Meeting Abstract

S9.6  Wednesday, Jan. 6  The role of the polar lobe and intracellular signaling in cell fate specification of the mud snail Ilyanassa. GHARBIAH, M; NAKAMOTO, A; NAGY, L*; University of Arizona lnagy@email.arizona.edu

The early embryo of the mud snail Ilyanassa is characterized by the presence of a polar lobe, a large anucleate structure protruded during the meiotic and first two cellular divisions. The contents of the polar lobe are exclusively partitioned into one cell, the D macromere, the descendants of which function to organize polarity within the larvae. Removal of the polar lobe results in larvae with radially positioned head structures as well as disruption of positional information along the anteroposterior axis. The polar lobe was assumed to house localized determinants required to establish axial polarity. We find that the polar lobe is necessary but not sufficient to establish polarity. The D quadrant macromere requires a signal from the micromeres to function as the organizing center of the embryo. Perturbations that disrupt cell contacts and gap junctions prevent organizer activity. In addition, early Notch and Wnt signaling are required to appropriately establish embryonic pattern. Treatment with selective inhibitors of these pathways result in larvae with severe defects in internal organs and cell fate specification. A reappraisal of the literature suggests evidence for additional early signaling to pattern the shell and endoderm. We use this data to build a model for a sequence of signaling events that regulate early embryonic development in Ilyanassa. When examined comparatively, the data suggests that underlying the conservative patterns of cleavage and fate maps within phyla with spiralian development is a surprising degree of diversity of molecular mechanisms specifying cell fate.