introgressions

Introgressions are the incorporation of genetic material from one species or population into another through hybridization followed by backcrossing. The introgressed sequences can range from single nucleotide changes to large chromosomal segments. If these segments persist in the recipient genome, they may contribute functional traits or influence genome structure, depending on selection, drift, and recombination.

Introgression occurs naturally in many taxa and can play a role in adaptation and diversification. In humans, ancestry from Neanderthals and Denisovans reflects ancient introgression events. In plants, introgressions from wild relatives into crops have historically provided genes for disease resistance, stress tolerance, and other agronomic traits, and remain an active avenue in breeding programs for wheat, maize, and related species.

Detecting introgressions relies on genome-scale data and methods that distinguish reticulate evolution from shared ancestry. Approaches include tests for asymmetric allele sharing such as ABBA-BABA (D-statistic) and f-statistics, as well as local ancestry inference using haplotype data and hidden Markov models. These analyses can identify ancestry tracts, estimate their age, and assess the functional significance of introgressed segments. Inference can be confounded by incomplete lineage sorting and by selection acting on linked variants.

Implications of introgressions include the introduction of adaptive variants and the reshaping of population histories, as well as potential linkage drag of deleterious alleles. Understanding introgression informs evolutionary biology, conservation, and plant and animal breeding, and highlights the reticulate nature of gene flow across species boundaries.