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Fossil range: Proterozoic - Recent
Ostreococcus is the smallest known free living eukaryote with an average size of 0.8 µm.
Scientific classification
Domain: Eukaryota
Whittaker & Margulis,1978
Animalia - Animals
Plantae - Plants
Alternative phylogeny

A eukaryote (pronounced /juːˈkæriɒt/ or /juːˈkærioʊt/) is an organism whose cells contain complex structures inside the membranes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried.[1][2][3] The presence of a nucleus gives eukaryotes their name, which comes from the Greek ευ (eu, "good", "noble" & "true") and κάρυον (karyon, "nut" & "kernel"). Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplasts and the Golgi apparatus. Almost all species of large organisms are eukaryotes, including animals, plants and fungi, although most species of eukaryotic protists are microorganisms.

Cell division in eukaryotes is different from that in organisms without a nucleus (prokaryotes). It involves separating the duplicated chromosomes, through movements directed by microtubules. There are two types of division processes. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, which is required in sexual reproduction, one diploid cell (having two instances of each chromosome, one from each parent) undergoes recombination of each pair of parental chromosomes, and then two stages of cell division, resulting in four haploid cells (gametes). Each gamete has just one complement of chromosomes, each a unique mix of the corresponding pair of parental chromosomes.

Eukaryotes appear to be monophyletic, and so make up one of the three domains of life. The two other domains, Bacteria and Archaea, are prokaryotes and have none of the above features.


Cell features

Eukaryotic cells are typically much larger than prokaryotes. They have a variety of internal membranes and structures, called organelles, and a cytoskeleton composed of microtubules, microfilaments, and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division.

Detail of the endomembrane system and its components

Internal membrane

Eukaryotic cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles or vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. It is probable that most other membrane-bound organelles are ultimately derived from such vesicles.

The nucleus is surrounded by a double membrane (commonly referred to as a nuclear envelope), with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form what is called the endoplasmic reticulum or ER, which is involved in protein transport and maturation. It includes the rough ER where ribosomes are attached, and the proteins they synthesize enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth ER. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, called Golgi bodies or dictyosomes.

Vesicles may be specialized for various purposes. For instance, lysosomes contain enzymes that break down the contents of food vacuoles, and peroxisomes are used to break down peroxide, which is toxic otherwise. Many protozoa have contractile vacuoles, which collect and expel excess water, and extrusomes, which expel material used to deflect predators or capture prey. In multicellular organisms, hormones are often produced in vesicles. In higher plants, most of a cell's volume is taken up by a central vacuole, which primarily maintains its osmotic pressure.

Mitochondria structure:
1) Inner membrane
2) Outer membrane
3) Crista
4) Matrix

Mitochondria and plastids

Mitochondria are organelles found in nearly all eukaryotes. They are surrounded by double membranes (known as the phospholipid bi-layer), the inner of which is folded into invaginations called cristae, where aerobic respiration takes place. Mitochondria contain their own DNA and ribosomes and are only formed by the fission of other mitochondria. They are now generally held to have developed from endosymbiotic prokaryotes, probably proteobacteria. The few protozoa that lack mitochondria have been found to contain mitochondrion-derived organelles, such as hydrogenosomes and mitosomes.

Plants and various groups of algae also have plastids. Again, these have their own DNA and developed from endosymbiotes, in this case cyanobacteria. They usually take the form of chloroplasts, which like cyanobacteria contain chlorophyll and produce energy through photosynthesis. Others are involved in storing food. Although plastids likely had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from others through secondary endosymbiosis or ingestion.

Endosymbiotic origins have also been proposed for the nucleus, for which see below, and for eukaryotic flagella, supposed to have developed from spirochaetes. This is not generally accepted, both from a lack of cytological evidence and difficulty in reconciling this with cellular reproduction.

Cytoskeletal structures

Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or similar, but shorter structures called cilia. Flagella and cilia are sometimes referred to as undulipodia, and are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin. These are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a basal body, also called a kinetosome or centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella also may have hairs, or mastigonemes, and scales connecting membranes and internal rods. Their interior is continuous with the cell's cytoplasm.

Microfilamental structures composed by actin and actin binding proteins, e.g., α-actinin, fimbrin, filamin are present in submembraneous cortical layers and bundles, as well. Motor proteins of microtubules, e.g., dynein or kinesin and actin, e.g., myosins provide dynamic character of the network.

