Abstract
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.
Highlights
Spiral cleavage is a form of early embryonic development that occurs widely within the lophotrochozoan superphylum [1]
The chirality of the oblique macromere divisions alternates with each cell cycle: e.g., the first quartet is budded to the northwest, the second to the northeast, and the third to the northwest once more
Owing to the practically identical cell division patterns in the four quadrant lineages, early cleavage generates multiple cell ‘tiers’, or sets of four synchronously formed and phenotypically similar cells that are radially disposed about the AV axis
Summary
Spiral cleavage is a form of early embryonic development that occurs widely within the lophotrochozoan superphylum [1]. The chirality of the oblique macromere divisions alternates with each cell cycle: e.g., the first quartet is budded to the northwest, the second to the northeast, and the third to the northwest once more. Through synchronous reiteration of obliquely oriented divisions, four founder cells give rise to lineages (A, B, C, D) that represent ‘quadrants’ distributed in a rotationally symmetric pattern about the AV axis (Figure 1F). Owing to the practically identical cell division patterns in the four quadrant lineages, early cleavage generates multiple cell ‘tiers’, or sets of four synchronously formed and phenotypically similar cells that are radially disposed about the AV axis. Most spiralian embryos exhibit tier-specific patterns of cell division at early stages; the significance of this mitotic asynchrony is unknown, but presumably reflects differential inheritance of factors regulating cell division. In 24-cell embryos of the marine snail Ilyanassa, each tier is distinguished by inheriting a unique set of cytoplasmic RNA species [5]
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