Horizontal gene transfer - AbsoluteAstronomy.com

Horizontal gene transfer (HGT), also lateral gene transfer (LGT), is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism. By contrast, vertical transfer occurs when an organism receives genetic material from its ancestor, e.g., its parent or a species from which it has evolved.

Most thinking in genetics

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

has focused upon vertical transfer, but there is a growing awareness that horizontal gene transfer is a highly significant phenomenon and amongst single-celled organisms perhaps the dominant form of genetic transfer. Artificial horizontal gene transfer is a form of genetic engineering

Genetic engineering

Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...

History

Horizontal gene transfer was first described in Seattle in 1951 in a publication which demonstrated that the transfer of a viral gene into Corynebacterium diphtheria created a virulent from a non-virulent strain, also simultaneously solving the riddle of diphtheria (that patients could be infected with the bacteria but not have any symptoms, and then suddenly convert later or never), and giving the first example for the relevance of the lysogenic cycle. Inter-bacterial gene transfer was first described in Japan in a 1959 publication that demonstrated the transfer of antibiotic resistance between different species of bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. In the mid-1980s, Syvanen predicted that lateral gene transfer existed, had biological significance, and was involved in shaping evolutionary history from the beginning of life on Earth.

As Jain, Rivera and Lake (1999) put it: "Increasingly, studies of genes and genomes are indicating that considerable horizontal transfer has occurred between prokaryote

Prokaryote

The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

s." (see also Lake and Rivera, 2007). The phenomenon appears to have had some significance for unicellular eukaryote

Eukaryote

A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s as well. As Bapteste et al. (2005) observe, "additional evidence suggests that gene transfer might also be an important evolutionary mechanism in protist

Protist

Protists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...

evolution."

There is some evidence that even higher plants and animals have been affected and this has raised concerns for safety. However, Richardson and Palmer (2007) state: "Horizontal gene transfer (HGT) has played a major role in bacterial evolution and is fairly common in certain unicellular eukaryotes. However, the prevalence and importance of HGT in the evolution of multicellular eukaryotes remain unclear."

Due to the increasing amount of evidence suggesting the importance of these phenomena for evolution (see below) molecular biologists such as Peter Gogarten have described horizontal gene transfer as "A New Paradigm for Biology".

It should also be noted that the process may be a hidden hazard of genetic engineering as it may allow dangerous transgenic

Genetically modified organism

A genetically modified organism or genetically engineered organism is an organism whose genetic material has been altered using genetic engineering techniques. These techniques, generally known as recombinant DNA technology, use DNA molecules from different sources, which are combined into one...

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

to spread from species to species.

Mechanism

There are several mechanisms for horizontal gene transfer:

Transformation
Transformation (genetics)
In molecular biology transformation is the genetic alteration of a cell resulting from the direct uptake, incorporation and expression of exogenous genetic material from its surroundings and taken up through the cell membrane. Transformation occurs naturally in some species of bacteria, but it can...

, the genetic alteration of a cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

resulting from the introduction, uptake and expression of foreign genetic material (DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

or RNA
RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

). This process is relatively common in bacteria, but less so in eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s. Transformation is often used in laboratories to insert novel genes into bacteria for experiments or for industrial or medical applications. See also molecular biology
Molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

and biotechnology
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...

.
Transduction
Transduction (genetics)
Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. Transduction does not require cell-to-cell contact , and it is DNAase resistant...

, the process in which bacterial DNA is moved from one bacterium to another by a virus (a bacteriophage, or phage).
Bacterial conjugation
Bacterial conjugation
Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...

