Protostome

Protostomia (from Greek πρωτο- proto- "first" and στόμα stoma "mouth") is a clade of animals containing phyla including the arthropods, annelids, and molluscs. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers.[1] The major distinctions between deuterostomes and protostomes are found in embryonic development and is based on the embryological origins of the mouth and anus.

Protostomes
Temporal range: Ediacaran - Recent
A centipede (Ecdysozoa)
Scientific classification
Kingdom: Animalia
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Grobben, 1908
Superphyla
  • Ecdysozoa
  • Spiralia
  • Chaetognatha

In most, but not all[2][3] protostomes, the mouth forms first, then the anus, whereas the reverse is true in deuterostomes.

Protostomy
Early development differences between protostomes versus deuterostomes. In protostomes blastula divisions occur as spiral cleavage because division planes are oriented obliquely to the polar major axis. In deuterostomes, the cleavage is radial because planes are parallel or perpendicular to the major polar axis. During gastrulation, protostome embryos' mouth is given first by the blastopore while the anus is formed later, and vice versa for the deuterostomes
Early development differences between deuterostomes versus protostomes. In deuterostomes, blastula divisions occur as radial cleavage because they occur parallel or perpendicular to the major polar axis. In protostomes the cleavage is spiral because division planes are oriented obliquely to the polar major axis. During gastrulation, deuterostome embryos' anus is given first by the blastopore while the mouth is formed secondarily, and vice versa for the protostomes


In animals at least as complex as earthworms, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it was once believed that in all cases the embryological dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. It is now known that the fate of the blastopore in protostomes is extremely variable. While the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name 'protostome' (in Greek: "first-mouth") is disputable.[2]

Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage.[4] Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.[5][2] Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.[6] Yet two of the most basal and ancient protostome phyla, the chaetognatha and priapulida, have deuterostome-like development.

Evolution

The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to about 60,000 deuterostome species.[7]

Protostomes are divided into the Ecdysozoa, e.g. arthropods, nematodes; the Spiralia, e.g. molluscs, annelids, platyhelminths, and rotifers. A modern consensus phylogenetic tree for the protostomes is shown below.[8][9][10][11][12][11] The timing of clades radiating into newer clades is given in mya (millions of years ago); less certain placements are indicated with dashed lines.[13]

Bilateria
 

Xenacoelomorpha

 
Nephrozoa
Deuterostomia
 

Chordata

 
 

Ambulacraria

 
 
Protostomia
Ecdysozoa
Scalidophora
 

Priapulida

 
 

Kinorhyncha

 
 
 
Nematoida
 

Nematoda

 
 

Nematomorpha

 
 
 
 

Loricifera

 
Panarthropoda
 

Onychophora

 
Tactopoda
 

Tardigrada

 
 

Arthropoda

 
 
 
 
 
>529 mya
Spiralia
Gnathifera
 

Rotifera and allies

 
 

Chaetognatha

 
 
Platytrochozoa
 

Platyhelminthes and allies

 
Lophotrochozoa
 

Mollusca

 
 

Annelida and allies

 
550 mya
580 mya
 
 

†Kimberella

 
610 mya
 
 

See also

References
  1. Hejnol, A.; Obst, M.; Stamatakis, A.; Ott, M.; Rouse, G. W.; Edgecombe, G. D.; et al. (2009). "Assessing the root of bilaterian animals with scalable phylogenomic methods". Proceedings of the Royal Society B: Biological Sciences. 276 (1677): 4261–4270. doi:10.1098/rspb.2009.0896. PMC 2817096. PMID 19759036.
  2. A. Hejnol M. Q. Martindale. "The mouth, the anus, and the blastopore - open questions about questionable openings". In M. J. Telford; D. T. J. Littlewood (eds.). Animal Evolution — Genomes, Fossils, and Trees. pp. 33–40.
  3. Martín-Durán, José M.; Passamaneck, Yale J.; Martindale, Mark Q.; Hejnol, Andreas (2016). "The developmental basis for the recurrent evolution of deuterostomy and protostomy". Nature Ecology & Evolution. 1 (1): 0005. doi:10.1038/s41559-016-0005. PMID 28812551.
  4. Valentine, James W. (July 1997). "Cleavage patterns and the topology of the metazoan tree of life". PNAS. The National Academy of Sciences. 94 (15): 8001–8005. Bibcode:1997PNAS...94.8001V. doi:10.1073/pnas.94.15.8001. PMC 21545. PMID 9223303.
  5. Peters, Kenneth E.; Walters, Clifford C.; Moldowan, J. Michael (2005). The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history. 2. Cambridge University Press. p. 717. ISBN 978-0-521-83762-0.
  6. Safra, Jacob E. (2003). The New Encyclopædia Britannica, Volume 1; Volume 3. Encyclopædia Britannica. p. 767. ISBN 978-0-85229-961-6.
  7. Dawkins, Richard. The ancestor’s tale. Boston. Mariner Books. 2004. p. 377–386
  8. Edgecombe, Gregory D.; Giribet, Gonzalo; Dunn, Casey W.; Hejnol, Andreas; Kristensen, Reinhardt M.; Neves, Ricardo C.; Rouse, Greg W.; Worsaae, Katrine; Sørensen, Martin V. (June 2011). "Higher-level metazoan relationships: recent progress and remaining questions". Organisms, Diversity & Evolution. 11 (2): 151–172. doi:10.1007/s13127-011-0044-4.
  9. Fröbius, Andreas C.; Funch, Peter (2017-04-04). "Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans". Nature Communications. 8 (1): 9. Bibcode:2017NatCo...8....9F. doi:10.1038/s41467-017-00020-w. PMC 5431905. PMID 28377584.
  10. Smith, Martin R.; Ortega-Hernández, Javier (2014). "Hallucigenia's onychophoran-like claws and the case for Tactopoda" (PDF). Nature. 514 (7522): 363–366. Bibcode:2014Natur.514..363S. doi:10.1038/nature13576. PMID 25132546.
  11. "Palaeos Metazoa: Ecdysozoa". palaeos.com. Retrieved 2017-09-02.
  12. Yamasaki, Hiroshi; Fujimoto, Shinta; Miyazaki, Katsumi (June 2015). "Phylogenetic position of Loricifera inferred from nearly complete 18S and 28S rRNA gene sequences". Zoological Letters. 1: 18. doi:10.1186/s40851-015-0017-0. PMC 4657359. PMID 26605063.
  13. Peterson, Kevin J.; Cotton, James A.; Gehling, James G.; Pisani, Davide (2008-04-27). "The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records". Philosophical Transactions of the Royal Society of London B: Biological Sciences. 363 (1496): 1435–1443. doi:10.1098/rstb.2007.2233. PMC 2614224. PMID 18192191.
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