Genome size

Last updated • 13 min readFrom Wikipedia, The Free Encyclopedia
Genome size ranges (in base pairs) of various life forms Genome Sizes.png
Genome size ranges (in base pairs) of various life forms

Genome size is the total amount of DNA contained within one copy of a single complete genome. It is typically measured in terms of mass in picograms (trillionths or10−12 of a gram, abbreviated pg) or less frequently in daltons, or as the total number of nucleotide base pairs, usually in megabases (millions of base pairs, abbreviated Mb or Mbp). One picogram is equal to 978 megabases. [1] In diploid organisms, genome size is often used interchangeably with the term C-value.

Contents

An organism's complexity is not directly proportional to its genome size; total DNA content is widely variable between biological taxa. Some single-celled organisms have much more DNA than humans, for reasons that remain unclear (see Junk DNA and C-value).

Variation in genome size and gene content

Since the 1950s, with the emergence of various molecular techniques, the genome sizes of thousands of eukaryotes have been analyzed, and these data are available in online databases for animals, plants, and fungi (see external links). Nuclear genome size is typically measured in eukaryotes using either densitometric measurements of Feulgen-stained nuclei (previously using specialized densitometers, now more commonly using computerized image analysis [2] ) or flow cytometry. In prokaryotes, pulsed field gel electrophoresis and complete genome sequencing are the predominant methods of genome size determination.

Nuclear genome sizes are well known to vary enormously among eukaryotic species. In animals they range more than 3,300-fold, and in land plants they differ by a factor of about 1,000. [3] [4] Protist genomes have been reported to vary more than 300,000-fold in size, but the high end of this range ( Amoeba ) has been called into question.[ by whom? ]

Genome size in eukaryotes was thought to be proportional to the complexity of an organism but by the mid-20th century it became apparent that closely-related species could differ substantially in the size of their genomes. This counter-intuitive observation gave rise to what became known as the "C-value paradox."

By the end of 1960s this "paradox" was resolved by the discovery of repetitive DNA and the realization that much of the differences in genomes sizes was due to the presence or absence of large amounts of repetitive DNA. Further advances at that time indicated that most of the DNA in large genomes was non-functional junk DNA and only a small fraction corresponded to functional DNA, including the functional parts of genes.

Genome size correlates with a range of measurable characteristics at the cell and organism levels, including cell size, cell division rate, and, depending on the taxon, body size, metabolic rate, developmental rate, organ complexity, geographical distribution, or extinction risk. [3] [4] Based on currently available completely sequenced genome data (as of April 2009), log-transformed gene number forms a linear correlation with log-transformed genome size in bacteria, archaea, viruses, and organelles combined, whereas a nonlinear (semi-natural logarithm) correlation is seen for eukaryotes. [5] Although the latter contrasts with the previous view that no correlation exists for the eukaryotes, the observed nonlinear correlation for eukaryotes may reflect disproportionately fast-increasing junk DNA in increasingly large eukaryotic genomes. Although sequenced genome data are practically biased toward small genomes, which may compromise the accuracy of the empirically derived correlation, and ultimate proof of the correlation remains to be obtained by sequencing some of the largest eukaryotic genomes, current data do not seem to rule out a possible correlation.

Human genome size

Schematic karyogram of a human. It shows 22 homologous chromosomes, both the female (XX) and male (XY) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (to scale at bottom left). The blue scale to the left of each chromosome pair (and the mitochondrial genome) shows its length in terms of millions of DNA base pairs.
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Further information: Karyotype Human karyotype with bands and sub-bands.png
Schematic karyogram of a human. It shows 22 homologous chromosomes, both the female (XX) and male (XY) versions of the sex chromosome (bottom right), as well as the mitochondrial genome (to scale at bottom left). The blue scale to the left of each chromosome pair (and the mitochondrial genome) shows its length in terms of millions of DNA base pairs.