, a process in which a bacterial cell transfers genetic material to another cell by cell-to-cell contact.
Gene transfer agent
Gene transfer agent
A gene transfer agent or "GTA" is a virus-like element that contains random pieces of the host chromosome and are found in most members of the alphaproteobacteria order Rhodobacterales. They are encoded by the host genome...

s, viruslike elements encoded by the host that are found in the alphaproteobacteria
Alphaproteobacteria
Alphaproteobacteria is a class of Proteobacteria. Like all Proteobacteria, they are Gram-negative.-Characteristics:The Alphaproteobacteria comprise most phototrophic genera, but also several genera metabolising C1-compounds , symbionts of plants and animals, and a group of pathogens, the...

order Rhodobacterales
Rhodobacterales
In taxonomy, the Rhodobacterales are an order of the Alphaproteobacteria.Gene transfer agents are viruslike elements produced by Rhodobacterales which transfer DNA and may be an important factor in their evolution.-External links:...

.

Viruses

The virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

called Mimivirus
Mimivirus
Mimivirus is a viral genus containing a single identified species named Acanthamoeba polyphaga mimivirus , or is a group of phylogenetically related large viruses . In colloquial speech, APMV is more commonly referred to as just “mimivirus”...

infects amoebae. Another virus, called Sputnik, also infects amoebae, but it cannot reproduce unless mimivirus has already infected the same cell. "Sputnik’s genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

reveals further insight into its biology. Although 13 of its gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s show little similarity to any other known genes, three are closely related to mimivirus and mamavirus

Mamavirus

Mamavirus is one of the most complex viruses discovered. It is a virus belonging to the family mimiviridae, which includes mimivirus, mamavirus, and marseillevirus The mimiviridae also belong to the family of the nucleocytoplasmic large DNA viruses , which includes the Poxviridae, the...

genes, perhaps cannibalized by the tiny virus as it packaged up particles sometime in its history. This suggests that the satellite virus could perform horizontal gene transfer between viruses, paralleling the way that bacteriophage

Bacteriophage

A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...

s ferry genes between bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

Prokaryotes

Horizontal gene transfer is common among bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, even amongst very distantly-related ones. This process is thought to be a significant cause of increased drug resistance

Drug resistance

Drug resistance is the reduction in effectiveness of a drug such as an antimicrobial or an antineoplastic in curing a disease or condition. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance. More commonly, the term is used...

when one bacterial cell acquires resistance and quickly transfers the resistance genes to many species.

Eukaryotes

"Sequence comparisons suggest recent horizontal transfer of many gene

Gene

s among diverse species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes."

Analysis of DNA sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

s suggests that horizontal gene transfer has also occurred within eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s from the chloroplast and mitochondrial genomes to the nuclear genome. As stated in the endosymbiotic theory
Endosymbiotic theory
The endosymbiotic theory concerns the mitochondria, plastids , and possibly other organelles of eukaryotic cells. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts...

, chloroplast
Chloroplast
Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s and mitochondria probably originated as bacterial endosymbiont
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...

s of a progenitor to the eukaryotic cell.
Horizontal transfer of genes from bacteria to some fungi, especially the yeast Saccharomyces cerevisiae
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...

, has been well documented.
There is also recent evidence that the azuki bean beetle has somehow acquired genetic material from its (non-beneficial) endosymbiont Wolbachia
Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...

. New examples have recently been reported demonstrating that Wolbachia bacteria represent an important potential source of genetic material in arthropods and filarial nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s.
There is also evidence for horizontal transfer of mitochondrial genes to parasites of the Rafflesiaceae
Rafflesiaceae
Rafflesiaceae is a family of parasitic plants found in east and southeast Asia, including Rafflesia arnoldii, the plant with the largest flower of all plants. The plants are endoparasites of vines in the genus Tetrastigma and lack stems, leaves, roots, and any photosynthetic tissue...

plant family from their hosts (also plants), from chloroplasts of a not-yet-identified plant to the mitochondria of the bean Phaseolus
Phaseolus
Phaseolus is a genus in the family Fabaceae of about fifty plant species, all native to the Americas.At least four of the species have been domesticated since pre-Columbian times for their beans. Most prominent among these is the common bean, P...

