Three-domain system
The three-domain system is a
Background
Woese argued, on the basis of differences in
Acceptance of the validity of Woese's phylogenetically valid classification was a slow process. Prominent biologists including Salvador Luria and Ernst Mayr objected to his division of the prokaryotes.[6][7] Not all criticism of him was restricted to the scientific level. A decade of labor-intensive oligonucleotide cataloging left him with a reputation as "a crank", and Woese would go on to be dubbed "Microbiology's Scarred Revolutionary" by a news article printed in the journal Science in 1997.[8] The growing amount of supporting data led the scientific community to accept the Archaea by the mid-1980s.[9] Today, very few scientists still accept the concept of a unified Prokarya.[10]
Classification
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or
Domain Archaea
The
Some examples of archaeal organisms are:
- methanogens – which produce the gas methane
- halophiles– which live in very salty water
- thermoacidophiles– which thrive in acidic high-temperature water
Domain Bacteria
The
Most known pathogenic prokaryotic organisms belong to bacteria (see[11] for exceptions). For that reason, and because the Archaea are typically difficult to grow in laboratories, Bacteria are currently studied more extensively than Archaea.
Some examples of bacteria include:
- "Cyanobacteria" – photosynthesizing bacteria that are related to the chloroplasts of eukaryotic plants and algae
- Gram-negativebacteria that include those causing syphilis and Lyme disease
- Gram-positive bacteria including Bifidobacterium animaliswhich is present in the human large intestine
Domain Eukarya
- Holomycota – mushrooms and allies
- Viridiplantae – green plants
- Holozoa – animals and allies
- Stramenopiles – includes brown algae
- Amoebozoa – solitary and social amoebae
- Discoba– includes euglenoids
Niches
Each of the three cell types tends to fit into recurring specialities or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources. Eukaryotes are the most flexible with regard to forming cooperative colonies, such as in multi-cellular organisms, including humans. In fact, the structure of a eukaryote is likely to have derived from a joining of different cell types, forming organelles.
Alternatives
Parts of the three-domain theory have been challenged by scientists including Ernst Mayr, Thomas Cavalier-Smith, and Radhey S. Gupta.[13][14][15]
Recent work has proposed that Eukaryota may have actually branched off from the domain Archaea. According to Spang et al. Lokiarchaeota forms a monophyletic group with eukaryotes in phylogenomic analyses. The associated genomes also encode an expanded repertoire of eukaryotic signature proteins that are suggestive of sophisticated membrane remodelling capabilities.[16] This work suggests a two-domain system as opposed to the three-domain system.[3][4][2] Exactly how and when archaea, bacteria, and eucarya developed and how they are related continues to be debated.[17][2][18]
See also
References
- ^ PMID 2112744.
- ^ S2CID 236916203. Retrieved 11 August 2022.
A rooted version of this three-domain tree placed Archaea and Eukarya as sister clades, suggesting that eukaryotes were very distantly related to archaea and not more related to any specific group. More recently, phylogenetic analyses using more sophisticated models and expanded gene data sets have provided increasing support for an alternative tree topology in which the eukaryotic clade branches within Archaea, rather than next to it.
- ^ S2CID 244823103.
- ^ PMID 32097647.
- PMID 270744.
- PMID 9707542.
- PMID 18053933.
- S2CID 84866217.
- ISBN 978-0-199-73438-2.
- PMID 24572480.
- PMID 12540534.
- PMID 23024290.
- PMID 9733652.
- PMID 9707542.
- PMID 11837318.
- PMID 25945739.
- . Retrieved 18 August 2022.
- S2CID 235786110.