Patterning a mollusc embryo: The roles of cell lineage and cell signaling in Ilyanassa obsoleta

Abstract

The experiments reported here describe mechanisms of cell fate specification in the embryo of the snail Ilyanassa obsoleta. During early cleavage, the animal-vegetal axis of the embryo is subdivided by a stereotyped series of asymmetric cell divisions that produce sets of cells with similar cleavage patterns and developmental fates. Here I show that, during these cell divisions, mRNAs for multiple patterning genes co-localize with particular centrosomes and are partitioned into specific cells during cleavage. The movement of the mRNAs to the centrosomes requires the microtubule cytoskeleton, and the subsequent movement, from the centrosome to a region on the cortex, requires actin filaments. These events localize different mRNAs to distinct sets of cells with similar developmental fates. In experimentally produced cells with two interphase centrosomes, mRNAs accumulate on the appropriate centrosome, indicating that the patterned accumulation of mRNAs is controlled at the level of individual centrosomes. In a second phase of cell fate specification, one cell in the embryo (called 3D) induces several ectodermal fates in target cells. Here I show that this signaling event involves two conserved cell signaling pathways, the MAPK pathway and the Dpp/BMP2&4 pathway. MAPK activation is detected first in 3D, and then in the cells that are predicted to require the signal. Ablation of 3D prevents the activation in the responding cells, and blocking MAPK activation with a specific inhibitor prevents the differentiation of the structures that require induction by 3D. My results indicate that 3D induces MAPK activation in the responding cells and that this is required for normal patterning. I examined embryos of several other molluscs and representatives of related protostome phyla and found that, in embryos with equal spiral cleavage, MAPK was activated in 3D, but not in the responding cells. The Ilyanassa ortholog of the secreted ligand Dpp (IoDpp) is expressed in the 3D macromere. Treatment of embryos with a closely related ligand (human BMP4) induces a high frequency of extra eyes in intact embryos, and rescues the lack of eyes in embryos where 3D is not specified. These results suggest that 3D specifies multiple fates by parallel Dpp and MAPK-activating signals.