Gene flow
Encyclopedia
In population genetics
, gene flow (also known as gene migration) is the transfer of alleles of gene
s from one population
to another.
Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of members carrying a particular variant of a gene). Immigration may also result in the addition of new genetic variants to the established gene pool
of a particular species or population.
There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind.
Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic variation between the two groups. It is for this reason that gene flow strongly acts against speciation
, by recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species.
For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side to the other and vice versa. If this pollen is able to fertilize the plant where it ends up and produce viable offspring, then the alleles in the pollen have effectively been able to move from the population on one side of the highway to the other.
, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba, Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China
where Ulmus pumila grows. This is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools.
Barriers to gene flow need not always be physical. Species can live in the same environment, yet show very limited gene flow due to limited hybridization or hybridization yielding unfit hybrids.
, gene flow was observed between a white European population and a black West African population, which were recently brought together. In West Africa, where malaria
is prevalent, the Duffy antigen
provides some resistance to the disease, and this allele is thus present in nearly all of the West African population. In contrast, Europeans have either the allele Fya or Fyb, because malaria is almost non-existent. By measuring the frequencies of the West African and European groups, scientists found that the allele frequencies became mixed in each population because of movement of individuals. It was also found that this gene flow between European and West African groups is much greater in the Northern U.S. than in the South.
Gene transfer, defined as the movement of genetic material across species boundaries, which includes horizontal gene transfer
, antigenic shift
, and reassortment
is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can incorporate genes from other dead bacteria, exchange genes with living bacteria, and can exchange plasmid
s across species boundaries.
"Sequence comparisons suggest recent horizontal transfer of many gene
s among diverse species
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."
Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists [should] instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.
"Using single gene
s as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT [horizontal gene transfer]. Combining the simple coalescence
model of cladogenesis
with rare HGT [horizontal gene transfer] events suggest there was no single last common ancestor that contained all of the genes ancestral to those shared among the three domains of life
. Each contemporary molecule
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."
can be threatened with extinction through the process of genetic pollution
, potentially causing uncontrolled hybridization, introgression
and genetic swamping. These processes can lead to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness
advantage of introduced plant or animal. Nonnative species can bring about a form of extinction of native plants and animals by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that drive the originally purebred native stock to complete extinction. The extent of this facet of gene flow is not always apparent from morphological
(outward appearance) observations alone. Some degree of gene flow may be due to normal, evolutionarily constructive processes, and all constellations of gene
s and genotypes cannot be preserved. That being said, hybridization with or without introgression may threaten a rare species' existence nonetheless.
, e.g. Sewall Wright
's neighborhood model, Wright's island model and the stepping stone model.
(GM) plants or livestock
, it becomes necessary to prevent "genetic pollution
" i.e. their genetic modification from reaching other conventionally hybridized or wild native plant and animal populations by using gene flow mitigation usually through unintentional cross pollination and crossbreeding. Reasons to limit gene flow may include biosafety
or agricultural co-existence
, in which GM and non-GM cropping systems work side by side.
Scientists in several large research programmes are investigating methods of limiting gene flow in plants. Among these programmes are Transcontainer
, which investigates methods for biocontainment
, SIGMEA, which focuses on the biosafety of genetically modified plants, and Co-Extra
, which studies the co-existence of GM and non-GM product chains.
Generally, there are three approaches to gene flow mitigation: keeping the genetic modification out of the pollen, preventing the formation of pollen, and keeping the pollen inside the flower.
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
, gene flow (also known as gene migration) is the transfer of alleles of 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 from one population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
to another.
Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of members carrying a particular variant of a gene). Immigration may also result in the addition of new genetic variants to the established gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...
of a particular species or population.
There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind.
Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic variation between the two groups. It is for this reason that gene flow strongly acts against speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
, by recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species.
For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side to the other and vice versa. If this pollen is able to fertilize the plant where it ends up and produce viable offspring, then the alleles in the pollen have effectively been able to move from the population on one side of the highway to the other.
Barriers to gene flow
Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of ChinaGreat Wall of China
The Great Wall of China is a series of stone and earthen fortifications in northern China, built originally to protect the northern borders of the Chinese Empire against intrusions by various nomadic groups...
, which has hindered the gene flow of native plant populations. One of these native plants, Ulmus pumila, demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba, Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of China
Great Wall of China
The Great Wall of China is a series of stone and earthen fortifications in northern China, built originally to protect the northern borders of the Chinese Empire against intrusions by various nomadic groups...
where Ulmus pumila grows. This is because Ulmus pumila has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects. Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools.
Barriers to gene flow need not always be physical. Species can live in the same environment, yet show very limited gene flow due to limited hybridization or hybridization yielding unfit hybrids.
Gene flow in humans
Gene flow has been observed in humans. For example, in the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, gene flow was observed between a white European population and a black West African population, which were recently brought together. In West Africa, where 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...
is prevalent, the Duffy antigen
Duffy antigen
Duffy antigen/chemokine receptor also known as Fy glycoprotein or CD234 is a protein that in humans is encoded by the DARC gene....
provides some resistance to the disease, and this allele is thus present in nearly all of the West African population. In contrast, Europeans have either the allele Fya or Fyb, because malaria is almost non-existent. By measuring the frequencies of the West African and European groups, scientists found that the allele frequencies became mixed in each population because of movement of individuals. It was also found that this gene flow between European and West African groups is much greater in the Northern U.S. than in the South.
Gene flow between species
Gene flow can occur between species, either through hybridization or gene transfer from bacteria or virus to new hosts.Gene transfer, defined as the movement of genetic material across species boundaries, which includes horizontal gene transfer
Horizontal gene transfer
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
, antigenic shift
Antigenic shift
Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains...
