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Introgression, also known as introgressive hybridization, in
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
is the transfer of genetic material from one species into the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
of another by the repeated
backcrossing Backcrossing is a crossing of a hybrid with one of its parent A parent is a caregiver of the offspring In biology, offspring are the young born of living organism, organisms, produced either by a single organism or, in the case of sexual repr ...

backcrossing
of an
interspecific hybrid In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
with one of its parent species. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of
gene flow In , gene flow (also known as gene migration or geneflow and flow) is the transfer of material from one to another. If the rate of gene flow is high enough, then two populations will have equivalent allele frequencies and therefore can be cons ...

gene flow
in that it occurs between two populations of different species, rather than two populations of the same species. Introgression also differs from simple hybridization. Simple hybridization results in a relatively even mixture; gene and allele frequencies in the first generation will be a uniform mix of two parental species, such as that observed in
mules
mules
. Introgression, on the other hand, results in a complex, highly variable mixture of genes, and may only involve a minimal percentage of the donor genome.


Definition

Introgression or introgressive hybridization is the incorporation (usually via hybridization and backcrossing) of novel genes and/or
alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
from one taxon into the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
of a second, distinct taxon. This introgression is considered 'adaptive' if the genetic transfer results in an overall increase in the recipient taxon's fitness. Ancient introgression events can leave traces of extinct species in present‐day genomes, a phenomenon known as ghost introgression.


Source of variation

Introgression is an important source of genetic variation in natural populations and may contribute to adaptation and even
adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic inte ...
. It can occur across
hybrid zoneA hybrid zone exists where the ranges of two interbreeding species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often ...
s due to chance, selection or hybrid zone movement. There is evidence that introgression is a ubiquitous phenomenon in plants and animals, including humans, in which it may have introduced the
microcephalin Microcephalin (MCPH1) is a gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ' ...

microcephalin
D allele. It has been proposed that, historically, introgression with wild animals is a large contributor to the wide range of diversity found in domestic animals, rather than multiple independent domestication events. Introgressive hybridization has also been shown to be important in the evolution of domesticated crop species, possibly providing genes that help in their expansion into different environments. A genomic study from New York University Abu Dhabi Center for Genomics and Systems Biology showed that domesticated date palm varieties from North Africa show introgressive hybridization of between 5-18% of its genome from the wild Cretan palm Phoenix theophrasti into Middle East date palms P. dactylifera. This process is also similar to the evolution of apples by hybridization of Central Asian apples with the European crabapple. It has also been shown that indica rice arose when Chinese japonica rice arrived in India about ~4,500 years ago and hybridized with an undomesticated proto-indica or wild O. nivara, and transferred key domestication genes from japonica to indica.


Examples


Humans

There is strong evidence for the introgression of
Neanderthal Neanderthals (, also Neandertals, ''Homo neanderthalensis'' or ''Homo sapiens neanderthalensis'') are an extinct species In biology Biology is the natural science that studies life and living organisms, including their anatomy, phys ...
genes and
Denisovan The Denisovans or Denisova hominins ) are an extinct species or subspecies of archaic human that ranged across Asia during the Lower Paleolithic, Lower and Middle Paleolithic. Denisovans are known from few remains, and, consequently, most of wha ...
genes into parts of the modern human gene pool (see more at
Archaic human admixture with modern humans There is evidence for interbreeding between archaic and modern humans during the Middle Paleolithic and early Upper Paleolithic. The interbreeding happened in several independent events that included Neanderthals and Denisovans, as well as se ...
).


Butterflies

One important example of introgression has been observed in studies of mimicry in the butterfly genus ''Heliconius''. This genus includes 43 species and many races with different color patterns. Congeners exhibiting overlapping distributions show similar color patterns. The subspecies ''H. melpomene amaryllis'' and ''H. melpomene timareta'' ssp. nov. overlap in distribution. Using the ABBA/BABA test, some researchers have observed that there is about 2% to 5% introgression between the pair of subspecies. Importantly, the introgression is not random. The researchers saw significant introgression in chromosomes 15 and 18, where important mimicry loci are found (loci B/D and N/Yb). They compared both subspecies with ''H. melpomene agalope'', which is a subspecies near ''H. melpomene amaryllis'' in entire genome trees. The result of the analysis was that there is no relation between those two species and ''H. melpomene agalope'' in the loci B/D and N/Yb. Moreover, they performed the same analysis with two other species with overlapping distributions, ''H. timareta florencia'' and ''H. melpomene agalope''. They demonstrated introgression between the two taxa, especially in the loci B/D and N/Yb. Finally, they concluded their experiments with sliding-window phylogenetic analyses, estimating different phylogenetic trees depending on the different regions of the loci. When a locus is important in the color pattern expression, there is a close phylogenetic relationship between the species. When the locus is not important in the color pattern expression, the two species are phylogenetically distant because there is no introgression at such loci.