Centrioles are often present even in cells and groups that do not have flagella. They generally occur in groups of one or two, called kinetids, that give rise to various microtubular roots. These form a primary component of the cytoskeletal structure, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles may also be associated in the formation of a spindle during nuclear division.

Significance of cytoskeletal structures is underlined in determination of shape of the cells, as well as their being essential components of migratory responses like chemotaxis and chemokinesis. Some protists have various other microtubule-supported organelles. These include the radiolaria and heliozoa, which produce axopodia used in flotation or to capture prey, and the haptophytes, which have a peculiar flagellum-like organelle called the haptonema.

Plant cell wall

Plant cells have a cell wall, a fairly rigid layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. The major carbohydrates making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.

Differences between eukaryotic cells

There are many different types of eukaryotic cells, though animals and plants are the most familiar eukaryotes, and thus provide an excellent starting point for understanding eukaryotic structure. Fungi and many protists have some substantial differences, however.

Animal cell

Structure of a typical animal cell
Structure of a typical plant cell

An animal cell is a form of eukaryotic cell that makes up many tissues in animals. The animal cell is distinct from other eukaryotes, most notably plant cells, as they lack cell walls and chloroplasts, and they have smaller vacuoles. Due to the lack of a rigid cell wall, animal cells can adopt a variety of shapes, and a phagocytic cell can even engulf other structures.

There are many different cell types. For instance, there are approximately 210 distinct cell types in the adult human body.

Plant cell

Plant cells are quite different from the cells of the other eukaryotic organisms. Their distinctive features are:

Fungal cell

Fungal cells are most similar to animal cells, with the following exceptions:

  • A cell wall containing chitin
  • Less definition between cells; the hyphae of higher fungi have porous partitions called septa, which allow the passage of cytoplasm, organelles, and, sometimes, nuclei. Primitive fungi have few or no septa, so each organism is essentially a giant multinucleate supercell; these fungi are described as coenocytic.
  • Only the most primitive fungi, chytrids, have flagella.

Other eukaryotic cells

Eukaryotes are a very diverse group, and their cell structures are equally diverse. Many have cell walls; many do not. Many have chloroplasts, derived from primary, secondary, or even tertiary endosymbiosis; and many do not. Some groups have unique structures, such as the cyanelles of the glaucophytes, the haptonema of the haptophytes, or the ejectisomes of the cryptomonads. Other structures, such as pseudopods, are found in various eukaryote groups in different forms, such as the lobose amoebozoans or the reticulose foraminiferans.


Nuclear division is often coordinated with cell division. This generally takes place by mitosis, a process that allows each daughter nucleus to receive one copy of each chromosome. In most eukaryotes, there is also a process of sexual reproduction, typically involving an alternation between haploid generations, wherein only one copy of each chromosome is present, and diploid generations, wherein two are present, occurring through nuclear fusion (syngamy) and meiosis. There is considerable variation in this pattern, however.

Eukaryotes have a smaller surface area to volume ratio than prokaryotes, and thus have lower metabolic rates and longer generation times. In some multicellular organisms, cells specialized for metabolism will have enlarged surface areas, such as intestinal vili.

Origin and evolution

Phylogenetic tree showing the relationship between the eukaryotes and other forms of life.[4] Eukaryotes are colored red, archaea green and bacteria blue.
One hypothesis of eukaryotic relationships

The origin of the eukaryotic cell was a milestone in the evolution of life, since they include all complex cells and almost all multi-cellular organisms. The timing of this series of events is hard to determine; Knoll (2006) suggests they developed approximately 1.6–2.1 billion years ago. Some acritarchs are known from at least 1650 million years ago, and the possible alga Grypania has been found as far back as 2100 million years ago.[5]


Fossils that are clearly related to modern groups start appearing around 1.2 billion years ago, in the form of a red alga, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back to 1.6 to 1.7 billion years ago.[6]

Biomarkers suggest that at least stem eukaryotes arose even earlier. The presence of steranes in Australian shales indicates that eukaryotes were present 2.7 billion years ago.[7][8]

rRNA trees constructed during the 1980s and 1990s left most eukaryotes in an unresolved "crown" group (not technically a true crown), which was usually divided by the form of the mitochondrial cristae; see crown eukaryotes. The few groups that lack mitochondria branched separately, and so the absence was believed to be primitive; but this is now considered an artifact of long-branch attraction, and they are known to have lost them secondarily.[9][10]

Six supergroup model and two clade model

More recent work has painted a different picture. Most eukaryotes are now included in one of the following supergroups, although the relationship between these groups, and the monophyly of each group, is not yet clear:[11][12]. Some groups position in the tree is still unclear, as in Kamera lens.

