Eukaryoter

Eukaryota
Eukaryota
	Ammonia tepida, en foraminifer
Videnskabelig klassifikation
Overdomæne	Biota
Domæne	Eukaryota
Riger
	Animalia (Dyr) Fungi (Svampe) Plantae (Planter) Chromista (Stramenopila) Protista (Protister)
	Hjælp til læsning af taksobokse

Fotografi af kæmpecellen *Xenophyophore* med en diameter på ca. 20 cm, som var verdens største i 2005 (NOAA).^[1]

Eukaryoter (af græsk eu god, ægte + karyon nød, kerne) er biologiske organismer, hvis celler indeholder en eller flere cellekerner^[2] – i modsætning til prokaryoter. Ud over cellekernen indeholder en eukaryot celle andre organeller, hvor de vigtigste er det endoplasmatiske reticulum, golgiapparatet, cytoskelet, mitokondrier, evt. grønkorn (kloroplaster) og fimrehår (flageller/cilier).

Eukaryoter har typisk en diameter på 10-100 μm. Der findes exceptionelt store enkeltcellede eukaryoter med mange cellekerner på op til 20 cm i diameter; fx Xenophyophore, som lever på havbunden.^[1]

Eukaryoter udgør et af de tre biologiske domæner, mens prokaryoterne udgør de to andre: bakterier og arkæer.

Oprindelse

De ældste fossiler fra eukaryoter er 1,8–1,9 milliarder år gamle og er fundet i Kina^[3]. Sandsynligvis ligger eukaryoternes oprindelse endnu tidligere, måske så tidligt som for tre milliarder år siden^[4].

Der findes flere hypoteser om, hvordan den eukaryote celle er opstået. I de fleste hypoteser indgår endosymbiose, dvs. at flere forskellige bakterier er gået sammen om at danne en fælles celle, som et fremtrædende element. Når det gælder organellerne mitokondrie og grønkorn (kloroplaster) er endosymbiosen velunderbygget, og i begge tilfælde er det lykkedes at identificere, hvilken bakterie der er optaget i den anden bakterie, hhv. (α-proteobakterier og cyanobakterier). Når det gælder cellekernen, er der ingen alment accepteret hypotese. En mulig oprindelse for cellekernen kunne være, at cellemembranen hos en prokaryot er bugtet indad, indtil den til sidst har omsluttet arvemassen og dannet kernemembranen. Andre prokaryote bakterier med en effektiv energiproduktion og cyanobakterier med fotosyntese er derefter trængt ind i cellen og har trukket lidt af cellemembranen med sig. Det forklarer, hvorfor mitokondriet og grønkornet har dobbelt membran. Derefter er mitokondrie, ribosom og andre organeller begyndt at arbejde sammen. Både mitokondrier og grønkorn har egen arvemasse og kopierer sig selvstændigt.

Eukaryoter (Bangiomorpha pubescens) udviklede meiose, kønnet formering, for cirka 1,2 milliarder år siden, hvorved evolutionen virkelig tog fart.^[5]^[6]^[7]

Bredt anvendt polyfyletisk opdeling

Der har været mange opdelinger af dette domæne.

Den polyfyletiske opdeling fra 1987 er:^[8]

Dyr (Animalia) — Alle flercellede dyr (heterotrofe, formerer sig anisogamt (æg og sæd) og udvikler sig som en embryo)
Svampe (Fungi) — Dækker svampe og gærsvampe, både flercellede og encellede. – ikke havsvampe (spongier) som er dyr (heterotrofe, formerer sig isogamt (plus- og minuskøn))
Planter (Plantae) — Alle planter (autotrofe, formerer sig anisogamt) – flercellede. Inkluderer også Grønalger
Chromista (incl. Stramenopila) – Bl.a. en række ikke-grønne alger – Kiselalger, Brunalger, kelp (tang) og andre eukaryotiske alger
Protozoa

En monofyletisk/parafyletisk opdeling

I en alternativ (fylogenetisk) opdeling består eukaryoterne af to underdomæner

Unikonta, der underinddeles i
- Amoebozoa
- Opisthokonta der bl.a. omfatter Dyreriget og Svamperiget samt forskellige protister
Bikonta, der bl.a. omfatter Planteriget og Stramenopila samt forskellige protozoer og enceller

