Introgression, also known as introgressive hybridization, in genetics is the movement of a gene (gene flow) from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Purposeful introgression is a long-term process; it may take many hybrid generations before the backcrossing occurs.
Introgression differs from simple hybridization. Introgression results in a complex mixture of parental genes, while simple hybridization results in a more uniform mixture, which in the first generation will be an even mix of two parental species. Natural introgression does not have human direct interference while the exotic introgression is induced intentionally (as for instance genetically modified organisms[clarification needed]) or not (human activities affecting local races of crops or human disturbances such as by introducing weeds).
Introgression (or "introgressive hybridization") describes the incorporation (usually via hybridization and backcrossing) of alleles from one entity (species) into the gene pool of a second, divergent entity (species).
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, animals, and even humans, in which it may have introduced the microcephalin D allele.
It has been proposed that historically, domestic animals have had a limited number of domestication situations followed by long periods of introgression where they have acquired the genetic material of wild animals in their DNA.
One important example of introgression has been observed in butterfly mimicry. Genus Heliconius has been studied. This genus includes 43 species and many races with different color patterns. Congeners exhibiting overlapping distributions show similar color patterns. The distribution of the subspecies H. melpomene amaryllis and H. melpomene timareta ssp. nov. overlap. Using the ABBA/BABA test, some researchers have observed that there is ≈2-5% introgression between the pair of subspecies. It is important to know that this is not random introgression. They saw important introgression in chromosomes 15 and 18, where important mimicry loci are located (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 this experiment 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 experiment 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.
Introgression could be an important conservation problem for wild species through hybridisation, for instance, between wild and domestic cats  or among wild canids and domestic dogs. Another important example has been studied by Arnold & Bennett 1993: iris species from southern Louisiana.
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 Lycopersicon pennellii (a wild variety of tomato) that was introgressed into Lycopersicum esculentu (a variety of 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.
- Gene flow
- Genetic engineering
- Genetically modified organism
- Transgenic plant
- Chimera (genetics)
- Gene pool
- Genetic pollution
- Genetic erosion
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