Cellularization: Difference between revisions
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The theory of cellularization, also known as the syncytial theory or ciliate-acoel theory, is a theory to explain the origin of [[Multicellular organism|Metazoa]]. The idea was proposed by Hadži <ref>{{Cite journal|last=Hadži|first=J.|date=1953-12-01|title=An Attempt to Reconstruct the System of Animal Classification|url=https://academic.oup.com/sysbio/article/2/4/145/1655104|journal=Systematic Biology|language=en|volume=2|issue=4|pages=145–154|doi=10.2307/sysbio/2.4.145|issn=1063-5157}}</ref> and Hanson<ref>{{Cite book|url=https://www.worldcat.org/oclc/2597099|title=The origin and early evolution of animals|last=Hanson, Earl D.|date=1977|publisher=Wesleyan University Press|isbn=0819550086|edition=1st ed|location=Middletown, Conn.|oclc=2597099}}</ref> . |
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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 ([[syncytium]])<ref>{{Citation|last=Klautau|first=M.|title=Metazoans, Origins of|date=2016|url=https://linkinghub.elsevier.com/retrieve/pii/B9780128000496002705|work=Encyclopedia of Evolutionary Biology|pages=1–6|publisher=Elsevier|language=en|doi=10.1016/b978-0-12-800049-6.00270-5|isbn=9780128004265|access-date=2019-09-18|last2=Russo|first2=C.A.M.}}</ref>. |
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{{Refimprove|date=December 2007}}'''The theory of cellularization''', also known as Ciliate-acoel theory, is one of the theories explaining the origin of the [[metazoan]]s. It was first based on [[Ernst Haeckel]]'s assumption that these beasts are formed from the jaws of fish and that the earliest animals derived from [[ciliate]] [[protozoan]]s. Haeckel later abandoned this idea which is revived by Hadzi in 1953.<ref>[https://books.google.com/books?id=LnPeZdz5X4sC&pg=PA357&lpg=PA357&dq=hadzi+cellularization&source=bl&ots=c8fPO6hKuS&sig=laEifAnxZ15HRCrBIkj7g8TfdoA&hl=en&ei=KQ4ATdzfFZLYngf38rnlDQ&sa=X&oi=book_result&ct=result&resnum=4&ved=0CCUQ6AEwAw#v=onepage&q=hadzi%20cellularization&f=false Strickberger's evolution: the integration of genes, organisms and populations. By Brian Keith Hall, Benedikt Hallgrímsson, Monroe W. Strickberger, pg357]</ref> According to the theory, a [[multinucleated]] unicellular ciliate ancestor would give rise to organisms similar to modern [[turbellaria]]n [[flatworm]]s by cellularization of the external layer. Recent molecular and morphologic data add increasing evidence against this view, and the alternative [[colonial theory]], also proposed by Haeckel in the 1870s is gaining widespread acceptance. |
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== Arguments == |
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=== Turbellarian flatworms === |
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By several cellularization processes, the ciliate ancestor evolved into the currently known [[Turbellaria|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 [[Cilium|cilia]], are [[bilaterally symmetric]], and [[Syncytium|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<ref>{{Cite book|url=http://worldcat.org/oclc/488084010|title=The origin and phylogeny of the metazoans and the theory of endoderm as secondary layer|last=Pilato, Giovanni|date=2007|publisher=Foxwell & Davies|isbn=1905868065|oclc=488084010}}</ref>. |
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By concluding that the first multicellular animals were flatworms, it is also suggested that simpler organisms as [[Sponge|sponges]], [[Ctenophora|ctenophores]] and [[Cnidaria|cnidarians]] would have derived from more complex animals<ref>{{Citation|last=Waggoner|first=Ben|title=Eukaryotes and Multicells: Origin|date=2001-04-25|url=http://dx.doi.org/10.1038/npg.els.0001640|work=eLS|publisher=John Wiley & Sons, Ltd|isbn=0470016175|access-date=2019-09-18}}</ref>. However, most current molecular research has shown that sponges are the most primitive metazoans <ref>{{Cite journal|last=Schütze|first=Joachim|last2=Krasko|first2=Anatoli|last3=Custodio|first3=Marcio Reis|last4=Efremova|first4=Sofla M.|last5=Müller|first5=Isabel M.|last6=Müller|first6=Werner E. G.|date=1999-01-07|title=Evolutionary relationships of Metazoa within the eukaryotes based on molecular data from Porifera|url=http://dx.doi.org/10.1098/rspb.1999.0605|journal=Proceedings of the Royal Society of London. Series B: Biological Sciences|volume=266|issue=1414|pages=63–73|doi=10.1098/rspb.1999.0605|issn=0962-8452}}</ref><ref>{{Cite journal|last=Manuel|first=Michaël|last2=Wörheide|first2=Gert|last3=Morgenstern|first3=Burkhard|last4=Erpenbeck|first4=Dirk|last5=Schreiber|first5=Fabian|last6=Jackson|first6=Daniel J.|last7=Leys|first7=Sally|last8=Guyader|first8=Hervé Le|last9=Wincker|first9=Patrick|date=2009-04-28|title=Phylogenomics Revives Traditional Views on Deep Animal Relationships|url=https://www.cell.com/current-biology/abstract/S0960-9822(09)00805-7|journal=Current Biology|language=English|volume=19|issue=8|pages=706–712|doi=10.1016/j.cub.2009.02.052|issn=0960-9822|pmid=19345102}}</ref> |
