Hybridization and introgression

SIGNIFICANCE: Hybridization and introgression are biological processes that are essential to creating genetic variation, and hence biodiversity, in plant and animal populations. These processes occur both in natural populations and in human-directed, controlled breeding programs.

Definitions and Types

Hybridization and introgression are natural biological processes. Natural hybridization is common among plant and animal species. Hybridization generally refers to the mating between genetically dissimilar individuals; parents may differ in a few or many genes. They may come from different populations or races of the same taxonomic species (interspecific hybridization) or of different species (intergeneric hybridization). In nature, hybridization can occur only if there is no barrier to crossbreeding, or when the usual barrier breaks down. Hybridization produces new genetic combinations or genetic variability. Through artificial means (controlled pollination), hybridization of both cross-pollinated and self-pollinated plants can be accomplished. Plant breeding encompasses hybridization within a species as well as hybridization between species and even genera (wide crosses). The latter are important for generating genetic variability or for incorporating a desirable gene not available within a species. There are crossing barriers, however, for accomplishing interspecific and intergeneric crosses. Joseph Gottlieb Kölreuter (c. 1761) was the first to report on hybrid vigor (heterosis) in interspecific crosses of various species of Nicotiana, concluding that cross-fertilization was generally beneficial and self-fertilization was not.

Introgression is the introduction of genes from one species or gene pool into another species or gene pool. Introgression follows hybridization and occurs when hybrids reproduce with members of one or both of the parental species that produced the hybrids. It usually involves transfer of a small amount of DNA from one species or genus to another. Both hybridization and introgression can cause rapid evolution, that is, speciation or extinction. When introgression occurs between a common species and a rare species, the rare species is frequently exterminated.

Scientific breakthroughs relative to species-specific molecular (DNA) markers allow quantitative assessment of introgression and hybridization in natural populations. A clear distinction among species is a prerequisite to guide efforts to conserve biodiversity. In 2023, a study showed that natural hybridization may help insulate species from the effects of climate change. Hybridization creates genetic diversity, and some of these genes may help a species better adapt to warmer environments.

Reproductive Isolation Barriers

Isolation barriers can be divided into two types: (1) external and (2) internal. External barriers to genetic interchange between related populations prevent pollen of plants in one population from falling on stigmas of plants in another. A combination of barriers, such as geographical and ecological or ecological and seasonal (flowering time), is more common than individual barriers.

Internal barriers to genetic interchange between related populations operate through incompatibilities between physiological or cytological systems of plants from different populations. They may (1) prevent the production of F₁ (first-generation) zygotes, even if the pollen from flowers in one population falls on stigmas of flowers in the other; (2) produce F₁ hybrids that are nonviable, weak, or sterile; or (3) cause hybrid breakdown in F₂ or later generations.

The promotion of natural hybridization and introgression has, across time, increased the genetic diversity available to farmers. Traditional farmers experiment with new varieties and breed plants purposely to create new strains. They generally plant experimental plots first and integrate new varieties into their main crops only when a variety has proven itself to be of value. This constant experimentation and breeding have created the diversity of crops on which people now depend.

Transgenic Crops and Controversy

Termed “gene flow,” the movement of genes between closely related plant species is quite natural and has been occurring ever since flowering plants evolved. Hybrids that are the offspring resulting from the mating of related species may then mate through pollen exchange with the wild-type (original) plants. Backcrossing, which is also called introgression, increases hybrids’ biological fitness.

The term “transgenic” or “genetically modified organism” (GMO) has been applied to plants and animals in which techniques of recombinant DNA have been used to modify specific parts of the genome of an organism. When the procedure is successful, the resulting organism may stably express a novel protein, express a protein with novel properties, or carry a change in the regulation of some of its genes. Usually, such a change is designed to improve the ability of the organism to grow (for instance, by resisting pests or using nutrients more efficiently) or to improve the usefulness of the organism (by improving its nutritive value, using it to manufacture pharmaceutically important molecules, or employing it to carry out environmentally important processes such as digesting environmental toxins).

Hybridization and introgression may introduce novel adaptive traits. The subjects have raised controversy, because transgenes introduced into crops have the potential for spreading into related weeds or wild plants. Scientists have hypothesized that transgenes might move from the genetically modified crop plants to weeds. The possibility of spreading transgenes via introgression and bridging, from genetically modified crops to related weed species, is a concern; introduction of herbicide-resistant cultivars into commercial agriculture could lead to the creation of superweeds.