Opisthokonts Animals, fungi, choanoflagellates, etc.
Amoebozoa Most lobose amoeboids and slime moulds
Rhizaria Foraminifera, Radiolaria, and various other amoeboid protozoa
Excavates Various flagellate protozoa
Archaeplastida (or Primoplantae) Land plants, green algae, red algae, and glaucophytes
Chromalveolates Heterokonts, Haptophytes, Cryptomonads, and Alveolates.

A further proposal is that eukaryotes can be classified into two larger clades, the unikonts and the bikonts,[13] that derive from an ancestral uniflagellar organism and a biflagellate, respectively. In this system, the opisthokonts and amoebozoans are considered unikonts, and the rest bikonts. The correct relationships of the organisms in the Archaeplastida and Chromalveolates is far from clear, and it seems likely that neither group, as defined above, is monophyletic.[14] Some small protist groups have not been related to any of these supergroups, in particular the characteristics of the anaerobic protist Breviata anathema seems to contradict the unikont/bikont division.[15]

Relationship to Archaea

Eukaryotes are more closely related to Archaea than Bacteria, at least in terms of nuclear DNA and genetic machinery, and one controversial idea is to place them with Archaea in the clade Neomura. However, in other respects, such as membrane composition, eukaryotes are similar to Bacteria. Three main explanations for this have been proposed:

  • Eukaryotes resulted from the complete fusion of two or more cells, wherein the cytoplasm formed from a eubacterium, and the nucleus from an archaeon,[16] from a virus,[17][18] or from a pre-cell.[19][20]
  • Eukaryotes developed from Archaea, and acquired their eubacterial characteristics from the proto-mitochondrion.
  • Eukaryotes and Archaea developed separately from a modified eubacterium.












One recent cladogram of Eukarya

Endomembrane system and mitochondria

The origins of the endomembrane system and mitochondria are also unclear.[21] The phagotrophic hypothesis proposes that eukaryotic-type membranes lacking a cell wall originated first, with the development of endocytosis, whereas mitochondria were acquired by ingestion as endosymbionts.[22] The syntrophic hypothesis proposes that the proto-eukaryote relied on the proto-mitochondrion for food, and so ultimately grew to surround it. Here the membranes originated after the engulfment of the mitochondrion, in part thanks to mitochondrial genes (the hydrogen hypothesis is one particular version).[23]

In a study using genomes to construct supertrees, Pisani et al. (2007) suggest that, along with evidence that there was never a mitochondrion-less eukaryote, eukaryotes evolved from a syntrophy between an archaea closely related to Thermoplasmatales and an α-proteobacterium, likely a symbiosis driven by sulfur or hydrogen. The mitochondrion and its genome is a remnant of the α-proteobacterial endosymbiont.[24]