In humans, the total female diploid nuclear genome per cell extends for 6.37 Gigabase pairs (Gbp), is 208.23 cm long and weighs 6.51 picograms (pg). [6] Male values are 6.27 Gbp, 205.00 cm, 6.41 pg. [6] Each DNA polymer can contain hundreds of millions of nucleotides, such as in chromosome 1. Chromosome 1 is the largest human chromosome with approximately 220 million base pairs, and would be 85 mm long if straightened. [7]

In eukaryotes, in addition to nuclear DNA, there is also mitochondrial DNA (mtDNA) which encodes certain proteins used by the mitochondria. The mtDNA is usually relatively small in comparison to the nuclear DNA. For example, the human mitochondrial DNA forms closed circular molecules, each of which contains 16,569 [8] [9] DNA base pairs, [10] with each such molecule normally containing a full set of the mitochondrial genes. Each human mitochondrion contains, on average, approximately 5 such mtDNA molecules. [10] Each human cell contains approximately 100 mitochondria, giving a total number of mtDNA molecules per human cell of approximately 500. [10] However, the amount of mitochondria per cell also varies by cell type, and an egg cell can contain 100,000 mitochondria, corresponding to up to 1,500,000 copies of the mitochondrial genome (constituting up to 90% of the DNA of the cell). [11]

Genome reduction

Genome reduction, also known as genome degradation , is the process by which an organism's genome shrinks relative to that of its ancestors. Genomes fluctuate in size regularly, and genome size reduction is most significant in bacteria.

The most evolutionarily significant cases of genome reduction may be observed in the eukaryotic organelles known to be derived from bacteria: mitochondria and plastids. These organelles are descended from primordial endosymbionts, which were capable of surviving within the host cell and which the host cell likewise needed for survival. Many present-day mitochondria have less than 20 genes in their entire genome, whereas a modern free-living bacterium generally has at least 1,000 genes. Many genes have apparently been transferred to the host nucleus, while others have simply been lost and their function replaced by host processes.

Other bacteria have become endosymbionts or obligate intracellular pathogens and experienced extensive genome reduction as a result. This process seems to be dominated by genetic drift resulting from small population size, low recombination rates, and high mutation rates, as opposed to selection for smaller genomes.[ citation needed ] Some free-living marine bacterioplanktons also shows signs of genome reduction, which are hypothesized to be driven by natural selection. [12] [13] [14] In contrast, terrestrial prokaryotes appear to have larger genome sizes than both aquatic and host-associated prokaryotes (average of 3.7 Mbp for terrestrial, 3.1 Mbp for aquatic and 3.0 Mbp for host-associated). [15]

In obligate endosymbiotic species

Obligate endosymbiotic species are characterized by a complete inability to survive external to their host environment. These species have become a considerable threat to human health, as they are often capable of evading human immune systems and manipulating the host environment to acquire nutrients. A common explanation for these manipulative abilities is their consistently compact and efficient genomic structure. These small genomes are the result of massive losses of extraneous DNA, an occurrence that is exclusively associated with the loss of a free-living stage. As much as 90% of the genetic material can be lost when a species makes the evolutionary transition from a free-living to an obligate intracellular lifestyle. During this process the future parasite subjected to an environment rich of metabolite where somehow needs to hide within the host cell, those factors reduce the retention and increase the genetic drift leading to an acceleration of the loss of non-essential genes. [16] [17] [18] Common examples of species with reduced genomes include Buchnera aphidicola , Rickettsia prowazekii, and Mycobacterium leprae . One obligate endosymbiont of leafhoppers, Nasuia deltocephalinicola , has the smallest genome currently known among cellular organisms at 112 kb. [19] Despite the pathogenicity of most endosymbionts, some obligate intracellular species have positive fitness effects on their hosts.

The reductive evolution model has been proposed as an effort to define the genomic commonalities seen in all obligate endosymbionts. [20] This model illustrates four general features of reduced genomes and obligate intracellular species:

  1. "genome streamlining" resulting from relaxed selection on genes that are superfluous in the intracellular environment;
  2. a bias towards deletions (rather than insertions), which heavily affects genes that have been disrupted by accumulation of mutations (pseudogenes); [21]
  3. very little or no capability for acquiring new DNA; and
  4. considerable reduction of effective population size in endosymbiotic populations, particularly in species that rely on vertical transmission of genetic material.

