Meghan Hodges
Transgene pollution, also known as genetic pollution, is the uncontrolled spread of genetic information, specifically transgenes, into the genomes of organisms in which such genes are not present in nature. Transgenes are genes that have been transferred, either naturally or through genetic engineering, from one organism to another. This transfer can be done through various methods, including microinjection, retroviruses, stem cells, cloning, and biolistics. The introduction of a transgene can potentially alter the phenotype of an organism and its normal genetic code. Genetic pollution can occur when genetically engineered organisms are introduced into a new environment, and it is considered undesirable by conservation biologists and conservationists. Transgene pollution has been documented in GMO crops, such as maize in Mexico and traditional corn in Brazil, and has raised concerns about its negative environmental, social, and economic impacts.
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What You'll Learn
Transgene escape
The potential risks and impacts of transgene escape have been widely discussed and have slowed the full utilisation of gene technology in crop improvement. There is concern that the escape and spread of transgenes could lead to genetic pollution, negatively impacting the fitness of wild populations and resulting in undesirable outcomes such as outbreeding depression and the introduction of unwanted phenotypes, which can lead to extinction. Conservation biologists and conservationists are particularly concerned about the potential for gene flow from domestic, feral, and non-native species into wild indigenous species, which could lead to hybridization and genetic pollution.
Several methods have been proposed and developed to prevent and mitigate transgene escape. These include the use of genetic use restriction technologies (GURTs) such as the "'Terminator' method", which involves producing seeds that give rise to sterile plants, thereby preventing the movement of transgenes into wild populations as hybridization would not be possible. Physical containment measures, such as filters in labs, screens in greenhouses, and isolation distances in fields, have also been utilised to prevent the escape of transgenes.
In addition to containment methods, there has been a growing recognition of the importance of monitoring transgene escape. Population geneticists have developed tools and methods to analyse the genetic makeup of individuals in hybrid zones, estimate migration and selection patterns, and assess the fitness of hybrids. These tools can aid in understanding the dynamics of escaped transgenes and inform strategies to mitigate their unintended spread into wild populations.
Despite efforts to prevent and contain transgene escape, there have been reported incidents of transgenes migrating from agricultural to wild populations. For example, transgene escape has been reported from field trials of creeping bentgrass (Agrostis stolonifera) into natural populations of compatible Agrostis species in Oregon. These incidents highlight the ongoing challenges in managing and containing transgene escape, particularly as the adoption of transgenic crop species continues to expand globally.
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Transgene construction
The construction of a transgene requires the assembly of several key components. Firstly, a promoter sequence is necessary to regulate the activity of the transgene, determining when and where it is expressed. Secondly, the transgene must include an exon, which is a protein-coding sequence typically derived from the cDNA of the protein of interest. This coding sequence can be chosen from transgenes with previously known functions. Additionally, a stop sequence is required to terminate the transgene expression.
Transgenic constructs can vary in size, with BAC transgenic constructs ranging from 70-250Kb. These constructs can be injected in either linearized or circular form. The transgene should ideally be designed as a complete transcriptional unit, containing all the necessary elements for gene expression. Regulatory elements, such as 5' and 3' untranslated regions, are crucial for controlling the expression of the transgene and can be provided separately or included endogenously.
To enhance transgene expression, additional sequences can be included in the construct. For example, intron sequences have been shown to significantly augment transgene expression. Furthermore, strategies for detecting the transgene or its product, such as epitope tags, should be considered during the design process. The choice of DNA origin for constructing a transgene is also important, and the addition of tags or markers can facilitate the discrimination of the transgene-encoded protein.
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Transgenesis techniques
Transgenesis is a method to generate genetically modified animals (GMAs) by adding foreign genes or replacing genes in animal genomes. Transgenesis can be carried out in the gonads, sperm, fertilized eggs, and embryos through DNA microinjection, retroviruses, stem cells, and cloning. The first transgenic animal was a mouse, into which researchers introduced a metallothionein human growth hormone fusion gene (mT-hGH), resulting in dramatic growth. The first transgenic fish was a goldfish, into which a novel gene was inserted through microinjection into its fertilized egg.