See also


  1. ^ Youngson, Robert M. (2006). Collins Dictionary of Human Biology. Glasgow: HarperCollins. ISBN 0-00-722134-7.  
  2. ^ Nelson, David L. & Michael M. Cox (2005), Lehninger Principles of Biochemistry (4th ed.), W.H. Freeman, ISBN 0716743396
  3. ^ Martin, E.A., ed (1983). Macmillan Dictionary of Life Sciences (2nd ed.). London: Macmillan Press. ISBN 0-333-34867-2.  
  4. ^ Ciccarelli FD, Doerks T, von Mering C, Creevey CJ, Snel B, Bork P (2006). "Toward automatic reconstruction of a highly resolved tree of life". Science 311 (5765): 1283–7. doi:10.1126/science.1123061. PMID 16513982.  
  5. ^ Knoll, Andrew H.; Javaux, E.J, Hewitt, D. and Cohen, P. (2006). "Eukaryotic organisms in Proterozoic oceans". Philosophical Transactions of the Royal Society of London, Part B 361 (1470): 1023–38. doi:10.1098/rstb.2006.1843. PMID 16754612.  
  6. ^ Bengtson S, Belivanova V, Rasmussen B, Whitehouse M. (2009). The controversial "Cambrian" fossils of the Vindhyan are real but more than a billion years older. Proc Natl Acad Sci U S A. 106: 7729–7734 PubMed
  7. ^ Brocks JJ, Logan GA, Buick R, Summons RE (August 1999). "Archean molecular fossils and the early rise of eukaryotes". Science 285 (5430): 1033–6. doi:10.1126/science.285.5430.1033. PMID 10446042.  
  8. ^ Ward P (9 Feb 2008). "Mass extinctions: the microbes strike back". New Scientist: 40–3.  
  9. ^ Tovar J, Fischer A, Clark CG (1999). "The mitosome, a novel organelle related to mitochondria in the amitochondrial parasite Entamoeba histolytica". Mol. Microbiol. 32 (5): 1013–21. doi:10.1046/j.1365-2958.1999.01414.x. PMID 10361303.  
  10. ^ Boxma B, de Graaf RM, van der Staay GW, et al. (2005). "An anaerobic mitochondrion that produces hydrogen". Nature 434 (7029): 74–9. doi:10.1038/nature03343. PMID 15744302.  
  11. ^ Burki F, Shalchian-Tabrizi K, Minge M, et al. (2007). "Phylogenomics reshuffles the eukaryotic supergroups". PLoS ONE 2 (8): e790. doi:10.1371/journal.pone.0000790. PMID 17726520.  
  12. ^ Laura Wegener Parfrey, Erika Barbero, Elyse Lasser, Micah Dunthorn, Debashish Bhattacharya, David J Patterson, and Laura A Katz (2006 December). "Evaluating Support for the Current Classification of Eukaryotic Diversity". PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMID 17194223.  
  13. ^ Burki F, Pawlowski J (October 2006). "Monophyly of Rhizaria and multigene phylogeny of unicellular bikonts". Mol. Biol. Evol. 23 (10): 1922–30. doi:10.1093/molbev/msl055. PMID 16829542.  
  14. ^ Kim E, Graham LE (2008). "EEF2 analysis challenges the monophyly of Archaeplastida and Chromalveolata". PLoS ONE 3 (7): e2621. doi:10.1371/journal.pone.0002621. PMID 18612431.  
  15. ^ Roger AJ, Simpson AGB. (2009). "Evolution: Revisiting the Root of the Eukaryote Tree". Current Biology 19 (4): R165–7. doi:10.1016/j.cub.2008.12.032. PMID 19243692.  
  16. ^ Martin W (December 2005). "Archaebacteria (Archaea) and the origin of the eukaryotic nucleus". Curr. Opin. Microbiol. 8 (6): 630–7. doi:10.1016/j.mib.2005.10.004. PMID 16242992.  
  17. ^ Takemura M (May 2001). "Poxviruses and the origin of the eukaryotic nucleus". J. Mol. Evol. 52 (5): 419–25. doi:10.1007/s002390010171. PMID 11443345.  
  18. ^ Bell PJ (September 2001). "Viral eukaryogenesis: was the ancestor of the nucleus a complex DNA virus?". J. Mol. Evol. 53 (3): 251–6. doi:10.1007/s002390010215. PMID 11523012.  
  19. ^ Wächtershäuser G (January 2003). "From pre-cells to Eukarya--a tale of two lipids". Mol. Microbiol. 47 (1): 13–22. doi:10.1046/j.1365-2958.2003.03267.x. PMID 12492850.  
  20. ^ Wächtershäuser G (October 2006). "From volcanic origins of chemoautotrophic life to Bacteria, Archaea and Eukarya". Philos. Trans. R. Soc. Lond. B Biol. Sci. 361 (1474): 1787–1808. doi:10.1098/rstb.2006.1904. PMID 17008219.  
  21. ^ Jékely G (2007). "Origin of eukaryotic endomembranes: a critical evaluation of different model scenarios". Adv. Exp. Med. Biol. 607: 38–51. doi:10.1007/978-0-387-74021-8_3. PMID 17977457.  
  22. ^ Cavalier-Smith T (1 March 2002). "The phagotrophic origin of eukaryotes and phylogenetic classification of Protozoa". Int. J. Syst. Evol. Microbiol. 52 (Pt 2): 297–354. PMID 11931142.  
  23. ^ Martin W, Müller M (March 1998). "The hydrogen hypothesis for the first eukaryote". Nature 392 (6671): 37–41. doi:10.1038/32096. PMID 9510246.  
  24. ^ Pisani D, Cotton JA, McInerney JO (2007). "Supertrees disentangle the chimerical origin of eukaryotic genomes". Mol Biol Evol. 24 (8): 1752–60. doi:10.1093/molbev/msm095. PMID 17504772.  