Based on this model, it is clear that endosymbionts face different adaptive challenges than free-living species and, as emerged from the analysis between different parasites, their genes inventories are extremely different, leading us to the conclusion that the genome miniaturization follows a different pattern for the different symbionts. [22] [23] [24]

Conversion from picograms (pg) to base pairs (bp)

or simply:

[1]

Drake's rule

In 1991, John W. Drake proposed a general rule: that the mutation rate within a genome and its size are inversely correlated. [25] This rule has been found to be approximately correct for simple genomes such as those in DNA viruses and unicellular organisms. Its basis is unknown.

It has been proposed that the small size of RNA viruses is locked into a three-part relation between replication fidelity, genome size, and genetic complexity. The majority of RNA viruses lack an RNA proofreading facility, which limits their replication fidelity and hence their genome size. This has also been described as the Eigen paradox. [26] An exception to the rule of small genome sizes in RNA viruses is found in the Nidoviruses. These viruses appear to have acquired a 3′-to-5′ exoribonuclease (ExoN) which has allowed for an increase in genome size. [27]

Genome miniaturization and optimal size

In 1972 Michael David Bennett [28] hypothesized that there was a correlation with the DNA content and the nuclear volume while Commoner and van’t Hoff and Sparrow before him postulated that even cell size and cell-cycle length were controlled by the amount of DNA. [29] [30] More recent theories have brought us to discuss about the possibility of the presence of a mechanism that constrains physically the development of the genome to an optimal size. [31]

Those explanations have been disputed by Cavalier-Smith’s article [32]   where the author pointed that the way to understand the relation between genome size and cell volume was related to the skeletal theory. The nucleus of this theory is related to the cell volume, determined by an adaptation balance between advantages and disadvantages of bigger cell size, the optimization of the ratio nucleus:cytoplasm (karyoplasmatic ratio) [33] [34] and the concept that larger genomes provides are more prone to the accumulation of duplicative transposons as consequences of higher content of non-coding skeletal DNA. [32] Cavalier-Smith also proposed that, as consequent reaction of a cell reduction, the nucleus will be more prone to a selection in favor for the deletion compared to the duplication. [32]

From the economic way of thinking, since phosphorus and energy are scarce, a reduction in the DNA should be always the focus of the evolution, unless a benefit is acquired. The random deletion will be then mainly deleterious and not selected due to the reduction of the gained fitness but occasionally the elimination will be advantageous as well. This trade-off between economy and accumulation of non-coding DNA is the key to the maintenance of the karyoplasmatic ratio.

Mechanisms of genome miniaturization

The base question behind the process of genome miniaturization is whether it occurs through large steps or due to a constant erosion of the gene content. In order to assess the evolution of this process is necessary to compare an ancestral genome with the one where the shrinkage is supposed to be occurred. Thanks to the similarity among the gene content of Buchnera aphidicola and the enteric bacteria Escherichia coli, 89% identity for the 16S rDNA and 62% for orthologous genes was possible to shed light on the mechanism of genome miniaturization. [35] The genome of the endosymbiont B. aphidicola is characterized by a genome size that is seven times smaller than E. coli (643 kb compared to 4.6 Mb) [36] [37] and can be view as a subset of the enteric bacteria gene inventory. [37] From the confrontation of the two genomes emerged that some genes persist as partially degraded. [37] indicating that the function was lost during the process and that consequent events of erosion shortened the length as documented in Rickettsia. [38] [39] [40] This hypothesis is confirmed by the analysis of the pseudogenes of Buchnera where the number of deletions was more than ten times higher compared to the insertion. [40]

In Rickettsia prowazekii, as with other small genome bacteria, this mutualistic endosymbiont has experienced a vast reduction of functional activity with a major exception compared to other parasites  still retain the bio-synthetic ability of production of amino acid needed by its host. [41] [42] [37] The common effects of the genome shrinking between this endosymbiont and the other parasites are the reduction of the ability to produce phospholipids, repair and recombination and an overall conversion of the composition of the gene to a richer A-T [43] content due to mutation and substitutions. [16] [41] Evidence of the deletion of the function of repair and recombination is the loss of the gene recA, gene involved in the recombinase pathway. This event happened during the removal of a larger region containing ten genes for a total of almost 10 kb. [37] [41] Same faith occurred uvrA, uvrB and uvrC, genes encoding for excision enzymes involved in the repair of damaged DNA due to UV exposure. [35]

