Dorsal-Ventral Patterning in the Mud Snail, Ilyanassa obsoleta

by Wandelt, Jessica Eve.

The experiments reported here describe mechanisms involved in the establishment of the dorsal-ventral axis in the mud snail, Ilyanassa obsoleta. Ilyanassa and other spiralians utilize an embryonic organizer to induce dorsal identity, and thus establish the bilateral axis. The D macromere embryonic organizer in Ilyanassa is specified at the four-cell stage by the inheritance of the polar lobe, but does not function as an inductive center until the 24-cell stage. Previously it was assumed that the D macromere of Ilyanassa functioned autonomously through its inheritance of the polar lobe. I have found this is not the case. Rather, I describe the role that the micromeres play in the activation of the D macromere organizer. Specifically, I have found that micromeres of the first and second quartet are necessary for at least three known characteristics of the D macromere: the activation of MAPK in the D macromere, the division of the D macromere, and the inductive capacity of the D macromere. Thus, while the polar lobe is necessary for D macromere function, its inheritance does not provide the D macromere with functional autonomy. The localized activation of MAPK was the first molecular component of dorsalventral patterning to be identified in Ilyanassa and other spiralians. In addition to being activated in the D macromere organizer, MAPK is also activated in the micromeres that are induced by the D macromere. I undertook a pharmacological screen to identify other components involved in dorsal-ventral patterning. I have found that a member of the Protein Kinase C (PKC) family is also involved in the establishment of the dorsal-ventral 12 axis in Ilyanassa. Inhibition of PKC disrupts patterning, resulting in a radialized animal. In addition, I have found that PKC functions in the same path as MAPK. PKC is necessary for the proper activation of MAPK in the D macromere organizer and the micromeres. These results suggest that either the same transduction pathway is used repeatedly in the establishment of the dorsal-ventral axis or that patterning is the result of one global signal. These results drastically change our view of dorsal-ventral patterning during spiralian development. 13