Se også

Referencer

1 2 oceanexplorer.noaa.gov: Xenophyophore Citat: "...The image may be one of a large 20-cm wide Xenophyophore. Xenophyophores are single cell animals called Protists. As benthic particulate feeders, xenophyophores normally sift through the sediments on the sea floor...", backup
↑ "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". Arkiveret fra originalen 2. oktober 2016. Hentet 25. september 2016.
↑ Hofmann & Chen (1981) Carbonaeceous megafossils from the Precambrian (1800 Ma) near Jixian, Northern China, Can J Earth Sci 18:443-447
↑ Ayala, F (1992) Wistar's view J Mol Evol 35:467-471
↑ Butterfield, Nicholas J. (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes". Paleobiology. 26 (3): 386-404. doi:10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2. Citat: "...Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datum point for constraining protistan phylogeny...Differential spore/gamete formation shows Bangiomorpha pubescens to have been sexually reproducing, the oldest reported occurrence in the fossil record..."
↑ Gibson, Timothy M; Shih, Patrick M; Cumming, Vivien M; Fischer, Woodward W; Crockford, Peter W; Hodgskiss, Malcolm S.W; Wörndle, Sarah; Creaser, Robert A; Rainbird, Robert H; Skulski, Thomas M; Halverson, Galen P (2017). "Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis" (PDF). Geology. 46 (2): 135-138. doi:10.1130/G39829.1.
↑ December 20, 2017, By McGill University: Researchers Date the Origins of Photosynthesis to 1.25 Billion Years Ago, backup Citat: "...The world’s oldest algae fossils are a billion years old, according to a new analysis by earth scientists at McGill University. Based on this finding, the researchers also estimate that the basis for photosynthesis in today’s plants was set in place 1.25 billion years ago. The study, published in the journal Geology, could resolve a long-standing mystery over the age of the fossilized algae, Bangiomorpha pubescens, which were first discovered in rocks in Arctic Canada in 1990...Once the researchers had gauged the fossils’ age at 1.047 billion years, they plugged that figure into a “molecular clock,” a computer model used to calculate evolutionary events based on rates of genetic mutations. Their conclusion: the chloroplast must have been incorporated into eukaryotes roughly 1.25 billion years ago..."
↑ Cavalier-Smith, Thomas (1987). "The origin of eukaryotic and archaebacterial cells". Annals of the New York Academy of Sciences. 503 (1): 17-54. Bibcode:1987NYASA.503...17C. doi:10.1111/j.1749-6632.1987.tb40596.x. PMID 3113314. S2CID 38405158.

Litteratur

Henderson's Dictionary of Biological Terms, 11th Edition, 1997, ISBN 0-582-22708-9, Longman
L. Margulis, Schwartz, K.V.:Five Kingdoms, An Illustrated Guide to the Phyla of Life on Earth, 2nd Edition, 1988, ISBN 0-7167-1912-6, Freeman.

Eksterne henvisninger

BBCNews, 19 June, 2003, Ancient organism challenges cell evolution Arkiveret 10. september 2017 hos Wayback Machine Citat: "..."It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both,"...Agrobacterium tumefaciens..."

Søsterprojekter med yderligere information:

Taksonomi fra Wikispecies

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[xenophyophore-1] 1 2 oceanexplorer.noaa.gov: Xenophyophore Citat: "...The image may be one of a large 20-cm wide Xenophyophore. Xenophyophores are single cell animals called Protists. As benthic particulate feeders, xenophyophores normally sift through the sediments on the sea floor...", backup

[2] "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". Arkiveret fra originalen 2. oktober 2016. Hentet 25. september 2016.

[3] Hofmann & Chen (1981) Carbonaeceous megafossils from the Precambrian (1800 Ma) near Jixian, Northern China, Can J Earth Sci 18:443-447

[Ayala-4] Ayala, F (1992) Wistar's view J Mol Evol 35:467-471

[butterfield2000-5] Butterfield, Nicholas J. (2000). "Bangiomorpha pubescens n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes". Paleobiology. 26 (3): 386-404. doi:10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2. Citat: "...Multicellular filaments from the ca. 1200-Ma Hunting Formation (Somerset Island, arctic Canada) are identified as bangiacean red algae on the basis of diagnostic cell-division patterns. As the oldest taxonomically resolved eukaryote on record Bangiomorpha pubescens n. gen. n. sp. provides a key datum point for constraining protistan phylogeny...Differential spore/gamete formation shows Bangiomorpha pubescens to have been sexually reproducing, the oldest reported occurrence in the fossil record..."

[Cumming2017-6] Gibson, Timothy M; Shih, Patrick M; Cumming, Vivien M; Fischer, Woodward W; Crockford, Peter W; Hodgskiss, Malcolm S.W; Wörndle, Sarah; Creaser, Robert A; Rainbird, Robert H; Skulski, Thomas M; Halverson, Galen P (2017). "Precise age of Bangiomorpha pubescens dates the origin of eukaryotic photosynthesis" (PDF). Geology. 46 (2): 135-138. doi:10.1130/G39829.1.

[McGill2017-7] December 20, 2017, By McGill University: Researchers Date the Origins of Photosynthesis to 1.25 Billion Years Ago, backup Citat: "...The world’s oldest algae fossils are a billion years old, according to a new analysis by earth scientists at McGill University. Based on this finding, the researchers also estimate that the basis for photosynthesis in today’s plants was set in place 1.25 billion years ago. The study, published in the journal Geology, could resolve a long-standing mystery over the age of the fossilized algae, Bangiomorpha pubescens, which were first discovered in rocks in Arctic Canada in 1990...Once the researchers had gauged the fossils’ age at 1.047 billion years, they plugged that figure into a “molecular clock,” a computer model used to calculate evolutionary events based on rates of genetic mutations. Their conclusion: the chloroplast must have been incorporated into eukaryotes roughly 1.25 billion years ago..."

[Cavalier1987-8] Cavalier-Smith, Thomas (1987). "The origin of eukaryotic and archaebacterial cells". Annals of the New York Academy of Sciences. 503 (1): 17-54. Bibcode:1987NYASA.503...17C. doi:10.1111/j.1749-6632.1987.tb40596.x. PMID 3113314. S2CID 38405158.

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