One of the most plausible mechanisms for the explanation of the genome shrinking is the chromosomal rearrangement because insertion/deletion of larger portion of sequence are more easily to be seen in during homologous recombination compared to the illegitimate, therefore the spread of the transposable elements will positively affect the rate of deletion. [32] The loss of those genes in the early stages of miniaturization not only this function but must played a role in the evolution of the consequent deletions. Evidences of the fact that larger event of removal occurred before smaller deletion emerged from the comparison of the genome of Bucknera and a reconstructed ancestor, where the gene that have been lost are in fact not randomly dispersed in the ancestor gene but aggregated and the negative relation between number of lost genes and length of the spacers. [35] The event of small local indels plays a marginal role on the genome reduction [44] especially in the early stages where a larger number of genes became superfluous. [45] [35]

Single events instead occurred due to the lack of selection pressure for the retention of genes especially if part of a pathway that lost its function during a previous deletion. An example for this is the deletion of recF, gene required for the function of recA, and its flanking genes. [46] One of the consequences of the elimination of such amount of sequences affected even the regulation of the remaining genes. The loss of large section of genomes could in fact lead to a loss in promotor sequences. This could in fact pushed the selection for the evolution of polycistronic regions with a positive effect for both size reduction [47] and transcription efficiency. [48]

Evidence of genome miniaturization

One example of the miniaturization of the genome occurred in the microsporidia, an anaerobic intracellular parasite of arthropods evolved from aerobic fungi.

During this process the mitosomes [49] was formed consequent to the reduction of the mitochondria to a relic voided of genomes and metabolic activity except to the production of iron sulfur centers and the capacity to enter into the host cells. [50] [51] Except for the ribosomes, miniaturized as well, many other organelles have been almost lost during the process of the formation of the smallest genome found in the eukaryotes. [32] From their possible ancestor, a zygomycotine fungi, the microsporidia shrunk its genome eliminating almost 1000 genes and reduced even the size of protein and protein-coding genes. [52] This extreme process was possible thanks to the advantageous selection for a smaller cell size imposed by the parasitism.

Another example of miniaturization is represented by the presence of nucleomorphs, enslaved nuclei, inside of the cell of two different algae, cryptophytes and chlorarachneans. [53]

Nucleomorphs are characterized by one of the smallest genomes known (551 and 380 kb) and as noticed for microsporidia, some genomes are noticeable reduced in length compared to other eukaryotes due to a virtual lack of non-coding DNA. [32] The most interesting factor is represented by the coexistence of those small nuclei inside of a cell that contains another nucleus that never experienced such genome reduction. Moreover, even if the host cells have different volumes from species to species and a consequent variability in genome size, the nucleomorph remain invariant denoting a double effect of selection within the same cell.

See also

Related Research Articles

<span class="mw-page-title-main">Endosymbiont</span> Organism that lives within the body or cells of another organism

An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria, which live in the root nodules of legumes, single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to insects.

<span class="mw-page-title-main">Genome</span> All genetic material of an organism

A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA. The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as regulatory sequences, and often a substantial fraction of junk DNA with no evident function. Almost all eukaryotes have mitochondria and a small mitochondrial genome. Algae and plants also contain chloroplasts with a chloroplast genome.

<span class="mw-page-title-main">Symbiogenesis</span> Evolutionary theory holding that eukaryotic organelles evolved through symbiosis with prokaryotes

Symbiogenesis is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells are descended from formerly free-living prokaryotes taken one inside the other in endosymbiosis. Mitochondria appear to be phylogenetically related to Rickettsiales bacteria, while chloroplasts are thought to be related to cyanobacteria.

<i>Rickettsia</i> Genus of bacteria

Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci, bacilli, or threads. The genus was named after Howard Taylor Ricketts in honor of his pioneering work on tick-borne spotted fever.

<span class="mw-page-title-main">Horizontal gene transfer</span> Transfer of genes from unrelated organisms

Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the evolution of many organisms. HGT is influencing scientific understanding of higher-order evolution while more significantly shifting perspectives on bacterial evolution.