, and from a heterokont
Heterokont
The heterokonts or stramenopiles are a major line of eukaryotes currently containing more than 100,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton...

alga to its predator, the sea slug Elysia chlorotica
Elysia chlorotica
Elysia chlorotica, common name the eastern emerald elysia, is a small-to-medium-sized species of green sea slug, a marine opisthobranch gastropod mollusc. This sea slug superficially resembles a nudibranch, yet it does not belong to that suborder of gastropods. Instead it is a member of the...

.
Striga hermonthica
Striga hermonthica
Striga hermonthica, commonly known as purple witchweed, is a parasitic plant belonging to the genus Striga. It is devastating to "major crops such as sorghum and rice . In subsaharan Africa in addition to S. bicolor, and O...

, a eudicot, has undergone a horizontal gene transfer from Sorghum
Sorghum
Sorghum is a genus of numerous species of grasses, one of which is raised for grain and many of which are used as fodder plants either cultivated or as part of pasture. The plants are cultivated in warmer climates worldwide. Species are native to tropical and subtropical regions of all continents...

(Sorghum bicolor) to its nuclear genome. The gene is of unknown functionality.
Researchers at the University of Arizona
University of Arizona
The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...

have found that the genome of pea aphid
Pea aphid
The pea aphid, Acyrthosiphon pisum is a sap-sucking insect in the Aphididae family. It feeds on severals 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...

s (Acyrthosiphon pisum) contains multiple genes that were horizontally transferred from fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

. Plants, fungi, and microorganisms can synthesize carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

s, but torulene made by pea aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...

s is the only carotenoid known to be synthesized by an organism in the animal kingdom.
It was recently suggested that the malaria
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

causing pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

Plasmodium vivax
Plasmodium vivax
Plasmodium vivax is a protozoal parasite and a human pathogen. The most frequent and widely distributed cause of recurring malaria, P. vivax is one of the four species of malarial parasite that commonly infect humans. It is less virulent than Plasmodium falciparum, which is the deadliest of the...

has horizontally acquired from humans genetic material that might help facilitate its long stay in the body.

Artificial horizontal gene transfer

Genetic engineering

is essentially horizontal gene transfer, albeit with synthetic expression cassettes. The Sleeping Beauty transposon system

Sleeping Beauty transposon system

The Sleeping Beauty transposon system is a synthetic DNA transposon that was constructed to introduce precisely defined DNA sequences into the chromosomes of vertebrate animals for the purposes of introducing new traits and to discover new genes and their functions.-Mechanism of Action:The Sleeping...

(SB) was developed as a synthetic gene transfer agent that was based on the known abilities of Tc1/mariner transposons to invade genomes of extremely diverse species. The SB system has been used to introduce genetic sequences into a wide variety of animal genomes.

Importance in evolution

Horizontal gene transfer in evolution

The fact that genes can move between distant branches of the tree of life even at low probabilities raises challenges to scientists who are trying to reconstruct evolution from studying genes and gene sequences in different organisms, because horizontal gene transfer effectively scrambles the...

Horizontal gene transfer is a potential confounding factor

Lurking variable

In statistics, a confounding variable is an extraneous variable in a statistical model that correlates with both the dependent variable and the independent variable...

in inferring phylogenetic tree

Phylogenetic tree

A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...

s based on the sequence

Sequence

In mathematics, a sequence is an ordered list of objects . Like a set, it contains members , and the number of terms is called the length of the sequence. Unlike a set, order matters, and exactly the same elements can appear multiple times at different positions in the sequence...

of one gene

Gene

. For example, given two distantly related bacteria that have exchanged a gene a phylogenetic tree

Phylogenetic tree

including those species will show them to be closely related because that gene is the same even though most other genes are dissimilar. For this reason it is often ideal to use other information to infer robust phylogenies such as the presence or absence of genes or, more commonly, to include as wide a range of genes for phylogenetic analysis as possible.