, and reassortment
Reassortment
Reassortment is the mixing of the genetic material of a species into new combinations in different individuals. Several different processes contribute to reassortment, including assortment of chromosomes, and chromosomal crossover. It is particularly used when two similar viruses that are infecting...
is sometimes an important source of genetic variation. Viruses can transfer genes between species. Bacteria can incorporate genes from other dead bacteria, exchange genes with living bacteria, and can exchange plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
s across species boundaries.
"Sequence comparisons suggest recent horizontal transfer of many 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 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."
Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research". Biologists [should] instead use the metaphor of a mosaic to describe the different histories combined in individual genomes and use the metaphor of an intertwined net to visualize the rich exchange and cooperative effects of horizontal gene transfer.
"Using single 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 as phylogenetic markers, it is difficult to trace organismal phylogeny in the presence of HGT [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 last common ancestor 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."
Genetic pollution
Purebred, naturally-evolved, region-specific, wild speciesSpecies
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...
can be threatened with extinction through the process of genetic pollution
Genetic pollution
Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. This gene flow is undesirable according to some environmentalists and conservationists, including groups such as Greenpeace, TRAFFIC, and GeneWatch UK.-Usage:...
, potentially causing uncontrolled hybridization, introgression
Introgression
Introgression, also known as introgressive hybridization, in genetics is the movement of a gene from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species...
and genetic swamping. These processes can lead to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
advantage of introduced plant or animal. Nonnative species can bring about a form of extinction of native plants and animals by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that drive the originally purebred native stock to complete extinction. The extent of this facet of gene flow is not always apparent from morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
(outward appearance) observations alone. Some degree of gene flow may be due to normal, evolutionarily constructive processes, and all constellations of 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 and genotypes cannot be preserved. That being said, hybridization with or without introgression may threaten a rare species' existence nonetheless.
Models of gene flow
Models of gene flow can be derived from population geneticsPopulation genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
, e.g. Sewall Wright
Sewall Wright
Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of...
's neighborhood model, Wright's island model and the stepping stone model.
Gene flow mitigation
When cultivating genetically modifiedGenetic 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...
(GM) plants or livestock
Livestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...
, it becomes necessary to prevent "genetic pollution
Genetic pollution
Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. This gene flow is undesirable according to some environmentalists and conservationists, including groups such as Greenpeace, TRAFFIC, and GeneWatch UK.-Usage:...
" i.e. their genetic modification from reaching other conventionally hybridized or wild native plant and animal populations by using gene flow mitigation usually through unintentional cross pollination and crossbreeding. Reasons to limit gene flow may include biosafety
Biosafety
Biosafety: prevention of large-scale loss of biological integrity, focusing both on ecology and human health .Biosafety is related to several fields:*In ecology ,...
or agricultural co-existence
Co-existence of genetically modified and conventional crops and derived food and feed
In the context of agriculture and food and feed production, co-existence means using cropping systems with and without genetically modified organisms in parallel. For co-existence to be assured, the separation and the identity of the respective food and feed products must be maintained at all...
, in which GM and non-GM cropping systems work side by side.
Scientists in several large research programmes are investigating methods of limiting gene flow in plants. Among these programmes are Transcontainer
TransContainer
TransContainer is an intermodal freight transport company based in Russia. It is a subsidiary of RZD.It is Russia's main intermodal container transport and integrated logistics company; it transports over a million TEUs per year...
, which investigates methods for biocontainment
Biological containment
Biological containment describes measures aimed at preventing genetically modified organisms and their transgenes from spreading into the environment ....
, SIGMEA, which focuses on the biosafety of genetically modified plants, and Co-Extra
Co-Extra
Co-Extra is an EU-funded research programme on co-existence and traceability of genetically modified crops and their edible derivatives. It has been granted €13.5 million under the Sixth Framework Programme of the European Union, and is conducted by more than 200 scientists in 52 organisations in...
, which studies the co-existence of GM and non-GM product chains.
Generally, there are three approaches to gene flow mitigation: keeping the genetic modification out of the pollen, preventing the formation of pollen, and keeping the pollen inside the flower.
- The first approach requires transplastomic plants. In transplastomic plants, the modified DNADNADeoxyribonucleic 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...
is not situated in the cell's nucleus but is present in plastidPlastidPlastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell...
s, which are cellular compartments outside the nucleus. An example for plastids are chloroplastChloroplastChloroplasts 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, in which photosynthesisPhotosynthesisPhotosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
occurs. In some plants, the pollenPollenPollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
does not contain plastids and, consequently, any modification located in plastids cannot be transmitted by the pollen. - The second approach relies on male sterile plants. Male sterile plants are unable to produce functioning flowers and therefore cannot release viable pollen. Cytoplasmic male sterileCytoplasmic male sterilityCytoplasmic male sterility is total or partial male sterility associated with plant biology as the result of specific nuclear and mitochondrial interactions...
plants are known to produce higher yields. Therefore, researchers are trying to introduce this trait to genetically modified crops. - The third approach works by preventing the flowers from opening. This trait is called cleistogamyCleistogamyCleistogamy or automatic self-pollination describes the trait of certain plants to propagate by using non-opening, self-pollinating flowers...
and occurs naturally in some plants. Cleistogamous plants produce flowers which either open only partly or not at all. However, it remains unclear how reliable cleistogamy is for gene flow mitigation: a Co-Extra research project on rapeseed investigating the matter has published preliminary results which cast doubt on the attainment of a high degree of reliability.