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Karyorelictea

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Karyorelictea
Illustration of Loxodes rostrum
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Diaphoretickes
Clade: SAR
Clade: Alveolata
Phylum: Ciliophora
Subphylum: Postciliodesmatophora
Class: Karyorelictea
Corliss, 1974 [1]
Orders [2]

Karyorelictea is a class of ciliates in the subphylum Postciliodesmatophora. Most species are members of the microbenthos community, that is, microscopic organisms found in the marine interstitial habitat, though one genus, Loxodes, is found in freshwater.

The majority of karyorelict taxa have not been cultivated in the laboratory, although clonal lines of Loxodes have been developed.

Systematics

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According to Lynn (2008), the Karyorelictea class is divided into three orders:[2]

These three orders were defined morphologically, and have been confirmed with molecular phylogenetics.[3]

An additional family, Wilbertomorphidae, is of uncertain affiliation and has not been assigned to an order.[4]

Nuclear dimorphism

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All ciliates, including karyorelicteans, possess two different kinds of nucleus, which separate the functions of gene expression and sexual recombination. The macronuclei, or somatic nuclei, are the site of transcription, while the smaller micronuclei, or germline nuclei, are only active during sexual reproduction, where they first undergo meiosis to form gametic nuclei, which are exchanged when two mating cells conjugate. Two gametic nuclei fuse to form a zygotic nucleus, which divides by mitosis into two daughter nuclei, one of which develops into a new micronucleus and the other into a macronucleus; the old macronucleus typically disintegrates (see main article).

In most ciliates, a macronucleus can divide during asexual reproduction to form new daughter macronuclei, through a process called amitosis. However, in karyorelicteans, the macronuclei are unable to divide. Instead, they must be produced by division and differentiation of a micronucleus every time, even during asexual reproduction.[5][6]

Because of their non-dividing somatic macronuclei, the karyorelicteans were thought to represent an intermediate evolutionary stage between the hypothetical ancestor of ciliates that did not have nuclear dualism, and the other more "advanced" ciliates which had both nuclear dualism and macronuclei that could divide by amitosis. The name of the group therefore makes reference to their supposedly "primitive" nuclei.[7] This theory has since been superseded, as molecular phylogenies have shown that the karyorelicteans are not the most "primitive" or basally-branching group of ciliates.[8]

Ecology

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Almost all karyorelictean species, except Loxodes, have been described from the marine interstitial habitat, where they live in the pore-water spaces between sediment grains.[9] Animals from such habitats are known as meiofauna, and karyorelicteans have many morphological similarities to meiofaunal animals despite being protists: most karyorelicteans are relatively large (1 mm or more in length), have a worm-like (vermiform) body shape with an elongated tail, and exhibit thigmotactic behavior.[10] Most karyorelicteans feed on bacteria or algae, and prefer microaerobic conditions.[11][12][13] However, one genus, Kentrophoros, lacks an oral apparatus and feeds instead on symbiotic sulfur-oxidizing bacteria that are attached to one side of the cell.[14][15]

Etymology

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The class name Karyorelictea derives from the ancient greek κάρυον (káruon), meaning "hard-shelled seed, or nucleus",[16][17] and from the Latin relictus, meaning 'abandoned'.[18]

Alternative genetic code

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An alternative genetic code is used by the nuclear genome of some karyorelictid ciliates (e.g. Parduczia sp.).[19] This code corresponds to translation table 27 and involves the reassignment of three codons:

  • UAA into Gln (Q) ;
  • UAG into Gln (Q) ;
  • UGA into Trp (W) or Termination (*).

