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 of alleles from one entity into the gene pool of a second, divergent entity. Ancient introgression events can leave traces of extinct species in present‐day genomes, a phenomenon known as ghost introgression.
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. 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 or among wild canids and domestic dogs. Another important example in iris species from southern Louisiana has been studied by Arnold & Bennett.
Introgression line
An introgression line 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 is the use of chromosome segments from Solanum pennellii that was introgressed into Solanum lycopersicum. 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.
