The mud snail Ilyanassa obsoleta has long been a model for the study of developmental mechanisms because it provides a spectacular example of localization of cytoplasmic determinants. Ilyanassa has a distinctive pattern of cell division known as spiral cleavage, which it shares with several other phyla. In addition, it extrudes a cytoplasmic bulb - the polar lobe- during its first two cleavages. When the polar lobe is removed, the embryo lacks a dorsal-ventral axis and has a radial symmetry. Not only are the direct physical descendants of the polar lobe - the D quadrant cells - missing in polar lobe ablated embryos, but lobeless embryos lack structures known to descend from other quadrants. Thus, through a process of cytoplasmic localization, the polar lobe provides one or more cells in the D quadrant with the ability to induce the fates of non-D cells. The localization of polar lobe determinants thus specifies the D quadrant as the organizer of the Ilyanassa dorsal-ventral axis. Although it has been nearly 100 years since the discovery of the effects of the polar lobe, (Crampton, 1896) the molecular mechanisms underlying the localization of the organizing activity to the D quadrant, as well as the nature of the organizing activity itself, remain unknown. Dr. Nagy and her lab have identified the first known component of the molluscan organizer: a localized activation of mitogen-activated protein kinase (MAPK) that spreads from the 3D macromere to all cells that require the D quadrant signal for normal development. Based on their results, they propose that the activation of MAPK in 3D results from inheriting polar lobe determinants, and that MAPK is a component of a signaling pathway that transduces the inductive signal from 3D to specify cell fate in other lineages. The aims of this grant are to probe the mechanisms by which MAPK is activated in the 3D macromere, the mechanisms by which the D quadrant signals to the rest of the embryo and the mechanism by which MAPK specifies cell fates in other lineages.
