Cellularization
In evolutionary biology, the term cellularization (cellularisation) has been used in theories to explain the evolution of
Processes of cell development in multinucleate cells (syncytium, plural syncytia) of animals and plants are also termed cellularization, often called syncytium cellularization.
The pre-cell theory
According to Otto Kandler's pre-cell theory,[1][2][3] early evolution of life and primordial metabolism (see Iron-Sulfur world hypothesis - metabolism first scenario, according to Wächtershäuser[5][6]) led to the early diversification of life through the evolution of a multiphenotypical population of pre-cells,[1][2][3] from which the three founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three domains of life[7] emerged successively.[1][2][3][8]
In this scenario the three domains of life did not originate from an ancestral nearly complete “first cell“ nor a cellular organism often defined as the last universal common ancestor (
His concept may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. Kandler’s pre-cell theory is supported by Wächtershäuser.[8]
According to Kandler, the protection of fragile primordial life forms from their environment by the invention of envelopes (i.e. membranes, walls) was an essential improvement. For instance, the emergence of rigid cell walls by the invention and elaboration of peptidoglycan[12] in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of the geosphere and hydrosphere.[3]
A coevolution of the biosphere and the geosphere is suggested: “The evolving life could venture into a larger variety of habitats, even into microaerobic habitats in shallow, illuminated surface waters. The continuous changes in the physical environment on the aging and cooling Earth led to further diversification of habitats and favored opportunistic radiation of primitive life into numerous phenotypes on the basis of each of the different chemolithoautotrophies. Concomitantly, with the accumulation of organic matter derived from chemolithoautotrophic life, opportunistic and obligate heterotrophic life may also have developed”.[1]: 155f
The details of Kandler's proposal for the early diversification of life are represented in a scheme, where numbers indicate evolutionary improvements.[3]
The syncytial theory or ciliate-acoel theory
This theory is also known as a theory of cellularization. It is a theory to explain the origin of the Metazoa. The idea was proposed by Hadži (1953)[4] and Hanson (1977).[13]
This cellularization (syncytial) 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 (syncytium).[14]
Example and Criticism
Turbellarian flatworms
According to the syncytial theory, the ciliate ancestor, by several cellularization processes, evolved into the currently known
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.[16] However, most current molecular research has shown that sponges are the most primitive metazoans.[17][18]
Germ layers are formed simultaneously
The syncytial theory rejects the theory of germ layers. During the development of the turbellaria (Acoela), 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 mesoderm (in more complex animals) build up the embryo.[19]
The macro and micronucleus of Ciliates
There is a lot of evidence against
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 novo trait in metazoans, the syncytial theory makes it almost impossible to explain the origin of flagellated sperm.[19]
Due to both the lack of molecular and morphological evidence for this theory, the alternative
For more theories see main article
Cellularization in a syncytium (syncytium cellularization)
The development of cells in a syncytium (multinucleate cells) is termed syncytium cellularization. Syncytia are quite frequent in animals and plants. Syncytium cellularization occurs for instance in the embryonic development of animals and in endosperm development of plants. Here two examples:
Drosophila melanogaster development
In the embryonic development of Drosophila melanogaster, first 13 nuclear divisions take place forming a syncytial blastoderm consisting of approximately 6000 nuclei. During the later gastrulation stage, membranes are formed between the nuclei, and cellularization is completed.[23]
Syncytium cellularization in plants
The term syncytium cellularization is used for instance for a process of cell development in the endosperm of the Poaceae, e.g. barley (Hordeum vulgare),[24] rice (Oryza sativa).[25]
See also
References
- ^ a b c d e f Kandler, Otto (1994). "The early diversification of life". In Stefan Bengtson (ed.). Early Life on Earth. Nobel Symposium 84. New York: Columbia U.P. pp. 152–160.
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