For example, the most common gene to be used for constructing phylogenetic relationships in prokaryote

Prokaryote

s is the 16s rRNA gene since its sequences tend to be conserved among members with close phylogenetic distances, but variable enough that differences can be measured. However, in recent years it has also been argued that 16s rRNA genes can also be horizontally transferred. Although this may be infrequent the validity of 16s rRNA-constructed phylogenetic trees must be reevaluated.

Biologist Johann Peter Gogarten

Johann Peter Gogarten

Johann Peter Gogarten was born in Bad Oeynhausen, Germany. He is a German-American biologist studying the early evolution of life. He studied plant physiology and membrane transport with Friedrich-Wilhelm Bentrup in Tübingen and Giessen. In 1987 he came to the US as a postdoc to work with Lincoln...

suggests "the original metaphor of a tree no longer fits the data from recent genome research" therefore "biologists should use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes." There exist several methods to infer such phylogenetic network

Phylogenetic network

A phylogenetic network is any graph used to visualize evolutionary relationships between nucleotide sequences, genes, chromosomes, genomes, or species . They are employed when reticulate events such as hybridization, horizontal gene transfer, recombination, or gene duplication and loss are...

s.

Using single gene

Gene

s as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of horizontal gene transfer. Combining the simple coalescence

Coalescence

Coalescence may refer to:* Coalescence , the merging of genetic lineages backwards time to a most recent common ancestor* Coalescence , the merging of two or more phonological segments into one...

model of cladogenesis

Cladogenesis

Cladogenesis is an evolutionary splitting event in a species in which each branch and its smaller branches forms a "clade", an evolutionary mechanism and a process of adaptive evolution that leads to the development of a greater variety of sister species...

with rare HGT horizontal gene transfer events suggest there was no single most recent common ancestor

Most recent common ancestor

In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...

that contained all of the genes ancestral to those shared among the three domains of life

Life

Life is a characteristic that distinguishes objects that have signaling and self-sustaining processes from those that do not, either because such functions have ceased , or else because they lack such functions and are classified as inanimate...

. Each contemporary molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

has its own history and traces back to an individual molecule cenancestor. However, these molecular ancestors were likely to be present in different organisms at different times."

Scientific American article (2000)

Uprooting the Tree of Life by W. Ford Doolittle

Ford Doolittle

Dr. W. Ford Doolittle is a biochemist., he is a professor at Dalhousie University in Halifax, Nova Scotia. He received his BA in Biochemical Sciences from Harvard University in 1963 and his PhD from Stanford University in 1967.Since joining the biochemistry department at Dalhousie in 1971, Dr...

(Scientific American
Scientific American
Scientific American is a popular science magazine. It is notable for its long history of presenting science monthly to an educated but not necessarily scientific public, through its careful attention to the clarity of its text as well as the quality of its specially commissioned color graphics...

, February 2000, pp 90–95) contains a discussion of the Last Universal Common Ancestor and the problems that arose with respect to that concept when one considers horizontal gene transfer. The article covers a wide area — the endosymbiont

Endosymbiont

An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...

hypothesis for eukaryote

Eukaryote

s, the use of small subunit ribosomal RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

(SSU rRNA) as a measure of evolutionary distances (this was the field Carl Woese

Carl Woese

Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...

worked in when formulating the first modern "tree of life

Tree of life (science)

Charles Darwin proposed that phylogeny, the evolutionary relatedness among species through time, was expressible as a metaphor he termed the Tree of Life...

", and his research results with SSU rRNA led him to propose the Archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

as a third domain of life

Life

) and other relevant topics. Indeed, it was while examining the new three-domain view of life that horizontal gene transfer arose as a complicating issue: Archaeoglobus fulgidus is cited in the article (p. 76) as being an anomaly with respect to a phylogenetic tree based upon the encoding for the enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

HMGCoA reductase — the organism in question is a definite Archaean, with all the cell lipids and transcription machinery that are expected of an Archaean, but whose HMGCoA genes are actually of bacterial origin.

