introgressed

Introgressed refers to genetic material that has moved from one population or species into another through hybridization and subsequent backcrossing. In population genetics, introgression describes the incorporation of donor DNA into a recipient genome, with the introgressed portions originating in the donor population but present in the recipient. The term is commonly used to describe introgressed alleles or haplotypes.

Mechanism and scope: Hybrids form when individuals from donor and recipient populations interbreed. Through repeated backcrossing of hybrids to the recipient, donor DNA is gradually diluted across the genome, yet certain segments can persist as introgressed regions. Selection can influence their fate: neutral, deleterious, or advantageous variants may rise in frequency, with beneficial ones contributing to adaptive introgression.

Examples: In humans, introgression from Neanderthals and Denisovans has left measurable tracts in modern genomes, including adaptive variants in immune function and other traits. The EPAS1 region in Tibetans is cited as an example of adaptive introgression related to high-altitude adaptation. In plants and animals, introgression occurs between cultivated species and wild relatives and can introduce traits such as disease resistance or environmental tolerance.

Detection and implications: Genomic tools identify introgressed segments through local ancestry inference, ABBA-BABA tests (D-statistics), and haplotype-based methods. Introgression informs studies of evolution, hybridization history, domestication, and conservation, and it can complicate notions of strict species boundaries by revealing historical gene flow.