Cell fate maps in the Ilyanassa obsoleta embryo beyond the third division

In the gastropod Ilyanassa obsoleta, early development is mediated by the polar lobe, which shunts determinants to the D lineage during the first two cleavages. These determinants act to specify cell fates directly within the D lineage and to specify cell fates inductively in other lineages of the embryo. Most fate maps in I. obsoleta have been inferred from previous cell ablation experiments, although Render (1991) completed natural fate maps for the first quartet of micromeres. Here this work has been extended to include second and third quartet micromeres as well as the mesentoblast cell (4d) and some of its derivatives and selected macromeres. Fate maps were created by injecting individual cells with Lucifer Yellow dextran via iontophoresis and by analyzing patterns of fluorescence in resulting veliger larvae. In general, second quartet micromeres make major contributions to the shell-forming mantle, the velum, the stomodeum and the heart. Third quartet micromeres give rise to large areas of the foot, velum, esophagus and heart. The mesentoblast cell, 4d, contributes to retractor muscles, heart, larval kidney and intestine. These results are discussed in terms of previous ablation experiments and mechanisms of cell fate specification in the I. obsoleta embryo.