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=== Germ layers are formed simultaneously === |
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The syncytial theory rejects the [[Germ layer|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 <ref name=":0">{{Cite book|url=https://books.google.nl/books?id=JuuWlZ7Llb8C&pg=PA191&lpg=PA191&dq=Hanson+(1958)+syncytial+theory&source=bl&ots=h_SfnBPkWf&sig=ACfU3U1ZKv-f6lkbdRDce-Endhh9f7ZDmw&hl=nl&sa=X&ved=2ahUKEwjIzdWrw83kAhWDZFAKHaeuDDAQ6AEwD3oECAYQAQ#v=onepage&q=Hanson%20(1958)%20syncytial%20theory&f=false|title=Modern Text Book of Zoology: Invertebrates|last=R.L.Kotpal|first=Prof|date=2012|publisher=Rastogi Publications|isbn=9788171339037|language=en}}</ref>. |
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=== ''Drosophila melanogaster development'' === |
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Evidence for the syncytial theory can also be found in the development of ''[[Drosophila melanogaster]]''. First 13 nuclear divisions take place forming a syncytial [[blastoderm]] consisting of approximately 6000 nuclei. During the later [[Gastrulation|gastrulation stage]], membranes are formed between the nuclei, and cellularization is completed<ref>{{Cite book|url=https://books.google.nl/books?hl=nl&lr=&id=A_vvCAAAQBAJ&oi=fnd&pg=PA1&dq=development+drosophila+embryo+blastula+syncytium&ots=nsZf-RX1cN&sig=Owtan8uS-O7zmC8I1A7uuLbLuXY&redir_esc=y#v=onepage&q=development%20drosophila%20embryo%20blastula%20syncytium&f=false|title=The Embryonic Development of Drosophila melanogaster|last=Campos-Ortega|first=Jose A.|last2=Hartenstein|first2=Volker|date=2013-11-11|publisher=Springer Science & Business Media|isbn=9783662224892|language=en}}</ref>. |
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<br /> |
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== Criticism == |
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=== The macro and micronucleus of Ciliates === |
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There is a lot of evidence against [[Ciliate|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<ref>{{Cite journal|last=Prescott|first=D M|date=1994-6|title=The DNA of ciliated protozoa.|url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC372963/|journal=Microbiological Reviews|volume=58|issue=2|pages=233–267|issn=0146-0749|pmid=8078435}}</ref>. This division of nuclei is a unique feature of the ciliates and is not found in any other members of the animal kingdom<ref>{{Cite journal|last=Lipscomb|first=Diana|date=1991-03|title=Protoctists Close at Hand Handbook of Protoctista L. Margulis J. O. Corliss M. Melkonian D. J. Chapman|url=http://dx.doi.org/10.2307/1311459|journal=BioScience|volume=41|issue=3|pages=169–170|doi=10.2307/1311459|issn=0006-3568}}</ref>. Therefore it would be unlikely that ciliates are indeed the ancestors of the metazoans. This is confirmed by molecular [[Phylogenetics|phylogenetic]] research. Ciliates were never found close to animals in any [[molecular phylogeny]]<ref>{{Cite journal|last=Schlegel|first=Martin|date=1994-09|title=Molecular phylogeny of eukaryotes|url=http://dx.doi.org/10.1016/0169-5347(94)90153-8|journal=Trends in Ecology & Evolution|volume=9|issue=9|pages=330–335|doi=10.1016/0169-5347(94)90153-8|issn=0169-5347}}</ref> |
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<br /> |
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=== Flagellated sperm === |
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Furthermore, the syncytial theory cannot explain the [[Flagellum|flagellated]] sperm of metazoans. Since the ciliate ancestor does not have any [[Flagellum|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<ref name=":0" /> |
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Due to both the lack of molecular and morphological evidence for this theory, the alternative [[colonial theory]] of Haeckel, is currently gaining widespread acceptance. |
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* [[Multicellular organism]] |
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* [[Ciliate]] |
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{{Portal|Evolutionary biology}} |
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* [[Evolutionary biology]] |
* [[Evolutionary biology]] |
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[[Category:Biological evolution]] |
[[Category:Biological evolution]] |
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{{Evolution-stub}} |
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Revision as of 15:24, 18 September 2019
The theory of cellularization, 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 [1] and Hanson[2] .
