Mesozoa
 | This articleneeds additional citations forverification.(June 2007) |
| Mesozoa | |
|---|---|
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| Dicyemamacrocephalum | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| (unranked): | Protostomia |
| (unranked): | Spiralia |
| Clade: | Platytrochozoa |
| (unranked): | Mesozoa van Beneden, 1876 |
| Phyla | |
TheMesozoaare minuscule,worm-likeparasitesof marine invertebrates. Generally, these tiny, elusive creatures consist of a somatoderm (outer layer) ofciliatedcells surrounding one or more reproductive cells.
A 2017 study recovered Mesozoa as a monophyletic group that emerged in theLophotrochozoaas sister of theRouphozoa.[1]
Some workers previously classified Mesozoa as the solephylumof the lonelysubkingdomAgnotozoa.Cavalier-Smithargued that at least some of the mesozoans are in factprotistans,not animals.[2]
In the 19th century, the Mesozoa were awastebasket taxonfor multicellular organisms which lacked theinvaginatinggastrulawhich was thought to define theMetazoa.[3]
Evolution
[edit]Mesozoa were once thought to beevolutionaryintermediate forms betweenProtozoansandMetazoans,but now they are thought to be degenerate or simplifiedmetazoa.Their ciliated larvae are similar to themiracidiumoftrematodes,and their internal multiplication is similar to what happens in the sporocysts of trematodes. MesozoanDNAhas a lowGC-content(40%). This amount is similar to ciliates, but ciliates tend to be binucleate. Others relate mesozoa to a group includingannelids,planariansandnemerteans.
Orthonectida have a very reduced muscular and nervous system, only consisting of a few cells, but so far no muscle cells or neurons have been found in Dicyemida.[4][5]
Groupings
[edit]The two main mesozoan groups are theDicyemidaand theOrthonectida.Other groups sometimes included in the Mesozoa are thePlacozoaand theMonoblastozoa.
Monoblastozoans consist of a single description written in the 19th century of a species that has not been seen since. As such, many workers doubt that they are a real group.[6]As described, the animal had only a single layer of tissue.[7]
Rhombozoan mesozoans
[edit]Rhombozoa, or dicyemid mesozoans, are found in thenephrid tractsofsquid and octopuses.[8] They range from a few millimeters long with twenty to thirty cells that include anterior attachment cells and a long central reproductive cell called an axial cell. This axial cell may develop asexually intovermiformjuveniles or it may produce eggs and sperm that self-fertilize to produce a ciliated infusiform larva.
There are three genera:Dicyema,PseudicyemaandDicyemennea.
Molecular evidence suggests that this phylum are derived from theLophotrochozoa.[9][10]
Orthonectid mesozoans
[edit]Orthonectida are found in the body spaces of various marine invertebrates including tissue spaces, gonads, genitorespiratory bursae. This pathogen causes host castration of different species.[11]
The best known of Orthonectida is the parasite of brittle stars. Themultinucleatesyncytial stage lives within tissues and spaces of the gonad but can spread into arms. It causes the destruction of starfish ovary and eggs to cause castration (the male gonads are usually unaffected). The stages of the plasmodium develop into more plasmodia by simple fragmentation; at some point, they decide to go sexual. The syncytia are dioecious (either male or female), but young syncytia can fuse to produce both male and female. The males are ciliated and smaller than the females. The females and the males leave the starfish and mate in the sea. Tailed sperm enters the female and fertilizes the numerous oocytes. Each oocyst produces a small ciliated larva which makes its way to another star.
The genome of one of these species –Intoshia linei– has been sequenced.[12]The Orthonectids may be degenerateannelid worms.[6][13]
References
[edit]- ^Lu, Tsai-Ming; Kanda, Miyuki; Satoh, Noriyuki; Furuya, Hidetaka (2017-05-29)."The phylogenetic position of dicyemid mesozoans offers insights into spiralian evolution".Zoological Letters.3:6.doi:10.1186/s40851-017-0068-5.ISSN2056-306X.PMC5447306.PMID28560048.
