Cleavage modification did not alter blastomere fates during bryozoan evolution

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Emergence of Embryo Shape During Cleavage Divisions

International audienceCells are arranged into species-specific patterns during early embryogenesis. Such cell division patterns are important since they often reflect the distribution of localized cortical factors from eggs/fertilized eggs to specific cells as well as the emergence of organismal form. However, it has proven difficult to reveal the mechanisms that underlie the emergence of cell positioning patterns that underlie embryonic shape, likely because a system-level approach is required that integrates cell biological, genetic, developmental and mechanical parameters. The choice of organism to address such questions is also important. Because ascidians display the most extreme form of invariant cleavage pattern amongst the metazoans, we have been analyzing the cell biological mechanisms that underpin three aspects of cell division (unequal cell division (UCD), oriented cell division (OCD), and asynchronous cell cycles) which affect the overall shape of the blastula-stage ascidian embryo composed of 64 cells. In ascidians, UCD creates two small cells at the 16-cell stage that in turn undergo two further successive rounds of UCD. Starting at the 16-cell stage, the cell cycle becomes asynchronous whereby the vegetal half divides before the animal half, thus creating 24, 32, 44 then 64-cell stages. Perturbing either UCD or the alternate cell division rhythm perturbs cell position. By analyzing cell shape, we discovered that cell shape propagates, via cell-cell contact, throughout the embryo following UCD and alternate/asynchronous cell division to create the ascidian-specific invariant cleavage pattern via OCD in the longest length of the apical surface of blastomeres