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 (syncytium)[3].
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[4].
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[5]. However, most current molecular research has shown that sponges are the most primitive metazoans [6][7]
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 [8].
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 blastoderm consisting of approximately 6000 nuclei. During the later gastrulation stage, membranes are formed between the nuclei, and cellularization is completed[9].
Criticism
The macro and micronucleus of Ciliates
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[10]. This division of nuclei is a unique feature of the ciliates and is not found in any other members of the animal kingdom[11]. 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[12]
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[8]
Due to both the lack of molecular and morphological evidence for this theory, the alternative colonial theory of Haeckel, is currently gaining widespread acceptance.
See also
References
- ^ Hadži, J. (1953-12-01). "An Attempt to Reconstruct the System of Animal Classification". Systematic Biology. 2 (4): 145–154. doi:10.2307/sysbio/2.4.145. ISSN 1063-5157.
- ^ Hanson, Earl D. (1977). The origin and early evolution of animals (1st ed ed.). Middletown, Conn.: Wesleyan University Press. ISBN 0819550086. OCLC 2597099.
{{cite book}}:|edition=has extra text (help) - ^ Klautau, M.; Russo, C.A.M. (2016), "Metazoans, Origins of", Encyclopedia of Evolutionary Biology, Elsevier, pp. 1–6, doi:10.1016/b978-0-12-800049-6.00270-5, ISBN 9780128004265, retrieved 2019-09-18
- ^ Pilato, Giovanni (2007). The origin and phylogeny of the metazoans and the theory of endoderm as secondary layer. Foxwell & Davies. ISBN 1905868065. OCLC 488084010.
- ^ Waggoner, Ben (2001-04-25), "Eukaryotes and Multicells: Origin", eLS, John Wiley & Sons, Ltd, ISBN 0470016175, retrieved 2019-09-18
- ^ Schütze, Joachim; Krasko, Anatoli; Custodio, Marcio Reis; Efremova, Sofla M.; Müller, Isabel M.; Müller, Werner E. G. (1999-01-07). "Evolutionary relationships of Metazoa within the eukaryotes based on molecular data from Porifera". Proceedings of the Royal Society of London. Series B: Biological Sciences. 266 (1414): 63–73. doi:10.1098/rspb.1999.0605. ISSN 0962-8452.
- ^ Manuel, Michaël; Wörheide, Gert; Morgenstern, Burkhard; Erpenbeck, Dirk; Schreiber, Fabian; Jackson, Daniel J.; Leys, Sally; Guyader, Hervé Le; Wincker, Patrick (2009-04-28). "Phylogenomics Revives Traditional Views on Deep Animal Relationships". Current Biology. 19 (8): 706–712. doi:10.1016/j.cub.2009.02.052. ISSN 0960-9822. PMID 19345102.
- ^ a b R.L.Kotpal, Prof (2012). Modern Text Book of Zoology: Invertebrates. Rastogi Publications. ISBN 9788171339037.
- ^ Campos-Ortega, Jose A.; Hartenstein, Volker (2013-11-11). The Embryonic Development of Drosophila melanogaster. Springer Science & Business Media. ISBN 9783662224892.
- ^ Prescott, D M (1994-6). "The DNA of ciliated protozoa". Microbiological Reviews. 58 (2): 233–267. ISSN 0146-0749. PMID 8078435.
{{cite journal}}: Check date values in:|date=(help) - ^ Lipscomb, Diana (1991-03). "Protoctists Close at Hand Handbook of Protoctista L. Margulis J. O. Corliss M. Melkonian D. J. Chapman". BioScience. 41 (3): 169–170. doi:10.2307/1311459. ISSN 0006-3568.
{{cite journal}}: Check date values in:|date=(help) - ^ Schlegel, Martin (1994-09). "Molecular phylogeny of eukaryotes". Trends in Ecology & Evolution. 9 (9): 330–335. doi:10.1016/0169-5347(94)90153-8. ISSN 0169-5347.
{{cite journal}}: Check date values in:|date=(help)