Introgressive hybridization, also known as

introgression Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Intro ...

, is the flow of genetic material between divergent lineages via repeated

backcrossing Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, to achieve offspring with a genetic identity closer to that of the parent. It is used in horticulture, animal breeding, and produ ...

. In plants, this backcrossing occurs when an

F_1

generation hybrid breeds with one or both of its parental species.

Source of variation

Although some genera of plants hybridize and introgress more easily than others, in certain scenarios, external factors may contribute to an increased rate of hybridization. The phenomenon known as Hybridization of the Habitat echoes this idea, explaining that disturbances in a natural habitat can lead to species which typically do not hybridize and backcross to do so with relative ease. Plant breeders also manipulate their subjects to hybridize in order to optimize their hardiness, appearance, or whatever desired traits they want to select for. This type of hybridization has been particularly impactful for the production of many crop species, including but not limited to: certain types of rice, corn, wheat, barley, and rye. Natural introgression can occur with many genera and species, but manipulating the gene pool with artificial/forced introgression is useful for honing in on desired characteristics, such as drought tolerance or pest resistance.

Background

In the early days of hybrid research, it was commonly believed that there was insufficient evidence of hybridization in nature because hybridization would mostly produce sterile or unfit offspring. Through experimentation and improved phylo

genetic testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...

capabilities, we now see that the ability to produce fertile hybrid offspring varies by genus, within the plant kingdom. A few examples of species with the capacity to produce fertile hybrids are given below.

Examples of natural introgression

Irises

One of the most significant early studies of plant hybridization involved three species of irises. Although they commonly form crosses where their natural habitats overlap, there is no evidence that '' Iris fulva'', '' Iris hexagona'', or I''ris brevicaulis'' are closely related and their phenotypic differences (color/pattern/size) are distinct. Once introgression occurs, the resulting offspring display a wide array of color combinations, as well as varying flower size. Iris fulva shows a tendency for asymmetrical introgression, where it transfers more genetic material into hybrid offspring than either ''Iris hexagona or Iris brevicaulis''.

Sunflowers

Differential introgression of

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it ...

s and nuclear genomes was first seen among the

common sunflower The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), a ...

(''Helianthus annuus ssp. texanus''). Within a particular region, the population showed differences in morphological features which indicated there may be hybridization with ''H. debilis ssp cucumenifolius''. Researchers discovered that these ''H. a. texanus'' contained chloroplast DNA from ''H. d. cucumennfolius'', indicating introgression had occurred in one direction.

Poplars

Hybridization among poplars is common where ever populations overlap, however the degree of introgression varies greatly depending on the species. One study exploring the extent of introgression among three species of poplar trees (P. balsamifera, P. angustifolia and P. trichocarpa) conducted along the Rock Mountain range in the U.S. and Canada found extensive introgression in areas of species converge. Genomic sequencing even showed a trispecies hybrid in these overlapping areas. Another study found a hybrid zone in Utah where there was a unidirectional flow of introgression between ''P. angustifolia and P. fremontii.''

Examples of artificial introgression

Wheat

Introgression has played a major role in the development of

wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeolog ...

for crop production. One of the ways crop species can be manipulated is by crossing them with wild type species. For instance, the wild wheat relative species Agropyron elongatum has been crossed and introgressed with the domesticated wheat

Triticum aestivum Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat species. About 95% of wheat produced worldwide is common wheat; it is the most widely grown of all crops and the cereal with the highest monetary yield. Ta ...

. Consequently, the resulting hybrids have a higher water stress adaptation and higher root and shoot biomass. Both of these modifications can improve the fitness of the crop.

Daffodils

Daffodils ''Narcissus'' is a genus of predominantly spring flowering perennial plants of the amaryllis family, Amaryllidaceae. Various common names including daffodil,The word "daffodil" is also applied to related genera such as '' Sternbergia'', ''Is ...

(genus ''Narcissus'') are able to produce semi-fertile or fertile offspring, even from wide crosses. The ability of daffodils, such as the yellow trumpet Narcissi and Poets’ Narcissi to hybridize and backcross allows for the vast variety of options modern-day gardeners have to select from. Although daffodils do hybridize and introgress in nature, artificial introgression allows for breeders to take species that are geographically separated and make unique crosses that would not appear naturally.

References

{{Reflist Hybridisation (biology) Plants