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.

Spiral cleavage and cell position contribute to the specification of the dorsoventral axis in the embryo of the archaeogastropod Haliotis tuberculata.

In the embryo of Haliotis tuberculata spiral cleavage induces size differences between the quadrants in the 4-cell embryo. These size differences, together with the formation of compact cell configurations, induce asymmetrical positions of equivalent cells in the 8- and 16-cell embryo. The asymmetries in size and position influence the final specification of the dorsoventral asymmetry in the 32-cell embryo, as well as formation of the mesentoblast.

Cleavage pattern in embryos of Haliotis tuberculata (Archaeogastropoda) and gastropod phylogeny.

The early cleavage pattern in embryos of the archaeogastropod Haliotis tuberculata strongly resembles the cleavage pattern of the archaeogastropods Trochus and Patella. It typically deviates from the cleavage patterns found in embryos of more advanced Archaeogastropoda, Caenogastropoda (the majority of the meso- and neogastropods), and Euthyneura (opisthobranch and pulmonate gastropods). It is assumed that the cleavage pattern found in Haliotis, Trochus, and Patella represents the ancestral pattern.

Actin genes expressed during early development ofPatella vulgata.

The actin gene family of the marine molluscPatella vulgata was chosen as a model system to study the regulation of genes expressed during early development in molluscs. Using a hamster actin cDNA clone as a probe, we isolated nine actin cDNA clones from trochophore larvae. The total nucleic acid sequence of three of these clones has been determined.

Dye-coupling between blastomeres in early embryos ofPatella vulgata (mollusca, gastropoda): Its relevance for cell determination.

During the early development of the molluscPatella, the dorsoventral axis is established after the fifth cleavage due to direct interaction between the animal micromeres and one of the vegetal macromeres. This vegetal macromere is thereby induced to become the mesentoblast mother cell (3D). In this study we have examined intercellular communication in earlyPatella embryos by monitoring the transfer of the fluorescent dye, Lucifer Yellow, upon iontophoretic injection into blastomeres between the second and sixth cleavage.