Again on p. 76, the article continues with:

"The weight of evidence still supports the likelihood that mitochondria in eukaryote

Eukaryote

s derived from alpha-proteobacterial cells and that chloroplast

Chloroplast

Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis.Chloroplasts are green...

s came from ingested cyanobacteria, but it is no longer safe to assume that those were the only lateral gene transfers that occurred after the first eukaryotes arose. Only in later, multicellular eukaryotes do we know of definite restrictions on horizontal gene exchange, such as the advent of separated (and protected) germ cell

Germ cell

A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces sexually. In many animals, the germ cells originate near the gut of an embryo and migrate to the developing gonads. There, they undergo cell division of two types, mitosis and meiosis, followed by...

s."

The article continues with:

"If there had never been any lateral gene transfer, all these individual gene trees would have the same topology (the same branching order), and the ancestral genes at the root of each tree would have all been present in the last universal common ancestor, a single ancient cell. But extensive transfer means that neither is the case: gene trees will differ (although many will have regions of similar topology) and there would never have been a single cell that could be called the last universal common ancestor.

"As Woese has written, 'the ancestor cannot have been a particular organism, a single organismal lineage. It was communal, a loosely knit, diverse conglomeration of primitive cells that evolved as a unit, and it eventually developed to a stage where it broke into several distinct communities, which in their turn became the three primary lines of descent (bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

and eukaryote

Eukaryote

s)' In other words, early cells, each having relatively few genes, differed in many ways. By swapping gene

Gene

s freely, they shared various of their talents with their contemporaries. Eventually this collection of eclectic and changeable cells coalesced into the three basic domains known today. These domains become recognisable because much (though by no means all) of the gene transfer that occurs these days goes on within domains."

With regard to how horizontal gene transfer affects evolutionary theory (common descent, universal phylogenetic tree) Carl Woese says:

"What elevated common descent to doctrinal status almost certainly was the much later discovery of the universality of biochemistry, which was seemingly impossible to explain otherwise. But that was before horizontal gene transfer (HGT), which could offer an alternative explanation for the universality of biochemistry, was recognized as a major part of the evolutionary dynamic. In questioning the doctrine of common descent, one necessarily questions the universal phylogenetic tree. That compelling tree image resides deep in our representation of biology. But the tree is no more than a graphical device; it is not some a priori form that nature imposes upon the evolutionary process. It is not a matter of whether your data are consistent with a tree, but whether tree topology is a useful way to represent your data. Ordinarily it is, of course, but the universal tree is no ordinary tree, and its root no ordinary root. Under conditions of extreme HGT, there is no (organismal) "tree." Evolution is basically reticulate."

However, in a May 2010 article in Nature, Douglas Theobald argued that there was indeed one Last Universal Common Ancestor to all existing life and that horizontal gene transfer has not destroyed our ability to infer this.

Genes

There is evidence for historical horizontal transfer of the following genes:

Lycopene
Lycopene
Lycopene is a bright red carotene and carotenoid pigment and phytochemical found in tomatoes and other red fruits and vegetables, such as red carrots, watermelons and papayas...

cyclase
Cyclase
A cyclase is an enzyme, almost always a lyase, that catalyzes a chemical reaction to form a cyclic compound. Important cyclase enzymes include:* Adenylyl cyclase, which forms cyclic AMP from adenosine triphosphate...

for carotenoid
Carotenoid
Carotenoids are tetraterpenoid organic pigments that are naturally occurring in the chloroplasts and chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some types of fungus. Carotenoids can be synthesized fats and other basic organic metabolic building...

biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...

, between Chlorobi and Cyanobacteria.
TetO gen conferring resistance to tetracycline, between Campylobacter jejuni.

History

Mechanism

Viruses

Prokaryotes

Eukaryotes

Artificial horizontal gene transfer

Importance in evolution

Scientific American article (2000)

Genes

See also

Further reading