The Role of the Mesentoblast in Gastropod Evolution
Each of the major gastropod taxa is characterized by a particular cleavage pattern. Van den Biggelaar and Haszprunar (1996) have proposed that the stage at which the mesentoblast forms is critical in determining the type of mollusc the embryo will become.
The pivitol aspect of gastropod development is the formation of the D lineage, especially the 3D and 4d blastomeres. Since the 3D acts as an inducer cell, minor changes in the timing of 3D formation can have profound effects on development. The later the 3D is formed, the longer the period of autonomous specification. In Figure 1, the major gastropod groups are arranged according to the cell stage at which the 3D forms the mesentoblast (4d) cell. The Polyplacophorans are used as an outlier group. In ancestral-like forms (such as the Docoglossa and Vetigastropoda) the mesentoblast is formed very late, at the 63-cell stage. In the Docoglossa, this stage lasts a long time, while in the Vetigastropoda, it is short. In the most derived groups (Caenogastropoda, Pulmonata, and Opisthobranchia) mesentoblast formation is accelerated up to the 24-cell stage. It appears that the radiation of gastropods from an archaeogastropod-like ancestor to each of the more derived clades has been associated with the early specification of the mesentoblast.
It appears that there is a heterochronic shift in the cells of the D lineage such that the macromeres divide faster than the micromeres. Moreover, in the more derived gastropods, 3D divides at the 24-cell stage, whereas 3A, 3B, and 3C divide only at the 41-cell stage. It is possible that one or more of the other cells is inducing this rapid rate of division.
Although the exact phylogeny of the gastropod clades is controversial, there is general agreement as to which of the clades diverged earlier or later than the others. One possible phylogeny is represented in Figure 2. Here, the more primitive gastropods (archaeogastropoda) eventually form two major lines, the Caenogastropods (those with polar lobes) and the Heterobranchia (which do not form lobes). The placement of the Neritimorpha is controversial but (on the basis of rRNA data) is placed here emerging from the Caenogastropoda lineage before polar lobes evolved. It appears that as the gastropods evolved, the specification of the mesentoblast became progressively earlier, and the embryos spend less time in an autonomous mode of development.
Van den Biggelaarm, J. A. M. and Haszprunar, G. 1996. Cleavage patterns and mesentoblast formation in the gastropoda: An evolutionary perspective. Evolution 50: 1520-1540.