle groups are specified were not resolved. In the present study we examined the early development and cell fate specification of the neural and muscular systems in this species. We characterized the normal development of the nervous system using immunohistochemical techniques for the detection of a variety of neuronal markers. During early embryonic stages an extensive nervous system was localized consisting of an apical organ, peripheral innervation and the first cells of the adult nervous system. F-actin localization revealed the developing musculature which consisted of a larval retractor muscle, accessory muscle bands and network of muscle fibers. To determine how this larval nervous system and musculature is specified during cleavage stages we employed classical cell deletions, lineage tracing experiments and chemical treatments. Similar to other spirilians with unequal cleavage, the early developmental program in these embryos produces an organizing center, the D quadrant, which directs development in adjacent lineages. Our results confirms the suggestion that the D quadrant is necessary for normal organization of the nervous system and the major muscle groups. In addition, we have shown that in the absence of this lineage a specific subset of the larval neurons are absent while others are supernumerary. Our research provides a valuable model for future investigations of the origins of neural and muscular systems as well as the inductive signals that specify these cells in molluscs.