Cleavage modification did not alter early blastomere fates during bryozoan evolution

Stereotypic cleavage patterns play a crucial role in cell fate determination by precisely positioning early embryonic blastomeres. Although misplaced cell divisions can alter blastomere fates and cause embryonic defects, cleavage patterns have changed several times during animal evolution. Here, we analyze the evolutionary transition from spiral cleavage – a stereotypic pattern remarkably conserved in many protostomes – to the biradial cleavage of bryozoans. We characterize the cell lineage, MAPK signaling and expression of several developmental genes in the bryozoan Membranipora membranacea , and found that the fate and the genes expressed in the early bryozoan blastomeres are similar to their putative homologous blastomeres in spiral-cleaving embryos. The data indicate that cleavage geometry evolved independent from other developmental traits during the transition from spiral to biradial cleavage in the bryozoan lineage, revealing that stereotypic cleavage patterns can be evolutionarily modified without major changes to the molecular identity and fate of embryonic blastomeres.