Introgressive hybridization, also known as introgression, is the flow of genetic material between divergent lineages via repeated backcrossing. In plants, this backcrossing occurs when an generation hybrid breeds with one or both of its parental species.
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. [1] 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. [2]
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 phylogenetic testing capabilities, we now see that the ability to produce fertile hybrid offspring varies by genus, within the plant kingdom. [3] A few examples of species with the capacity to produce fertile hybrids are given below.
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 Iris 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. [4]
Differential introgression of chloroplasts and nuclear genomes was first seen among the common sunflower (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. [5]
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. [6] Another study found a hybrid zone in Utah where there was a unidirectional flow of introgression between P. angustifolia and P. fremontii. [7]
Introgression has played a major role in the development of wheat 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. 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. [8]
Daffodils (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. [3] 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.[ citation needed ]
In biology, a hybrid is the offspring resulting from combining the qualities of two organisms of different varieties, species or genera through sexual reproduction. Generally, it means that each cell has genetic material from two different organisms, whereas an individual where some cells are derived from a different organism is called a chimera. Hybrids are not always intermediates between their parents, but can show hybrid vigor, sometimes growing larger or taller than either parent. The concept of a hybrid is interpreted differently in animal and plant breeding, where there is interest in the individual parentage. In genetics, attention is focused on the numbers of chromosomes. In taxonomy, a key question is how closely related the parent species are.
Triticale is a hybrid of wheat (Triticum) and rye (Secale) first bred in laboratories during the late 19th century in Scotland and Germany. Commercially available triticale is almost always a second-generation hybrid, i.e., a cross between two kinds of primary (first-cross) triticales. As a rule, triticale combines the yield potential and grain quality of wheat with the disease and environmental tolerance of rye. Only recently has it been developed into a commercially viable crop. Depending on the cultivar, triticale can more or less resemble either of its parents. It is grown mostly for forage or fodder, although some triticale-based foods can be purchased at health food stores and can be found in some breakfast cereals.
The mandarin orange, also known as mandarin or mandarine, is a small, rounded citrus tree fruit. Treated as a distinct species of orange, it is usually eaten plain or in fruit salads. Tangerines are a group of orange-coloured citrus fruit consisting of hybrids of mandarin orange with some pomelo contribution.
Cicer is a genus of the legume family, Fabaceae, and the only genus found in tribe Cicereae. It is included within the IRLC, and its native distribution is across the Middle East and Asia. Its best-known and only domesticated member is Cicer arietinum, the chickpea.
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 production of gene knockout organisms.
An F1 hybrid (also known as filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types. F1 hybrids are used in genetics, and in selective breeding, where the term F1 crossbreed may be used. The term is sometimes written with a subscript, as F1 hybrid. Subsequent generations are called F2, F3, etc.
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. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of gene flow in that it occurs between two populations of different species, rather than two populations of the same species.
A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form in situ due to the evolution of a new lineage but generally they result from secondary contact of the parental forms after a period of geographic isolation, which allowed their differentiation. Hybrid zones are useful in studying the genetics of speciation as they can provide natural examples of differentiation and (sometimes) gene flow between populations that are at some point between representing a single species and representing multiple species in reproductive isolation.
Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thought to be particularly difficult to achieve, and thus hybrid species were thought to be very rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a somewhat common phenomenon, particularly in plants. In botanical nomenclature, a hybrid species is also called a nothospecies. Hybrid species are by their nature polyphyletic.
Genetic pollution is a term for uncontrolled gene flow into wild populations. It is defined as "the dispersal of contaminated altered genes from genetically engineered organisms to natural organisms, esp. by cross-pollination", but has come to be used in some broader ways. It is related to the population genetics concept of gene flow, and genetic rescue, which is genetic material intentionally introduced to increase the fitness of a population. It is called genetic pollution when it negatively impacts the fitness of a population, such as through outbreeding depression and the introduction of unwanted phenotypes which can lead to extinction.
A hybrid swarm is a population of hybrids that has survived beyond the initial hybrid generation, with interbreeding between hybrid individuals and backcrossing with its parent types. Such population are highly variable, with the genetic and phenotypic characteristics of individuals ranging widely between the two parent types. Hybrid swarms thus blur the boundary between the parent taxa. Precise definitions of which populations can be classified as hybrid swarms vary, with some specifying simply that all members of a population should be hybrids, while others differ in whether all members should have the same or different levels of hybridization.
Louisiana iris is a taxonomic group of five iris species native to Louisiana and surrounding regions of the southeastern United States: Iris fulva, Iris hexagona, Iris brevicaulis, Iris giganticaerulea, and Iris nelsonii.
Iris brevicaulis is a species in the genus Iris, it is also in the subgenus Limniris and in the series Hexagonae. It is a rhizomatous perennial, from North America. It has bright green, glossy long leaves, a long zig-zagged stem and 3–6 flowers per stem, which are come in blue shades from violet-blue, to lavender, to purple-blue, to bright blue to blue, and pale blue.
Iris fulva, also known as copper iris, is a species in the genus Iris, it is also in the subgenus Limniris and in the series Hexagonae. It is a rhizomatous perennial, endemic to the southern and central United States. It has copper-red to deep red flowers and bright green leaves.
Iris nelsonii is a species in the genus Iris, it is also in the subgenus Limniris and in the series hexagonae. It is a rhizomatous perennial, from northern America. It has long drooping, grass-like leaves, tall stems, 10 red-purple flowers.
Lathyrus belinensis, also known as the Belin pea is a flowering plant species in the genus Lathyrus under the family Fabaceae. The species was discovered in Turkey by botanists Nigel Maxted and David John Goyder and was first described in 1988. The species is a highly localized endemic found only in the Turkish province of Antalya. L. belinensis was listed among the top one hundred most endangered species of the world by the IUCN in 2012.
Secondary contact is the process in which two allopatricaly distributed populations of a species are geographically reunited. This contact allows for the potential for the exchange of genes, dependent on how reproductively isolated the two populations have become. There are several primary outcomes of secondary contact: extinction of one species, fusion of the two populations back into one, reinforcement, the formation of a hybrid zone, and the formation of a new species through hybrid speciation.
Eukaryote hybrid genomes result from interspecific hybridization, where closely related species mate and produce offspring with admixed genomes. The advent of large-scale genomic sequencing has shown that hybridization is common, and that it may represent an important source of novel variation. Although most interspecific hybrids are sterile or less fit than their parents, some may survive and reproduce, enabling the transfer of adaptive variants across the species boundary, and even result in the formation of novel evolutionary lineages. There are two main variants of hybrid species genomes: allopolyploid, which have one full chromosome set from each parent species, and homoploid, which are a mosaic of the parent species genomes with no increase in chromosome number.
Citrus ryukyuensis is a newly characterized wild citrus species native to the Ryukyus and adjacent islands, most closely related to the mainland mandarin orange, C. reticulata. As with other citrus, it is a member of the Rutaceae family. The Ryukyu and mainland species have diverged for more than 2 million years, and unlike the mainland mandarin, the Ryukyu mandarin reproduces sexually. Its hybridization with the mainland species has given rise to the unique mandarin hybrids of the islands.
Hybridization, when new offspring arise from crosses between individuals of the same or different species, results in the assemblage of diverse genetic material and can act as a stimulus for evolution. Hybrid species are often more vigorous and genetically differed than their ancestors. There are primarily two different forms of hybridization: natural hybridization in an uncontrolled environment, whereas artificial hybridization occurs primarily for the agricultural purposes.
{{cite book}}: CS1 maint: location missing publisher (link)