- ^Hanelt, B.; Van Schyndel, D.; Adema, C.M.; Lewis, L.A.; Loker, E.S. (November 1996)."The Phylogenetic Position of Rhopaluva ophiocomae (Orthonectida) Based on 18s Ribosomal DNA Sequence Analysis"(PDF).Molecular Biology and Evolution.13(9): 1187–1191.doi:10.1093/oxfordjournals.molbev.a025683.PMID8896370.Archived(PDF)from the original on 2022-10-09.Retrieved2013-03-14.
- ^Syed, Tareq; Schierwater, Bernd (December 2002)."Trichoplax adhaerens: discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution".Vie et Milieu.52(4): 177–187.
- ^Slyusarev, George S.; Nesterenko, Maksim A.; Starunov, Viktor V. (2019)."The structure of the muscular and nervous systems of the maleIntoshialinei(Orthonectida) ".Acta Zoologica.100(4): 451–458.doi:10.1111/azo.12279.
- ^Zverkov, O. A.; Mikhailov, K. V.; Isaev, S. V.; Rusin, L. Y.; Popova, O. V.; Logacheva, M. D.; Penin, A. A.; Moroz, L. L.; Panchin, Y. V.; Lyubetsky, V. A.; Aleoshin, V. V. (2019)."Dicyemida and Orthonectida: Two Stories of Body Plan Simplification".Frontiers in Genetics.10:443.doi:10.3389/fgene.2019.00443.PMC6543705.PMID31178892.
- ^abSchiffer, Philipp; Robertson, Helen; Telford, Maximilian (2018-04-03). Orthonectids are highly degenerate annelid worms (Data set).doi:10.5281/zenodo.1206208.[better source needed]
- ^de Meeûs, Thierry; Renaud, François (July 2002)."Parasite within the new phylogeny of eukaryotes".Trends in Parasitology.18(6): 247–51.doi:10.1016/S1471-4922(02)02269-9.PMID12036736.
- ^Hochberg, F. G. (30 June 1983)."The parasites on cephalopods: A review".Memoirs of the National Museum of Victoria.44:109–145.doi:10.24199/j.mmv.1983.44.10.
- ^Kobayash, M.; Furuya, H.; Wada, H. (September–October 2009). "Molecular markers comparing the extremely simple body plan of dicyemids to that of lophotrochozoans: insight from the expression patterns ofHox,Otx,andbrachyury".Evolution & Development.11(5): 582–589.doi:10.1111/j.1525-142X.2009.00364.x.PMID19754714.S2CID6070504.
- ^Suzuki, T. G.; Ogino, K.; Tsuneki, K.; Furuya, H. (June 2010)."Phylogenetic Analysis of Dicyemid Mesozoans (Phylum Dicyemida) From Innexin Amino Acid Sequences: Dicyemids Are Not Related to Platyhelminthes".Journal of Parasitology.96(3): 614–625.doi:10.1645/GE-2305.1.PMID20557208.S2CID25877334.
- ^Barnes, Robert D. (1982).Invertebrate Zoology.Philadelphia, PA: Holt-Saunders International. pp. 247–248.ISBN978-0-03-056747-6.
- ^Mikhailov, K. V.; Slyusarev, G. S.; Nikitin, M. A.; Penin, A. A.; Aleoshin, V. V.; Panchin, Y. V. (11 July 2016)."The Genome of Intoshia linei Affirms Orthonectids as Highly Simplified Spiralians".Curr Biol.26(13): 1768–74.Bibcode:2016CBio...26.1768M.doi:10.1016/j.cub.2016.05.007.PMID27374341.S2CID3917921.
- ^Telford, Maximilian J.; Robertson, Helen E.; Schiffer, Philipp H. (2018-06-18)."Orthonectids Are Highly Degenerate Annelid Worms".Current Biology.28(12): 1970–1974.e3.Bibcode:2018CBio...28E1970S.doi:10.1016/j.cub.2018.04.088.ISSN0960-9822.PMID29861137.S2CID44166754.
External links
[edit]- .Encyclopædia Britannica(11th ed.). 1911.
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