The theory ofcellularization, also known as the syncytial theory or ciliate-acoel theory, is a theory to explain the origin of Metazoa. The idea was proposed by Hadži and Hanson. The cellularization theory states that metazoans evolved from a unicellular ciliate with multiple nuclei that went through cellularization. Firstly, the ciliate developed a ventral mouth for feeding and all nuclei moved to one side of the cell. Secondly, an epithelium was created by membranes forming barriers between the nuclei. In this way, a multicellular organism was created from one multinucleate cell.
Arguments
Turbellarian flatworms
By several cellularization processes, the ciliate ancestor evolved into the currently known turbellarian flatworms, which are therefore the most primitive metazoans according to the theory. The theory of cellularization is based on the large similarities between ciliates and flatworms. Both ciliates and flatworms have cilia, are bilaterally symmetric, and syncytial. Therefore the theory assumes that bilateral symmetry is more primitive than radial symmetry. However, current biological evidence shows that the most primitive forms of metazoans show radial symmetry, and thus radially symmetrical animals like cnidaria cannot be derived from bilateral flatworms. By concluding that the first multicellular animals were flatworms, it is also suggested that simpler organisms as sponges, ctenophores and cnidarians would have derived from more complex animals. However, most current molecular research has shown that sponges are the most primitive metazoans.
The syncytial theory rejects the theory of germ layers. During the development of the turbellaria, three regions are formed without the formation of germ layers. From this, it was concluded that the germ layers are simultaneously formed during the cellularization process. This is in contrast to germ layer theory in which ectoderm, endoderm and mesodermbuild up the embryo.
''Drosophila melanogaster'' development
Evidence for the syncytial theory can also be found in the development of Drosophila melanogaster. First 13 nuclear divisions take place forming a syncytial blastodermconsisting of approximately 6000 nuclei. During the later gastrulation stage, membranes are formed between the nuclei, and cellularization is completed.
There is a lot of evidence against ciliates being the metazoan ancestor. Ciliates have two types of nuclei: a micronucleus which is used as germline nucleus and a macronucleus which regulates the vegetative growth. This division of nuclei is a unique feature of the ciliates and is not found in any other members of the animal kingdom. Therefore it would be unlikely that ciliates are indeed the ancestors of the metazoans. This is confirmed by molecular phylogenetic research. Ciliates were never found close to animals in any molecular phylogeny.
Flagellated sperm
Furthermore, the syncytial theory cannot explain the flagellated sperm of metazoans. Since the ciliate ancestor does not have any flagella and it is unlikely that the flagella arose as a de novotrait in metazoans, the syncytial theory makes it almost impossible to explain the origin of flagellated sperm. Due to both the lack of molecular and morphological evidence for this theory, the alternative colonial theory of Haeckel, is currently gaining widespread acceptance.
Related Theories
Iron-sulfur world hypothesis
Cellularization occurs in several stages. It begins with the formation of primitive lipids in the surface metabolism. These lipids accumulate on or in the mineral base. This lipophilizes the outer or inner surfaces of the mineral base, which promotes condensation reactions over hydrolytic reactions by lowering the activity of water and protons. In the next stage lipid membranes are formed. While still anchored to the mineral base they form a semi-cell bounded partly by the mineral base and partly by the membrane. Further lipid evolution leads to self-supporting lipid membranes and closed cells. The earliest closed cells are pre-cells because they allow frequent exchange of genetic material. According to Woese, this frequent exchange of genetic material is the cause for the existence of the common stem in the tree of life and for a very rapid early evolution.