PD-icon.svg This article incorporates public domain material from the NCBI document "Science Primer".

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Definition from Wiktionary, a free dictionary




Greek εὖ (good-, well-, true) + καρυωτός (having nuts) < κάρυον (nut).


Wikipedia has an article on:




eukaryote (plural eukaryotes)

  1. Any of the single-celled or multicellular organisms, of the taxonomic domain Eukaryota, whose cells contain at least one distinct nucleus.

Alternative spellings

Related terms


Simple English

File:Eukaryota diversity
Eukaryote diversity
Scientific classification
Domain: Eukarya
A typical animal cell

A eukaryote is an organism with complex cells, or a single cell with a complex structure.

In these cells the genetic material is organized into chromosomes in membrane-bound nuclei.

Animals, plants, algae and fungi are all eukaryotes. There are also eukaryotes amongst the protists.

In contrast, simpler organisms, such as bacteria and archaea, do not have nuclei and other complex cell structures. Such organisms are called prokaryotes.

The eukaryotes are often treated as a superkingdom, or domain.

The oldest known eukaryote is Grypania, a coiled, unbranched filament up to 30mm long. It was found in strata 2.1 billion years old. Another ancient group is the acritarchs, believed to be the cysts or reproductive stages of algal plankton. They are found 1400 million years ago, in the Proterozoic eon.[1]p57



Eukaryotic cells are usually much bigger than prokaryotes. A eukaryotic cell can be up to 10 times bigger than a prokaryote. Eukaryotic cells have many different internal membranes and structures, called organelles. They also have a cytoskeleton. The cytoskeleton is made up of microtubules and microfilaments. Those parts are very important in the cell's shape. Eukaryotic DNA is put in bundles called chromosomes, which are separated by a microtubular spindle during cell division. Most eukaryotes have some sort of sexual reproduction through fertilisation, which prokaryotes do not use.

Prokayotes do not have sexes, but they can pass DNA to other bacteria. Their cell division is asexual. Bacterial conjugation is when bacteria give each each DNA by touching each other or making something like a bridge to go between them. [2]

Eukaryotes have sets of linear chromosomes located in the nucleus and the number of chromosomes is usually typical for each species.

Internal membrane

In eukaryotic cells, there are many things with membranes around them. All of them together are called the endomembrane system. Simple bags, called vesicles or vacuoles, are sometimes made by budding off other membranes, just like how children make bubbles with their toys. Many cells take in food and other things using something called endocytosis. In endocytosis, the membrane closest to the outside bends inwards and then pinches off to make a vesicle. Many other organelles that have membranes probably started off as vesicles.

The nucleus is surrounded by two membranes membrane that has holes in it so things can go in and out. The nuclear envelope has things sticking out of it that look like tubes and sheets. These are called the endoplasmic reticulum which is often shortened to ER. The ER is works with moving proteins around and allowing them to mature.

The ER has two parts, the rough ER and the smooth ER. The rough ER has ribosomes attached to it. The proteins made by the ribosomes attached to the rough ER go to the inside the rough ER, called the lumen. After that, they usually go into vesicles, which grow and pinch off from the smooth ER. In most eukaryotes, the vesicles with proteins inside fuse with piles of flattened vesicles called the Golgi bodies, where the proteins inside are changed again.

Vesicles are sometimes changed so they can do one thing very well. This is called specialization, or differentiation. For example, lysosomes have enzymes inside them that break down the food the comes from food vacuoles, and peroxisomes have enzymes that break down peroxide, a poison, so it is not poisonous anymore.

