Cleavage Patterns and the Topology of the Metazoan Tree of Life

Several major alliances of metazoan phyla have been identified by small subunit rRNA sequence comparisons. It is possible to arrange the phyla to produce a parsimonious distribution of cleavage types, requiring only one change from a radial ancestral condition to spiral cleavage and one other to "idiosyncratic" cleavage; this arrangement is consistent with most of the recent molecular phylogenies. The cleavage shifts are correlated with changes in many of the features that once were used to distinguish Protostomia and Deuterostomia. It is hypothesized that changes in cleavage direction are causally associated with changes in blastomere fates and thus that cleavage type correlates with such features as the identity of mesoderm founder cells, which in turn can constrain the mode of origination of the eucelom. Cleavage changes may also affect the timing of cell fate specification. In a tree that emphasizes cleavage parsimony, radial cleavage, regulative development, and enterocely are ancestral within the Bilateria, and spiral or idiosyncratic cleavages, mosaic development, and schizocely are associated with a change in cleavage direction. Deuterostomy is presumably ancestral and is correlated with radial cleavage for this reason, rather than mechanistically.