Karyorelictea

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.

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

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

Systematics

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

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 The name of the group therefore makes reference to their supposedly "primitive" nuclei.[7] This theory has since been superseded, as molecular phylogenies have shown that the karyorelicteans are not the most "primitive" or basally-branching group of ciliates.[8]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

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

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

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
  1. WoRMS (2009). "Karyorelictea". World Ciliophora Database. World Register of Marine Species. Retrieved July 21, 2010.
  2. Lynn, Denis (2008-06-24). The Ciliated Protozoa: Characterization, Classification, and Guide to the Literature. Springer Science & Business Media. ISBN 9781402082399.
  3. Ilaria Andreoli; Lara Mangini; Filippo Ferrantini; Giovanni Santangelo; Franco Verni; Giulio Petroni (2009). "Molecular phylogeny of unculturable Karyorelictea (Alveolata, Ciliophora)". Zoologica Scripta. 38 (6): 651–662. doi:10.1111/j.1463-6409.2009.00395.x. S2CID 84951188.
  4. Xu, Yuan; Li, Jiamei; Song, Weibo; Warren, Alan (September 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". The Journal of Eukaryotic Microbiology. 60 (5): 480–489. doi:10.1111/jeu.12055. ISSN 1550-7408. PMID 23829190. S2CID 37210061.
  5. Yan, Ying; Rogers, Anna J.; Gao, Feng; Katz, Laura A. (2017-05-22). "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 1618-0429. PMC 5831164. PMID 28673471.
  6. Raikov, I. B. (1985). "Primitive never-dividing macronuclei of some lower ciliates". Int. Rev. Cytol. 95: 297–325.
  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. Gao, Feng; Warren, Alan; Zhang, Qianqian; Gong, Jun; Miao, Miao; Sun, Ping; Xu, Dapeng; Huang, Jie; Yi, Zhenzhen (2016-04-29). "The All-Data-Based Evolutionary Hypothesis of Ciliated Protists with a Revised Classification of the Phylum Ciliophora (Eukaryota, Alveolata)". Scientific Reports. 6 (1): 24874. doi:10.1038/srep24874. ISSN 2045-2322. PMC 4850378. PMID 27126745.
  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. London: Chapman and Hall. pp. 305–325.
  10. Giere, Olav (2009). Meiobenthology : the microscopic motile fauna of aquatic sediments (2nd rev. and extended ed.). Berlin: Springer. ISBN 9783540686613. OCLC 310352202.
  11. Fenchel, Tom (1969). "The ecology of marine microbenthos IV. Structure and function of the benthic ecosystem, its chemical and physical factors and the microfauna communities with special reference to the ciliated protozoa". Ophelia. 6: 1–182. doi:10.1080/00785326.1969.10409647.
  12. Fenchel, Tom; Finlay, Bland (2008-11-01). "Oxygen and the Spatial Structure of Microbial Communities". Biological Reviews. 83 (4): 553–569. doi:10.1111/j.1469-185x.2008.00054.x. ISSN 1469-185X. PMID 18823390. S2CID 21908644.
  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. Swart, Estienne Carl; Serra, Valentina; Petroni, Giulio; Nowacki, Mariusz (28 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. PMC 4967479. PMID 27426948.
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