Introgression

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, 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) or not. 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). 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. 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. There is strong evidence for the introgression of Neanderthal genes and Denisovan genes into parts of the modern human gene pool (see more at Archaic human admixture with modern humans). 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. 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 in iris species from southern Louisiana has been studied by Arnold & Bennett (1993).