Mesentoblast and Trochoblast Specification in Species With Spiral Cleavage Predict Their Phyletic Relations

Historically closely related species have the same body plan. Because of more remote historical relations we can distinguish related body plans. The greater the uniformity in the early morphogenesis of species, the closer their phylogenetic relationship. The earlier the developmental patterns of species start to deviate, the more they are only remotely related. It may be hypothesised that if different species have a great number of cell lines with comparable cleavage patterns and comparable developmental fates in common, they will have close phylogenetic relations. This contribution is an attempt to determine the phyletic relations between representatives of the various phyla with spiral cleavage, the Spiralia. We will concentrate on the specification of two typical spiralian cell lines: the mesodermal stem cell and the trochoblasts. First, mesentoblast formation within different classes of Gastropoda will be compared. Then, a comparison will be made between the various molluscan classes, followed by a preliminary comparison between the phylum Mollusca and two other spiralian phyla, the Annelida and the Platyhelminthes. Finally, trochoblast differentiation and the regulation of the expression of a trochoblast specific gene in the spiralian embryo will be discussed.