Various methods for producing transgenic animals have been developed over the years. The procedures and methodologies used depend on the animal’s intended use. Transgenesis can be used to research gene function, serve as bioreactors, and create novel animal breeding techniques. For example, the pronuclear DNA microinjection technique has been the most effective method for producing transgenic pigs, although the success rate of this technique varies between species. Transgenesis can also be used to modify the composition of milk or produce novel proteins in milk.
To prevent the spread of transgenes, physical containment methods such as filters in labs, screens in greenhouses, and isolation distances in fields can be used. GeneSafe technologies have also introduced a method called "Terminator," which involves using seeds that produce sterile plants to prevent the movement of transgenes into wild populations.
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Transgene contamination
One well-known example of transgene contamination is the discovery of transgenes from genetically engineered (GE) maize in landraces of maize in Oaxaca, Mexico. This incident raised concerns about the irreversible contamination of the maize gene pool, as Mexico is the center of crop origin and diversity for maize. Other examples include the detection of transgenes in wild cotton in Mexico and the presence of up to seven transgenic genes in traditional corn seeds in Brazil, indicating cross-contamination.
To prevent transgene contamination, various strategies have been proposed and implemented. These include physical containment measures such as filters in labs, screens in greenhouses, and isolation distances in fields. Genetic use restriction technologies (GURTs) have also been developed, including the "Terminator" method, which involves creating seeds that produce sterile plants to prevent the spread of transgenes. However, this method has faced criticism for disproportionately impacting farmers in developing countries who rely on saved seeds for annual planting.
The potential consequences of transgene contamination are far-reaching and can include negative environmental, social, and economic impacts. Conservation biologists and conservationists are particularly concerned about the introduction of genes from domestic, feral, and non-native species into wild indigenous species, which can lead to hybridization and a loss of genetic purity.
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Transgene mitigation
TM strategies focus on compromising the fitness of weeds that have acquired positive survival traits from crop genes. The basic premises of TM are:
- Tandem constructs act as tightly linked genes, and their segregation from each other is rare.
- TM traits are neutral or positive for crops but deleterious for weeds.
- Mildly harmful TM traits will be eliminated from weed populations due to strong competition and large seed output.
Examples of processes targeted by TM include seed dormancy, seed ripening, shattering, and growth. By targeting these processes, TM aims to reduce the fitness of weeds and decrease the possibility of transgene propagation.
While TM shows potential in crop-weed systems, field studies have not yet validated its effectiveness in minimizing gene flow. To ensure the retention of the TM trait, two flanking TM genes can be used to lower the probability of losing the trait. Additionally, an RNA interference cassette can be included in the transgenic construct to allow for the targeted killing of transgenic plants through herbicide application.
Other strategies for transgene containment include maternal inheritance, male sterility, seed sterility, apomixis, cleistogamy, genome incompatibility, and fruit-specific excision of transgenes. These approaches aim to prevent the spread of transgenes to non-target organisms and reduce concerns about their impact on biodiversity and non-GM crops.
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Frequently asked questions
Transgene pollution is the uncontrolled spread of transgenes, or genes that have been transferred from one organism to another, into the genomes of organisms in which such genes are not present in nature. Transgene escape has been documented for GMO crops since 2001.
Transgene pollution can occur through various means, including pollen spread, seed escape, and the mixing of food and feed. Transgenes can also escape through hybridization with wild relatives.
Transgene pollution can have negative environmental, social, and economic impacts. It can lead to outbreeding depression and the introduction of unwanted phenotypes, which can cause the extinction of certain species. Transgene pollution can also reduce the resilience of ecosystems, making them more susceptible to disturbances such as toxic pollutants or species invasions.