References

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  1. ^ WoRMS (2009). "Karyorelictea". World Ciliophora Database. World Register of Marine Species. Retrieved July 21, 2010.
  2. ^ a b Lynn, Denis (2008). The Ciliated Protozoa: Characterization, Classification, and Guide to the Literature. Springer. ISBN 978-1-4020-8239-9.
  3. ^ Ilaria Andreoli; Lara Mangini; Filippo Ferrantini; Giovanni Santangelo; Franco Verni; Giulio Petroni (16 October 2009). "Molecular phylogeny of unculturable Karyorelictea (Alveolata, Ciliophora)". Zoologica Scripta. 38 (6): 651–662. doi:10.1111/J.1463-6409.2009.00395.X. ISSN 0300-3256. S2CID 84951188. Wikidata Q124486881.
  4. ^ Yuan Xu; Jiamei Li; Weibo Song; Alan Warren (6 July 2013). "Phylogeny and establishment of a new ciliate family, Wilbertomorphidae fam. nov. (Ciliophora, Karyorelictea), a highly specialized taxon represented by Wilbertomorpha colpoda gen. nov., spec. nov". Journal of Eukaryotic Microbiology. 60 (5): 480–489. doi:10.1111/JEU.12055. ISSN 1066-5234. PMID 23829190. Wikidata Q30653609.
  5. ^ Ying Yan; Anna J Rogers; Feng Gao; Laura A. Katz (22 May 2017). "Unusual features of non-dividing somatic macronuclei in the ciliate class Karyorelictea". European Journal of Protistology. 61 (Pt B): 399–408. doi:10.1016/J.EJOP.2017.05.002. ISSN 0932-4739. PMC 5831164. PMID 28673471. Wikidata Q34558450.
  6. ^ I B Raikov (1 January 1985). "Primitive never-dividing macronuclei of some lower ciliates". International Review of Cytology. 95: 267–325. doi:10.1016/S0074-7696(08)60584-7. ISSN 0074-7696. PMID 2414246. Wikidata Q38154201.
  7. ^ Corliss, J. O.; Hartwig, E. (1977). "The "primitive" interstitial ciliates: their ecology, nuclear uniquenesses, and postulated place in the evolution and systematics of the phylum Ciliophora". Mikrofauna Meeresbodens. 61: 65–88.
  8. ^ Feng Gao; Alan Warren; Qianqian Zhang; et al. (29 April 2016). "The All-Data-Based Evolutionary Hypothesis of Ciliated Protists with a Revised Classification of the Phylum Ciliophora (Eukaryota, Alveolata)". Scientific Reports. 6: 24874. doi:10.1038/SREP24874. ISSN 2045-2322. PMC 4850378. PMID 27126745. Wikidata Q28833826.
  9. ^ Foissner, Wilhelm (1998). "The karyorelictids (Protozoa: Ciliophora), a unique and enigmatic assemblage of marine, interstitial ciliates: a review emphasizing ciliary patterns and evolution". In Coombs, G.H.; Vickerman, K.; Sleigh, M.A.; Warren, A. (eds.). Evolutionary relationships among Protozoa. Springer. pp. 305–325. ISBN 978-0-412-79800-9.
  10. ^ Giere, Olav (2009). Meiobenthology : the microscopic motile fauna of aquatic sediments (2nd ed.). Springer. ISBN 978-3-540-68661-3. OCLC 310352202.
  11. ^ Tom Fenchel (31 July 1969). "The ecology of marine microbenthos IV. Structure and function of the benthic ecosystem, its chemical and physical factors and the microfauna commuities with special reference to the ciliated protozoa". Ophelia. 6 (1): 1–182. doi:10.1080/00785326.1969.10409647. ISSN 0078-5326. Wikidata Q114642373.
  12. ^ Tom Fenchel; Bland Finlay (24 September 2008). "Oxygen and the spatial structure of microbial communities". Biological Reviews. 83 (4): 553–569. doi:10.1111/J.1469-185X.2008.00054.X. ISSN 1464-7931. PMID 18823390. S2CID 21908644. Wikidata Q37281625.
  13. ^ Fauré-Fremiet, E (1950). "Écologie des ciliés psammophiles littoraux". Bull Biol Fr Belg. 84 (1): 35–75. PMID 15420543.
  14. ^ Finlay, Bland; Fenchel, Tom (1 July 1989). "Everlasting picnic for protozoa". New Scientist: 66–69.
  15. ^ Fenchel, Tom; Finlay, Bland (1989). "Kentrophoros: A mouthless ciliate with a symbiotic kitchen garden". Ophelia. 30: 75–93.
  16. ^ Bailly, Anatole (1981-01-01). Abrégé du dictionnaire grec français. Paris: Hachette. ISBN 978-2010035289. OCLC 461974285.
  17. ^ Bailly, Anatole. "Greek-french dictionary online". www.tabularium.be. Retrieved 2017-01-24.
  18. ^ Gaffiot, Félix (1934). Dictionnaire illustré Latin-Français (in French). Paris: Librairie Hachette. p. 1278. Retrieved 14 October 2017.
  19. ^ Estienne Carl Swart; Valentina Serra; Giulio Petroni; Mariusz Nowacki (14 July 2016). "Genetic Codes with No Dedicated Stop Codon: Context-Dependent Translation Termination". Cell. 166 (3): 691–702. doi:10.1016/J.CELL.2016.06.020. ISSN 0092-8674. PMC 4967479. PMID 27426948. Wikidata Q34534392.
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