Some researchers believe that if herbicide-resistant genes were to become more common in weeds as a result of widespread use of herbicide-resistant crops, farmers who rely on herbicides to manage weeds would be forced to use greater amounts and a larger number of herbicides.

To “solve” problem of horizontal gene transfer, the producers of transgenic crops naturally turn to gene technology. They propose to reduce the risk of creating transgenic uncontrollable weeds and volunteer cultivars by linking herbicide-resistance genes to other genes that are harmless to the crop but damaging to a weed, such as genes that affect seed dormancy or prevent flowering in the next generation. Thus, if a weed did acquire an herbicide-resistance gene from a transgenic crop, its offspring would not survive to spread the herbicide resistance through the weed population. Several of the newly patented techniques sterilize seeds so that farmers cannot replant them. In addition, patent protection and intellectual property rights keep farmers from sharing and storing seeds. Thus, genetic seed sterility could increase seed industry profits; farmers would need to buy seed every season.

Maternal Inheritance

Most crops are genetically modified via insertion of genes into the nucleus. The genes can, therefore, spread to other crops or wild relatives by movement of pollen. By engineering tolerance to the herbicide glyphosate into the tobacco chloroplast genome, however, researchers not only have obtained high levels of transgene expression but also, because chloroplasts are inherited maternally in many species, have prevented transmission of the gene by pollen—closing a potential escape route for transgenes into the environment. Glyphosate (Roundup) is the most widely used herbicide in the world. It interferes with 5-enol-pyruvyl shikimate-3-phosphate synthase (EPSPS), an enzyme that is encoded by a nuclear gene and catalyzes a step in the biosynthesis of certain (aromatic) amino acids in the chloroplasts. Conventional strategies for producing glyphosate-tolerant plants are to insert, into the nucleus, an EPSPS gene from a plant or a glyphosate-tolerant bacterium (the bacterial gene is modified so that the enzyme is correctly targeted to the chloroplasts), or a gene that inactivates the herbicide.

Putting GMOs in Perspective

The prestigious Genetics Society of America has weighed in on the issue of GMOs. Part of its statement reads:

Every year, thousands of Americans become ill and die from food contamination. This is not a consequence of using GMOs, but instead reflects contamination from food-borne bacteria. “Natural” food supplements are widely used but are generally not well-defined, purified, or studied. Although some reports of contamination of corn meal by GMOs not approved for human consumption led to several claims of allergic response, to date, none of those individuals has been shown to contain antibodies to the GM protein.

Key terms

• genetically modified organisms (GMOs)plants and animals in which techniques of recombinant DNA have been used to introduce, remove, or modify specific parts of the genome of an organism
• hybridizationthe process of mating or crossing two genetically different individuals; the resultant progeny is called a hybrid
• introgressionthe transfer of genes from one species to another or the movement of genes between species (or other well-marked genetic populations) mediated by backcrossing
• transgenea gene introduced into a cell or organism by means other than sexual

Bibliography

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Brauer, Chris J., et al. "Natural Hybridization Reduces Vulnerability to Climate Change." Nature Climate Change, 2023, www.nature.com/articles/s41558-022-01585-1. Accessed 5 Sept. 2024.

Brown, Jack, and Peter D. S. Caligari. “Developing Hybrid Cultivars.” An Introduction to Plant Breeding. Ames: Blackwell, 2008. Print.

Galun, Esra, and Adina Breiman. Transgenic Plants. London: Imperial College P, 1997. Print.

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Kole, Chittaranjan. Wild Crop Relatives: Genomic and Breeding Resources: Plantation and Ornamental Crops. Heidelberg: Springer, 2011. Print.

Moran, Benjamin M. "The Genomic Consequences of Hybridization." eLife, 4 Aug. 2021, elifesciences.org/articles/69016. Accessed 3 Nov. 2022.

OECD. Safety Assessment of Transgenic Organisms: OECD Consensus Documents. Paris: OECD, 2010. Print.

Parekh, Sarad R., ed. The GMO Handbook: Genetically Modified Animals, Microbes, and Plants in Biotechnology. Totowa: Humana, 2004. Print.

Xu, Yunbi. Molecular Plant Breeding. Wallingford: CABI, 2010. Print.