The following points highlight the top six types of hypothesis with respect to origin of eukaryotic cell. The hypothesis are: 1. Independent Hypothesis 2. Endogenous Theory (Filiation Theory) 3. Chimera Hypothesis 4. Endosymbiotic Theory 5. Serial Endosymbiotic Theory (Set) 6. Syntrophy Hypothesis.
Type # 1. Independent Hypothesis:
The unique nature of the eukaryotic nucleus with structurally complex chromosomes is thought to be derived independently from pre-prokaryotes, neither from archaebactria nor from eubacteria; however there is no concrete evidence in support.
Type # 2. Endogenous Theory (Filiation Theory):
Eukaryotic cells originated from ‘Proto-eukaryote’, a large anaerobic bacterium, that formed nucleus, mitochondria, chloroplasts by invagination of plasma-membrane and enclosed genetic material inside double membrane.
Type # 3. Chimera Hypothesis:
According to this concept, eukaryotic cells originated as chimera of two or more prokaryotic cells.
Though there is no intermediate organisms between prokaryotes and eukaryotes, eukaryotes are more closely related to archaebacteria in certain respects, particularly to thermophilic archaebacteria of hot spring which do not possess cell wall, looking like amoeba, with cytoskeleton-like structure, having sulphur compound based energy metabolism, Fe3+ or Mn4+ acting as respiratory oxidants, with aerobic respiration.
One suggestion may be that eukaryotes originated as a chimera between an archaebacterium and a eubacterium (Fig. 2.21).
Of the different chimera hypotheses, fusion model and engulfment model are mechanistically problematic (Fig. 2.22). By contrast, symbiotic model relies on intimate relationships over extended periods of time that allowed symbionts to co-evolve and become dependent on each other.
Type # 4. Endosymbiotic Theory:
The more well documented and generally accepted theory for the origin of eukaryotic organelles is endosymbiotic theory. Recent evidences justify that organelles have originated from the endosymbiotic association of ingested aerobic and photosynthetic prokaryotes, the precursors of mitochondria and chloroplast respectively.
Molecular data have played an important role in supporting xenogenous origin (from outside of cell) rather than autogenous origin (from within the cell) of organelles. Recent phylogenetic analyses reveal that many eukaryotic organellar and nuclear genes whose prokaryotic ancestry can be pinned down are of bacterial origin.
Phylogenetic analyses reveal that many eukaryotic orgnallear and nuclear genes whose prokaryotic ancestry can be pinned down are of bacterial origin. In the case of endosymbiosis one type of cell (symbiont) entered into another type of cell (host) through phagocytosis.
The ingested cell under some circumstances could survive and reproduce within cytoplasm of the host cell. The relationship is stabilized by their mutual benefits of metabolic symbiosis and becomes obligatory.
Horizontal gene transfer from symbiont to host genome causes the loss of corresponding protein synthesizing ability of the symbiont and is likely to be selectively favoured. The development from symbiont to organelle is completed by the loss of its independent survival ability.
This idea is based on the fact that organelles like mitochondria and chloroplasts:
(i) Are replicators, i.e., can divide independently.
(ii) Carry genetic information, i.e., DNA.
(iii) With protein synthesizing machinery, i.e., ability of transcription and translation.
(iv) Have own ribosomes of prokaryotic type, i.e., 70S type.
The evidences supporting bacterial origin of mitochondria and chloroplasts are convincing.
a. Mitochondria and Chloroplasts contain their own DNA
(i) DNA simple, closed circular supercoiled dsDNA with single origin point.
(ii) DNA controls the synthesis of their rRNA 2 and tRNA, ribosomal proteins and certain proteins of respiratory chain (mitochondria) and similar genes for PSI, PSIl, cytochrome of complex, ATP synthase and ribulose bisphosphate carboxylase of choroplastids.
b. They contain their own ribosomes:
(i) 30S + 50S = 70S
{16S rRNA + 21 PP}{5S & 23S rRNA + 34 PP}
(ii) Shine-Dalgarno sequence on 16S rRNA.
c. Antibiotic specificity:
Ribosomes are sensitive to chloramphenicol (SOS), streptomycin and tetracycline (30S) like bacteria but eukaryotic ribosomes are insensitive to these antibiotics.
d. Molecular phylogeny:
16S rRNA and tRNA sequencing have shown that chloroplasts and mitochondria are evolutionarily related to bacteria.
