Introgression, also known as introgressive hybridization, in

genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar work ...

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 surv ...

of another by the repeated backcrossing of an interspecific hybrid 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 population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent a ...

in that it occurs between two populations of different species, rather than two populations of the same species. Introgression also differs from simple

hybridization Hybridization (or hybridisation) may refer to: *Hybridization (biology), the process of combining different varieties of organisms to create a hybrid *Orbital hybridization, in chemistry, the mixing of atomic orbitals into new hybrid orbitals *Nu ...

. 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. 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

is the incorporation (usually via hybridization and backcrossing) of novel genes and/or alleles from one taxon into the

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 in ...

. It can occur across hybrid zones 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 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 ''Homo neanderthalensis'' and erroneously ''Homo sapiens neanderthalensis''), also written as Neandertals, are an Extinction, extinct species or subspecies of archaic humans who lived in Eurasia until about 40,000 years ag ...

genes and Denisovan genes into parts of the modern human gene pool.

Birds

The Mallard duck is possibly the world's most capable bird to hybridise with other duck species, often to the point of the loss of genetic identity of these species. For example, feral mallard populations have significantly reduced wild populations of the Pacific black duck in New Zealand and Australia through cross-breeding.

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 non-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 from southern

Louisiana Louisiana , group=pronunciation (French: ''La Louisiane'') is a state in the Deep South and South Central regions of the United States. It is the 20th-smallest by area and the 25th most populous of the 50 U.S. states. Louisiana is bord ...

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'' 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. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropon ...

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 genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

segments from '' Solanum pennellii'' (a wild species of

tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word ...

) 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 and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...

of the donor. Introgression lines allow the study of quantitative trait loci, but also the creation of new varieties by introducing exotic traits.

Lineage fusion

Lineage fusion is an extreme variant of introgression that results from the merging of two 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.