Many protozoa have contractile vacuoles, which are vacuoles that can fuse or pinch off from the outer membrane. Contractile vesicles are often used to get and get rid of unneeded water. Extrusomes shoot out stuff that make predators go away or catch food. In multicellular organisms, hormones are often made in vesicles. In the complicated plants, most of the inside of a plant cell is taken up by a central vacuole. That central vacuole is the main thing that keeps osmotic pressure so the cell can hold its shape.


Because the cell organelles of eukaryotes have different (polyphyletic) origins, the question arises as to whether the group is a unified clade or not.[3] It is certain that the protists are not.[4][5] Cell organelles are specialised units which carry out well-defined functions, like mitochondria and plastids. It is fairly clear now that all or most of these organelles have their origin in once-independent prokaryotes (bacteria or archaea), and that the eukaryote cell is a 'community of micro-organisms' working together in 'a marriage of convenience'.[6] The first such events took place between ancient bacteria to produce the double-membrane class known as gram-negative bacteria.[7] Since the gram-negative bacteria include the cyanobacteria, this was the first of several such events in the history of the eukaryotes.


Protista is a group of different single-celled organisms. More accurate taxonomies have been proposed, but scientists are still discussing them. For this reason, Protista is still useful for talking about these organisms. One modern scheme for the classification of the Eukarya is as follows:[8][9]

Opisthokonts Animals, fungi, choanoflagellates, etc.
Amoebozoa Most lobose amoeboids and slime moulds
Rhizaria Foraminifera, Radiolaria, and various other amoeboid protozoa
Excavates Various flagellate protozoa
Archaeplastida (or Primoplantae) Land plants, green algae, red algae, and glaucophytes
Chromalveolates Heterokonts, Haptophytes, Cryptomonads, and Alveolates.

The implication of this might be that the Eukarya are only unified in the fact that the cells are a community derived from bacteria and archaea; but the derivations vary. The conclusion may be that, like the Protista, the Eukarya is a polyphyletic assembly, though a useful one. However, as mentioned above, all branches of the Eukarya have sexual reproduction.[10] That, and the general organisation of the nucleus, are defining features. It is on this that the issue of monophyletic origin rests.[11]


  1. Clarkson E.N.K. 1998. Invertebrate palaeontology and evolution. Blackwell, Oxford.
  2. Holmes R.K. & Jobling M.G. (1996). Genetics: conjugation. In: Baron's medical microbiology (Baron S. et al. eds.) (4th ed.). Univ of Texas Medical Branch. ISBN 0-9631172-1-1. 
  3. Dyer B.D. and Obar R.A. 1994. Tracing the history of eukaryotic cells. Columbia N.Y.
  4. Margulis L & Dolan M.F. 2002. Early life: evolution on the Precambrian Earth. 2nd ed, Jones & Bartlett, Boston. p89
  5. Margulis L. Schwartz K.V. & Dolan M. 1999. Diversity of life: the illustrated guide to the five kingdoms. Jones & Bartlett, Boston, p94. In this work the authors propose 19 phyla for the Protista, and call this 'Kingdom' the 'Protoctista', a term which is unfortunately almost unpronounceable.
  6. Margulis L. and McMenamin 1990. Marriage of convenience. The Sciences 30, 31-36.
  7. Lake, James A. 2009. Evidence for an early prokaryotic endosymbiosis. Nature 460: p967.
  8. Burki F, Shalchian-Tabrizi K, Minge M, et al. (2007). [Expression error: Unexpected < operator "Phylogenomics reshuffles the eukaryotic supergroups"]. PLoS ONE 2 (8): e790. doi:10.1371/journal.pone.0000790. PMID 17726520. 
  9. Parfrey, Laura Wegener; Erika Barbero; Elyse Lasser; Micah Dunthorn; Debashish Bhattacharya; David J Patterson; and Laura A Katz (2006 December). [Expression error: Unexpected < operator "Evaluating support for the current classification of eukaryotic diversity"]. PLoS Genet. 2 (12): e220. doi:10.1371/journal.pgen.0020220. PMID 17194223. 
  10. Secondarily lost in some lines.
  11. Woese, Carl R.; Kandler, Otto; Wheelis, Mark L. 1990. "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the U.S.A. 87, 4576–9 [1] pmid = 2112744 doi:10.1073/pnas.87.12.4576



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