Introgression, also known as introgressive hybridization, in 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 surv ...

of another by the repeated

backcrossing Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. It is used in horticulture, animal breeding, and product ...

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 A mule is the offspring of a female horse and a male donkey. Mule, Mules, MULE or The Mule can also refer to: Animals * Mule (sheep), in British sheep farming, a cross between a meat ram and a hardy mountain ewe * Mule deer (''Odocoileus hemio ...

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

Birds

The

Mallard The mallard () or wild duck (''Anas platyrhynchos'') is a dabbling duck that breeds throughout the temperate and subtropical Americas, Eurasia, and North Africa, and has been introduced to New Zealand, Australia, Peru, Brazil, Uruguay, Arge ...

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 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 Troglomorphism is the morphological adaptation of an animal to living in the constant darkness of caves, characterised by features such as loss of pigment, reduced eyesight or blindness, and frequently with attenuated bodies and/or appendages. The ...

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 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 segments from '' Solanum pennellii'' (a wild species of tomato) that was introgressed into '' Solanum lycopersicum'' (the cultivated tomato). The lines of an IL-library usually cover the complete genome 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.