Introgression

Introgression, also known as introgressive hybridization, in genetics is the movement of a gene 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.

Definition

Introgression or introgressive hybridization is the incorporation (usually via hybridization and backcrossing) of alleles from one entity (species) into the gene pool of a second, divergent entity (species).[1][2][3][4] Ancient introgression events can leave traces of extinct species in present‐day genomes, a phenomenon known as ghost introgression.[5]

Source of variation

Introgression is an important source of genetic variation in natural populations and may contribute to adaptation and even adaptive radiation.[6] It can occur across hybrid zones due to chance, selection or hybrid zone movement.[7] There is evidence that introgression is a ubiquitous phenomenon in plants and animals,[8][9] including humans,[10] in which it may have introduced the microcephalin D allele.[11]

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.[12]

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.[13] 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.[14]

Examples

Humans

There is strong evidence for the introgression of Neanderthal genes[15] and Denisovan genes[16] into parts of the modern human gene pool (see more at Archaic human admixture with modern humans).

Butterflies

One important example of introgression has been observed in studies of mimicry in the butterfly genus Heliconius.[17] 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.

Wild species

Introgression could be an important conservation problem for wild species through hybridisation, for instance, between wild and domestic cats[18] or among wild canids and domestic dogs.[19] Another important example in iris species from southern Louisiana has been studied by Arnold & Bennett (1993).[20]

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 variety of tomato) that was introgressed into Solanum lycopersicum (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.[21]

See also

References
  1. Anderson E, Hubricht L (1938). "Hybridization in Tradescantia. III. The evidence for introgressive hybridization". Am J Bot. 25 (6): 396–402. doi:10.2307/2436413. JSTOR 2436413.
  2. Anderson E, 1949. Introgressive hybridization. New York: Wiley & Sons
  3. Harrison, R (2014). "Hybridization, Introgression, and the Nature of Species Boundaries". Journal of Heredity. 105: 795–809. doi:10.1093/jhered/esu033. PMID 25149255.
  4. Ottenburghs, Jente; Kraus, Robert H. S.; van Hooft, Pim; van Wieren, Sipke E.; Ydenberg, Ronald C.; Prins, Herbert H. T. (2017-11-30). "Avian introgression in the genomic era". Avian Research. 8 (1): 30. doi:10.1186/s40657-017-0088-z. ISSN 2053-7166.
  5. Jente Ottenburghs (2020) Ghost Introgression: Spooky Gene Flow in the Distant Past. BioEssays. https://doi.org/10.1002/bies.202000012
  6. Grant P.R., Grant B.R., Petren K. (2005). "Hybridization in the Recent Past". The American Naturalist. 166 (1): 56–67. doi:10.1086/430331. PMID 15937789.CS1 maint: multiple names: authors list (link)
  7. Richard Buggs (2007). "Empirical study of hybrid zone movement". Heredity. 99 (3): 301–312. doi:10.1038/sj.hdy.6800997. PMID 17611495.
  8. 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. S2CID 52367016.
  9. 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.
  10. Holliday T. W. (2003). "Species concepts, reticulations, and human evolution". Current Anthropology. 44 (5): 653–673. doi:10.1086/377663.
  11. 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 1635020. PMID 17090677.
  12. Blaustein, R. (2015). "Unraveling the Mysteries of Animal Domestication". BioScience. 65: 7–13. doi:10.1093/biosci/biu201.
  13. Flowers, Jonathan; et al. (2019). "Cross-species hybridization and the origin of North African date palms". Proceedings of the National Academy of Sciences USA. 116 (5): 1651–1658. doi:10.1073/pnas.1817453116. PMC 6358688. PMID 30642962.
  14. Choi, Jae; et al. (2017). "The Rice Paradox: Multiple Origins but Single Domestication in Asian Rice". Molecular Biology and Evolution. 34 (4): 969–979. doi:10.1093/molbev/msx049. PMC 5400379. PMID 28087768.
  15. Wills, Christopher (2011). Genetic and Phenotypic Consequences of Introgression Between Humans and Neanderthals. Advances in Genetics. 76. pp. 27–54. doi:10.1016/B978-0-12-386481-9.00002-X. ISBN 9780123864819. PMID 22099691.
  16. 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): 194–197. Bibcode:2014Natur.512..194H. doi:10.1038/nature13408. PMC 4134395. PMID 25043035.
  17. The Heliconius Genome Consortium (2012). "Butterfly genome reveals promiscuous exchange of mimicry adaptations among species". Nature. 487 (7405): 94–98. Bibcode:2012Natur.487...94T. doi:10.1038/nature11041. PMC 3398145. PMID 22722851.
  18. Review of scientific papers on gene introgression between wild and domestic cats
  19. Review and link to scientific papers regarding introgression of dog genes into wild canid populations
  20. Arnold, M. L. & Bennett, B. D. (1993). Natural Hybridization in Louisiana irises: genetic variation and ecological determinants. In: Harrison, R. G. ed. (1993). Hybrid Zones and Evolutionary Process, pp. 115–139. New York: Oxford University Press. ISBN 978-0-19-506917-4
  21. Eshed, Y (1995) An Introgression Line Population of Lycopersicon pennellii in the Cultivated Tomato Enables the Identification and Fine Mapping of Yield-Associated QTL

Further reading
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