<span class="mw-page-title-main">Nucleomorph</span>

Nucleomorphs are small, vestigial eukaryotic nuclei found between the inner and outer pairs of membranes in certain plastids. They are thought to be vestiges of red and green algal nuclei that were engulfed by a larger eukaryote. Because the nucleomorph lies between two sets of membranes, nucleomorphs support the endosymbiotic theory and are evidence that the plastids containing them are complex plastids. Having two sets of membranes indicate that the plastid, a prokaryote, was engulfed by a eukaryote, an alga, which was then engulfed by another eukaryote, the host cell, making the plastid an example of secondary endosymbiosis.

<span class="mw-page-title-main">Rickettsiales</span> Order of bacteria

The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans, and Ehrlichia, which causes diseases in livestock. Another genus of well-known Rickettsiales is the Wolbachia, which infect about two-thirds of all arthropods and nearly all filarial nematodes. Genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group.

<i>Buchnera aphidicola</i> Species of bacterium

Buchnera aphidicola, a member of the Pseudomonadota and the only species in the genus Buchnera, is the primary endosymbiont of aphids, and has been studied in the pea aphid, Acyrthosiphon pisum. Buchnera is believed to have had a free-living, Gram-negative ancestor similar to a modern Enterobacterales, such as Escherichia coli. Buchnera is 3 μm in diameter and has some of the key characteristics of its Enterobacterales relatives, such as a Gram-negative cell wall. However, unlike most other Gram-negative bacteria, Buchnera lacks the genes to produce lipopolysaccharides for its outer membrane. The long association with aphids and the limitation of crossover events due to strictly vertical transmission has seen the deletion of genes required for anaerobic respiration, the synthesis of amino sugars, fatty acids, phospholipids, and complex carbohydrates. This has resulted not only in one of the smallest known genomes of any living organism, but also one of the most genetically stable.

<span class="mw-page-title-main">Alphaproteobacteria</span> Class of bacteria

Alphaproteobacteria or α-proteobacteria, also called α-Purple bacteria in earlier literature, is a class of bacteria in the phylum Pseudomonadota. The Magnetococcales and Mariprofundales are considered basal or sister to the Alphaproteobacteria. The Alphaproteobacteria are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like all Proteobacteria, its members are gram-negative, although some of its intracellular parasitic members lack peptidoglycan and are consequently gram variable.

<i>Paulinella</i> Genus of single-celled organisms

Paulinella is a genus of at least eleven species including both freshwater and marine amoeboids. Like many members of euglyphids it is covered by rows of siliceous scales, and use filose pseudopods to crawl over the substrate of the benthic zone.

<span class="mw-page-title-main">Prokaryote</span> Unicellular organism lacking a membrane-bound nucleus

A prokaryote is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word prokaryote comes from the Ancient Greek πρό (pró), meaning 'before', and κάρυον (káruon), meaning 'nut' or 'kernel'. In the earlier two-empire system arising from the work of Édouard Chatton, prokaryotes were classified within the empire Prokaryota. However, in the three-domain system, based upon molecular phylogenetics, prokaryotes are divided into two domains: Bacteria and Archaea. A third domain, Eukaryota, consists of organisms with nuclei.

The CoRR hypothesis states that the location of genetic information in cytoplasmic organelles permits regulation of its expression by the reduction-oxidation ("redox") state of its gene products.

<i>Acyrthosiphon pisum</i> Species of true bug

Acyrthosiphon pisum, commonly known as the pea aphid, is a sap-sucking insect in the family Aphididae. It feeds on several species of legumes worldwide, including forage crops, such as pea, clover, alfalfa, and broad bean, and ranks among the aphid species of major agronomical importance. The pea aphid is a model organism for biological study whose genome has been sequenced and annotated.

<span class="mw-page-title-main">Genome evolution</span> Process by which a genome changes in structure or size over time

Genome evolution is the process by which a genome changes in structure (sequence) or size over time. The study of genome evolution involves multiple fields such as structural analysis of the genome, the study of genomic parasites, gene and ancient genome duplications, polyploidy, and comparative genomics. Genome evolution is a constantly changing and evolving field due to the steadily growing number of sequenced genomes, both prokaryotic and eukaryotic, available to the scientific community and the public at large.

