Introgression

Abstract

Biologists have forever sought to understand how species arise and persist. Historically, species that rarely interbreed, or are reproductively isolated, were considered the norm, while those with incomplete reproductive isolation were considered less common. Over the last few decades, advances in genomics have transformed our understanding of the frequency of gene flow between species and with it our ideas about reproductive isolation in nature. These advances have uncovered a rich and often complicated history of genetic exchange between species - demonstrating that such genetic introgression is an important evolutionary process widespread across the tree of life (Figure 1).

Figure 1.. Examples of species that have experienced introgression.

(A) Heliconius butterflies (photo: iNaturalist/Mike Melton). (B) Helianthus sunflowers (photo: iNaturalist/Grace Stark). (C) Brewer’s yeasts (Saccharomyces cerevisiae; photo: Quinn K Langdon). (D) Humans (Homo sapiens; photo: mizmareck/Flickr). (E) Snowshoe hares (Lepus americanus; photo: Van Alstine). (F) Fire ants (Solenopsis sp.; photo: iNaturalist/ Eric Blomberg). (G) Darwin’s finches (Geospiza species). (H) African Great Lake cichlids (family Cichlidae; photo: Catherine E Wagner). (I) Anopheles gambiae mosquitos (photo: CDC/Wikicommons).

Figure 2.. The integration of segments of DNA through introgression.

(A) The process of introgression occurs through hybridization and subsequent backcrossing. An initial hybridization event between two distinct species — blue and red — produces an F1 hybrid. If this F1 hybrid backcrosses into the blue species and their offspring subsequently breed with the blue species as well, this will result in introgression of the red species’ DNA into the blue species’ genome. (B) The results of this introgression event are shown in the phylogeny (the red arrow pointing from the lineage of the red species into the lineage of the blue species), phenotypes, and genotypes of the group of organisms. The red species and the outgroup are unaffected by this unidirectional introgression event.

Figure 3.. Potential outcomes after an initial introgression event.

After initial introgression of DNA segments from the red species into the blue species occurs, there are different potential outcomes in the genome. Each bar represents a schematic of part of the genome with the color representing the ancestry of origin and the dashed lines denoting a particular region of interest. (A) If introgression occurs in a neutral region of the genome (e.g., a region with low gene density), the introgressed segments will be broken into smaller pieces by recombination and their frequencies will be influenced by genetic drift over time. (B) If introgression is maladaptive in a particular region of the genome (e.g., a region with high gene density or one containing loci involved in genetic incompatibilities between genes from red and blue ancestry), the introgressed segments are likely to be purged from the population. (C) Finally, if an allele in an introgressed region provides an adaptive advantage (e.g., an ecological advantage), the introgressed segments containing that allele will likely increase in frequency in the population.