Type # 5. Serial Endosymbiotic Theory (Set):
Serial Endosymbiotic Theory, supported by Taylor 1974, Gray 1983, Doolittle and Daniels 1988, Margulis 1995, proposes the following steps of evolutionary origin of eukaryotic cell (Fig. 2.23).
SE I (Origin of Flagella):
A thermo acidophil, fermenting, Gram(-ve) bacterium merged with Spirochaete through phagocytosis to develop so- called undulipodium flagellated cells.
SE II (Origin of Nucleus):
The resulting pre- eukaryote went through secondary endosymbiosis by engulfing archaebacterium with membranous folds. The archaebacterium becomes nucleus, losing cell membrane, while the membranous folds develop nuclear envelope and endoplasmic reticulum. The genome of bacterium is transferred to the nucleus through membrane pores. Classical example of such eukaryote is Giardia lamblia.
SE III (Origin of Mitochondria):
Mitochondria is surrounded by a double membrane representing outer and inner membrane of bacteria. The inner membrane is invaginated forming tubular or discoid cristae. The biochemistry of energy metabolism in mitochondria is very much similar to that of purple non-sulphur bacteria.
The theory implies that the aerobic bacterium established itself as a symbiont within an anaerobic fermenting proto-eukaryote and lost the ability of photosynthesis and become mitochondrion (Fig. 2.24). Strombidium purpureum is an example, where mitochondrial rRNA sequence shows analogy to eubacterial rRNA.
The serial endosymbiotic theory postulated that the capture of an proteobacterial endosymbiont by a nucleus containing eukaryotic host resembling extant amitochondriate protists, results in the origin of mitochondria.
Giardiai like anaerobic primitive eukaryotes by engulfment of an aerobic Gram(-ve) eubacterium like Paracoccus denitrificans resulted protista (unicellular eukaryote) with mitochondria; classical example is Pelomyxa palustris.
SE IV (Origin of Chloroplast):
Chloroplasts in mitochondria containing eukaryotic cell evolved by association of photosynthetic endosymbionts like photosynthetic bacteria or cyanobacteria (Mereschowsky). Plastid genes are strikingly similar to cyanobacteria in sequence organization and mode ‘of expression. Phylogenetic analysis of rRNA and tufA sequences indicates cyanobacterial origin of all plastids.
A well-studied example of endosymbiotic cyanobacteria (cyanelles) is Cyanophora paradoxum. In cryptomonad flagellates and dinoflagellates chloroplasts represent a second generation endosymbiont. This type of secondary/tertiary endosymbiosis (Fig. 2.25) results in several sets of membranes around the chloroplast in which the outermost membrane represents the cell membrane of the latest endosymbiont.
Origin of Peroxisomes:
Peroxisomes may have been formed through endocytosis of prokaryotes with detoxifying capabilities.
Origin of CERL system:
Lysosomes are developed from invaginated vesicles with enzymes. Further extension of invaginations into the cytoplasm formed tubular network to form Golgi bodies and endoplasmic reticulum.
Type # 6. Syntrophy Hypothesis:
A novel symbiotic hypothesis states that eukaryotic cells arose through metabolic symbiosis or syntrophy between eubacteria and methanogenic archaea.
The hydrogen hypothesis holds that eukaryotic cells originated through a symbiotic metabolic association in anaerobic environments between a fermentive α-proteobacterium that generated hydrogen and CO2 as waste products, and a strict anaerobic autotrophic archaeon that depended on hydrogen and might have been a methanogen.
The syntrophy hypothesis as proposed by Moreira and Lopez-Garcia (1998) is also based on symbiosis mediated by interspecies hydrogen transfer but the organisms involved were δ pro-teobacteria (ancestral sulphate-reducing myxo-bacteria) and a methanogenic archaea (Fig. 2.26 and 2.27).
Margulis (2000) proposed the origin of eukaryotic nucleus via symbiogenesis by syntrophic merger between a thermoacidophil archaebacterium and heterotrophic swimmer eubacterium under selective pressure of oxygen avoidance and speed swimming; the former generated hydrogen sulfide to protect the later, the chimera emerged was an amitocondriate protists with nucleus as a component of the karyomastigont.
Eukaryotic nucleus with introns and spliceosomes, originated through mitochondrial endosymbiont, created a strong selective pressure to exclude ribosomes from the vicinity of chromosomes and forcing nucleus-cytosol compartmentalization — thus breaking the prokaryotic paradigm of co-transcriptional translation — allowing the proper maturation of mRNA.