Bacterial genomes are generally smaller and less variant in size among species when compared with genomes of eukaryotes. Bacterial genomes can range in size anywhere from about 130 kbp to over 14 Mbp. A study that included, but was not limited to, 478 bacterial genomes, concluded that as genome size increases, the number of genes increases at a disproportionately slower rate in eukaryotes than in non-eukaryotes. Thus, the proportion of non-coding DNA goes up with genome size more quickly in non-bacteria than in bacteria. This is consistent with the fact that most eukaryotic nuclear DNA is non-gene coding, while the majority of prokaryotic, viral, and organellar genes are coding. Right now, we have genome sequences from 50 different bacterial phyla and 11 different archaeal phyla. Second-generation sequencing has yielded many draft genomes ; third-generation sequencing might eventually yield a complete genome in a few hours. The genome sequences reveal much diversity in bacteria. Analysis of over 2000 Escherichia coli genomes reveals an E. coli core genome of about 3100 gene families and a total of about 89,000 different gene families. Genome sequences show that parasitic bacteria have 500–1200 genes, free-living bacteria have 1500–7500 genes, and archaea have 1500–2700 genes. A striking discovery by Cole et al. described massive amounts of gene decay when comparing Leprosy bacillus to ancestral bacteria. Studies have since shown that several bacteria have smaller genome sizes than their ancestors did. Over the years, researchers have proposed several theories to explain the general trend of bacterial genome decay and the relatively small size of bacterial genomes. Compelling evidence indicates that the apparent degradation of bacterial genomes is owed to a deletional bias.

<span class="mw-page-title-main">Minimal genome</span> Concept in genetics

The minimal genome is a concept which can be defined as the set of genes sufficient for life to exist and propagate under nutrient-rich and stress-free conditions. Alternatively, it may be defined as the gene set supporting life on an axenic cell culture in rich media, and it is thought what makes up the minimal genome will depend on the environmental conditions that the organism inhabits.

Hamiltonella defensa is a species of bacteria. It is maternally or sexually transmitted and lives as an endosymbiont of whiteflies and aphids, meaning that it lives within a host, protecting its host from attack. It does this through bypassing the host's immune responses by protecting its host against parasitoid wasps. However, H. defensa is only defensive if infected by a virus. H. defensa shows a relationship with Photorhabdus species, together with Regiella insecticola. Together with other endosymbionts, it provides aphids protection against parasitoids. It is known to habitate Bemisia tabaci.

Genomic streamlining is a theory in evolutionary biology and microbial ecology that suggests that there is a reproductive benefit to prokaryotes having a smaller genome size with less non-coding DNA and fewer non-essential genes. There is a lot of variation in prokaryotic genome size, with the smallest free-living cell's genome being roughly ten times smaller than the largest prokaryote. Two of the free-living bacterial taxa with the smallest genomes are Prochlorococcus and Pelagibacter ubique, both highly abundant marine bacteria commonly found in oligotrophic regions. Similar reduced genomes have been found in uncultured marine bacteria, suggesting that genomic streamlining is a common feature of bacterioplankton. This theory is typically used with reference to free-living organisms in oligotrophic environments.

<span class="mw-page-title-main">Plastid evolution</span> Evolution

A plastid is a membrane-bound organelle found in plants, algae and other eukaryotic organisms that contribute to the production of pigment molecules. Most plastids are photosynthetic, thus leading to color production and energy storage or production. There are many types of plastids in plants alone, but all plastids can be separated based on the number of times they have undergone endosymbiotic events. Currently there are three types of plastids; primary, secondary and tertiary. Endosymbiosis is reputed to have led to the evolution of eukaryotic organisms today, although the timeline is highly debated.

Reductive evolution is the process by which microorganisms remove genes from their genome. It can occur when bacteria found in a free-living state enter a restrictive state or are completely absorbed by another organism becoming intracellular (symbiogenesis). The bacteria will adapt to survive and thrive in the restrictive state by altering and reducing its genome to get rid of the newly redundant pathways that are provided by the host. In an endosymbiont or symbiogenesis relationship where both the guest and host benefit, the host can also undergo reductive evolution to eliminate pathways that are more efficiently provided for by the guest.

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