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
- Anderson E, Hubricht L (1938). "Hybridization in Tradescantia. III. The evidence for introgressive hybridization". Am J Bot. 25: 396–402.
- Anderson E, 1949. Introgressive hybridization. New York: Wiley & Sons
- Harrison, R (2014). "Hybridization, Introgression, and the Nature of Species Boundaries". Journal of Heredity. 105: 795–809. doi:10.1093/jhered/esu033.
- Grant, P.R., Grant, B.R. & Petren, K. (2005). "Hybridization in the Recent Past". The American Naturalist 166: 56–67. (available online at The American Naturalist)
- Richard Buggs (2007). "Empirical study of hybrid zone movement". Heredity. 99 (3): 301–312. doi:10.1038/sj.hdy.6800997.
- Dowling T. E.; Secor C. L. (1997). "The role of hybridization and introgression in the diversification of animals". Annual Review of Ecology and Systematics. 28: 593–619. doi:10.1146/annurev.ecolsys.28.1.593.
- Bullini L (1994). "Origin and evolution of animal hybrid species". Trends in Ecology and Evolution. 9 (11): 422–426. doi:10.1016/0169-5347(94)90124-4. PMID 21236911.
- Holliday T. W. (2003). "Species concepts, reticulations, and human evolution". Current Anthropology. 44 (5): 653–673. doi:10.1086/377663.
- Evans, Pd; Mekel-Bobrov, N; Vallender, Ej; Hudson, Rr; Lahn, Bt (Nov 2006). "Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage". Proceedings of the National Academy of Sciences of the United States of America. 103 (48): 18178–83. Bibcode:2006PNAS..10318178E. doi:10.1073/pnas.0606966103. ISSN 0027-8424. PMC . PMID 17090677.
- Blaustein, R. (2015). "Unraveling the Mysteries of Animal Domestication". BioScience. 65: 7–13. doi:10.1093/biosci/biu201.
- Wills, Christopher (2011). "Genetic and Phenotypic Consequences of Introgression Between Humans and Neanderthals". Advances in Genetics. 76: 27. doi:10.1016/B978-0-12-386481-9.00002-X. ISBN 9780123864819.
- Huerta-Sánchez, Emilia; Jin, Xin; Asan; Bianba, Zhuoma; Peter, Benjamin M.; Vinckenbosch, Nicolas; Liang, Yu; Yi, Xin; He, Mingze; Somel, Mehmet; Ni, Peixiang; Wang, Bo; Ou, Xiaohua; Huasang; Luosang, Jiangbai; Cuo, Zha Xi Ping; Li, Kui; Gao, Guoyi; Yin, Ye; Wang, Wei; Zhang, Xiuqing; Xu, Xun; Yang, Huanming; Li, Yingrui; Wang, Jian; Wang, Jun; Nielsen, Rasmus (2014). "Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA". Nature. 512 (7513): 194. doi:10.1038/nature13408. PMID 25043035.
- The Heliconius Genome Consortium (2012). "Butterfly genome reveals promiscuous exchange of mimicry adaptations among species". Nature. 487: 94–98. doi:10.1038/nature11041.
- Review of scientific papers on gene introgression between wild and domestic cats
- Review and link to scientific papers regarding introgression of dog genes into wild canid populations
- Arnold, M. L. & Bennett, B. D. (1993). "Natural Hybridization in Louisiana irises: genetic variation and ecological determinants". In: Harrison, R. G. (ed.) Hybrid Zones and Evolutionary Process, pp. 115-139. Oxford University Press, New York. ISBN 978-0-19-506917-4
- Eshed, Y (1995) An Introgression Line Population of Lycopersicon pennellii in the Cultivated Tomato Enables the Identification and Fine Mapping of Yield-Associated QTL
- Arnold, M. L. (2007). Evolution through Genetic Exchange. New York: Oxford University Press. ISBN 0-19-922903-1.
- Anderson, E. (1949). Introgressive Hybridization. New York: Wiley.
- Eyal Friedman; et al. (2004). "Zooming In on a Quantitative Trait for Tomato Yield Using Interspecific Introgressions". Science. 305: 1786–1798. doi:10.1126/science.1101666. PMID 15375271.
- Rieseberg, L. H. & Wendel, J. F. (1993). "Introgression and its consequences in plants". In Harrison, R. G. Hybrid Zones and Evolutionary Process. New York: Oxford University Press. pp. 70–109. ISBN 978-0-19-506917-4.
- Martinsen G. D.; Whitham R. J. Turek; Keim P. (2001). "Hybrid populations selectively filter gene introgression between species". Evolution. 55 (7): 1325–1335. doi:10.1554/0014-3820(2001)055[1325:hpsfgi]2.0.co;2. PMID 11525457.
- Whitney, K.D., Ahern J.R.,Campbell L.G, Albert L.P., King M.S. (2010). "Patterns of hybridization in plants" (PDF). Evolution and Systematics. 12: 175–182. doi:10.1016/j.ppees.2010.02.002. ("Forbidden" - No Access 2015-04-06) Alternate Link: Patterns of Hybridization in Plants