Domestic species

Introgression can have a significant impact on human populations through hybridization, for instance, between wild and domestic populations of animals. This includes household pets, as seen in cats or in dogs.


Plants

Introgression has been observed in several plant species. For instance, a species of
iris
iris
from southern
Louisiana Louisiana (Standard French Standard French (in French: ''le français standard'', ''le français normé'', ''le français neutre'' eutral Frenchor ''le français international'' nternational French is an unofficial term for a standard ...

Louisiana
has been studied by Arnold & Bennett (1993) regarding the increased fitness of hybrid variants.


Fish

Espinasa et al. found that introgression between a surface-dwelling members of ''
Astroblepus ''Astroblepus'' is a genus of fish in the family (biology), family Astroblepidae found in South America and Panama. This genus is the only member of its family. These catfishes are primarily found in torrential streams in the Andes, Andean area. ' ...
'' and a troglomorphic species, ''Astroblepus pholeter,'' resulted in the development of previously lost traits in offspring, such as distinct eyes and optic nerves.


Introgression line

An introgression line (IL) is a
crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. Crops may refer either to the harvested parts or to the harvest in a more refined state. Most crops are cultivated in agriculture Agriculture is th ...

crop
species that contains genetic material artificially derived from a wild relative population through repeated backcrossing. An example of a collection of ILs (called an ''IL-Library'') is the use of
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
segments from '' Solanum pennellii'' (a wild species of
tomato The tomato is the edible berry A berry is a small, pulpy, and often edible fruit In botany, a fruit is the seed-bearing structure in flowering plants (also known as angiosperms) formed from the ovary after flowering. Fruits are ...

tomato
) that was introgressed into '' Solanum lycopersicum'' (the cultivated tomato). The lines of an IL-library usually cover the complete
genome In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
of the donor. Introgression lines allow the study of
quantitative trait loci A quantitative trait locus (QTL) is a locus (section of DNA) that correlates with variation of a quantitative trait in the phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the c ...
, but also the creation of new varieties by introducing exotic
trait Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
s.


Lineage fusion

Lineage fusion is an extreme variant of introgression that results from the merging of two taxonomically distinct species or populations. This eventually results in a single population that displaces or replaces the parental species in the region. Some lineage fusion occurs soon after two taxa diverge or speciate, especially if there are few reproductive barriers between lineages. It is not strictly necessary for the two lineages to be closely related, but rather have the ability to produce viable offspring.


See also

*
Chimera (genetics) A genetic chimerism or chimera ( /kaɪˈmɪərə/ or /kɪˈmɪərə/ ) is a single organism composed of cells with more than one distinct genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used ...
*
Genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the direct manipulation of an organism's gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_in ...
*
Genetic erosion Genetic erosion (also known as genetic depletion) is a process where the limited gene pool of an endangered species diminishes even more when reproductive individuals die off before reproducing with others in their endangered low population. The ...
*
Genetic pollution Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. It is defined as "the dispersal of contaminated altered genes from genetically engineered organisms to natural organisms, esp. by cross-pollination", but ha ...
*
Transgene A transgene is a gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiol ...
*
Transgenic plant A transgene is a gene that has been transferred naturally, or by any of a number of genetic engineering techniques from one organism to another. The introduction of a transgene, in a process known as transgenesis, has the potential to change the p ...


References


Further reading

* * * Décobert, O. (2017). Complément à l’inventaire des Carabini du Midi toulousain (Coleoptera, Carabidae) - Carnets natures, 2017, vol. 4 : 33-38 (ISSN 2427-6111) https://carnetsnatures.fr/volume4/carabidae-decobert.pdf * * * * {{cite journal , author = Whitney, K.D., Ahern J.R.,Campbell L.G, Albert L.P., King M.S. , year = 2010 , title = Patterns of hybridization in plants , url = http://www.ruf.rice.edu/~kwhitney/Whitney%20Reprints/Whitney%20et%20al.%202010%20PPEES%202.pdf , journal = Perspectives in Plant Ecology, Evolution and Systematics, volume = 12, issue = 3 , pages = 175–182 , doi=10.1016/j.ppees.2010.02.002 ("Forbidden" - No Access 2015-04-06) Alternate Link
Patterns of Hybridization in Plants
Classical genetics