Cellular arrangements and surface topography during early development in embryos ofIlyanassa obsoleta

CLEAVAGE PATTERNS AND MESENTOBLAST FORMATION IN THE GASTROPODA: AN EVOLUTIONARY PERSPECTIVE

The larger gastropod taxa are characterized by distinctive cleavage patterns. The cell stage at which the mesentoblast is formed appears to be crucial. In none of the taxa is it formed earlier than the 24- and not later than the 63-cell stage. A heterochronic shift from late to early mesentoblast formation appears to coincide with successive steps in gastropod evolution. Comparison of the early cleavage patterns appears to be a powerful method for investigating the evolutionary relations between major gastropod taxa.

Cell lineage of the Ilyanassa embryo: evolutionary acceleration of regional differentiation during early development

Cell lineage studies in mollusk embryos have documented numerous variations on the lophotrochozoan theme of spiral cleavage. In the experimentally tractable embryo of the mud snail Ilyanassa, cell lineage has previously been described only up to the 29-cell stage. Here I provide a chronology of cell divisions in Ilyanassa to the stage of 84 cells (about 16 hours after first cleavage at 23°C), and show spatial arrangements of identified nuclei at stages ranging from 27 to 84 cells. During this period the spiral cleavage pattern gives way to a bilaterally symmetric, dorsoventrally polarized pattern of mitotic timing and geometry. At the same time, the mesentoblast cell 4d rapidly proliferates to form twelve cells lying deep to the dorsal ectoderm. The onset of epiboly coincides with a period of mitotic quiescence throughout the ectoderm. As in other gastropod embryos, cell cycle lengths vary widely and predictably according to cell identity, and many of the longest cell cycles occur in small daughters of highly asymmetric divisions. While Ilyanassa shares many features of embryonic cell lineage with two other caenogastropod genera, Crepidula and Bithynia, it is distinguished by a general tendency toward earlier and more pronounced diversification of cell division pattern along axes of later differential growth.

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.