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.1997 Jul 22;94(15):8001-5.
doi: 10.1073/pnas.94.15.8001.

Cleavage patterns and the topology of the metazoan tree of life

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Cleavage patterns and the topology of the metazoan tree of life

J W Valentine. Proc Natl Acad Sci U S A..

Abstract

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.

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Figure 1
Figure 1
A phylogenetic hypothesis of some metazoan phyla based on major groupings suggested by SSU rRNA data, modified so as to produce a parsimonious distribution of cleavage types. Although the general topology is stable under a variety of SSU alignments and algorithms, the positioning and branching patterns of the phyla within the major alliances vary somewhat with algorithms, with different associations of taxa, or with different exemplars. (The major clades are recognized from data in refs. , , , , , and –.) The chief placements that are not directly supported by SSU rRNA data are of Rotifera and Gastrotricha, placed at the base of the Eutrochozoa, and of the Lophophorata, united as a sister group to the Eutrochozoa. It is possible that the Rotifera and Gastrotricha are basal to the entire Lophotrochozoa. (Data for cleavages are from refs. and or as cited in the text.) Cleavage type abbreviations: I, idiosyncratic; NA, not applicable; R, radial; S, spiral.
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