Genetics Articles
Genetically Modified Organisms: Animals, Plants, Bacteria
Transgenic critters (genetically modified organisms) are all over the place. Animals, insects, and bacteria have all gotten in on the fun. In this article, you take a trip to the transgenic zoo to learn a little bit about the menagerie.
Transgenic animals
Mice were the organisms of choice in the development of transgenic methods. One of the ways that transgenic mice can be created is by inserting a transgene into a mouse’s genome during the process of fertilization. When a sperm enters an egg, there’s a brief period before the two sets of DNA (maternal and paternal) fuse to become one. The two sets of DNA existing during this intermission are called
pronuclei. Geneticists discovered that by injecting many copies of the transgene (with its promoter and sometimes with a marker gene, too) directly into the paternal pronucleus (see the following figure), the transgene was sometimes integrated into the embryo’s chromosomes.
If the transgene integrates into one of the mouse chromosomes when still at the one-cell stage, it will end up in all the cells of the mouse’s body. If it integrates after several rounds of cell division, then not all the embryo’s cells will contain the transgene. The cells that do have the transgene often have multiple copies (oddly, these end up together in a head-to-tail arrangement), and the transgenes are inserted into the mouse’s chromosomes at random. The resulting, partly transgenic mouse is mosaic. Mosaicism is when there are two population of cells in the animal. In this case, there is one that contains and expresses the transgene and one that does not. To get a fully transgenic animal, many mosaic animals are mated in the hope that non-mosaic transgenic offspring will be produced from one or more matings.
One of the first applications of the highly successful mouse transgenesis method used growth hormone genes. When introduced into the mouse genome, rat, human, and bovine growth hormone genes all produced mice that were much larger than normal. The result encouraged the idea that growth hormone genes engineered into meat animals would allow faster production of larger, leaner animals.
However, transgenic pigs with the human growth hormone gene didn’t fare very well; in studies, they grew faster than their nontransgenic counterparts but only when fed large amounts of protein. And female transgenic pigs turned out to be sterile. All the pigs showed muscle weakness, and many developed arthritis and ulcers. Unfortunately, cows didn’t fare any better. In contrast, fish do swimmingly with transgenes and transgenic salmon with the growth hormone gene grow six times faster than their nontransgenic cousins and convert their food to body weight much more efficiently, meaning that less food makes a bigger fish.
Primates have also been targeted for transgenesis as a way to study human disorders including aging, neurological diseases, and immune disorders. The first transgenic monkey was born in 2000. This rhesus monkey was endowed with a simple marker gene because the purpose of the study was simply to determine whether transgenesis in monkeys was possible. Since then, transgenic primates that model human disease, such as a transgenic monkey that can be used to model Huntington disease have been created and are showing great promise as a tool for studying treatments for these conditions.
Transgenic pets: Glow-in-dark fish
Ever have one of those groovy posters that glows under a black light? Well, move that black light over to the aquarium — there’s a new fish in town. Originally derived from zebrafish, a tiny, black-and-white–striped native of India’s Ganges River, these glowing versions bear a gene that makes them fluorescent. The little, red, glow-in-the-dark wonders (referred to as GloFish) are the first commercially available transgenic pets.
Zebrafish are tried-and-true laboratory veterans — they even have their own scientific journal! Developmental biologists love zebrafish because their transparent eggs make it simple to observe development. Geneticists use zebrafish to study the functions of all sorts of genes, many of which have direct counterparts in other organisms, including humans. And genetic engineers have taken advantage of these easy-to-keep fish, too; scientists in Singapore saw the potential to use zebrafish as little pollution indicators. The Singapore geneticists used a gene from jellyfish to make their zebrafish glow in the dark. The action of the fluorescent gene is set up to respond to cues in the environment (like hormones, toxins, or temperature). The transgenic zebrafish then provide a quick and easy to read signal: If they glow, a pollutant is present.
Of course, glowing fish are so unique that some enterprising soul couldn’t let lab scientists have all the fun. Thus, these made-over zebrafish have hit the market. Currently, GloFish are available in more than 10 different colors, including green, red, orange, pink, purple, or blue! Initially, when GloFish were introduced in 2003, the state of California banned their sale outright. However, they changed their decision in 2015, after GloFish sales were approved by the FDA, the U.S. Fish and Wildlife Service, and a variety of state-level regulators.
Transgenic insects
A number of uses for transgenic insects appear to be on the horizon. Malaria and other mosquito-borne diseases are a major health problem worldwide, but the use of pesticides to combat mosquito populations is problematic because resistant populations rapidly replace susceptible ones. And in fact, the problem isn’t really the mosquitoes themselves (despite what you may think when you’re being buzzed and bitten). The problem is the parasites and viruses the mosquitoes carry and transmit through their bites. In response to these problems, at least one research group has developed a transgenic mosquito in which expression of the transgene results in an early death. Research trials with these mosquitos in Brazil were designed to see if they may help decrease the population of mosquitoes that spreads dengue fever or the Zika virus. However, it is still unclear whether the release of these transgenic mosquitos is decreasing the population without the unintended consequence of creating hybrid mosquitos.
Another potential use for transgenic insects would be to release of millions of transgenically infertile bugs that attract the mating attentions of fertile ones. The matings result in infertile eggs, reducing the reproduction of the target insect population. This is an especially appealing idea when used to combat invasive species that can sweep through crops with economically devastating results.
Transgenic bacteria
Bacteria are extremely amenable to transgenesis. Unlike other transgenic organisms, genes can be inserted into bacteria with great precision, making expression far easier to control. As a result, many products can be produced using bacteria, which can be grown under highly controlled conditions, essentially eliminating the danger of transgene escape.
Many important drugs are produced by recombinant bacteria, such as insulin for treatment of diabetes, clotting factors for the treatment of hemophilia, and human growth hormone for the treatment of some forms of short stature. These sorts of medical advances can have important side benefits as well:
- Transgenic bacteria can produce much greater volumes of proteins than traditional methods.
- Transgenic bacteria are safer than animal substitutes, such as pig insulin, which are slightly different from the human version and may therefore cause allergic reactions.
To use bacteria to make human proteins for the treatment of disease, the gene for the human protein must be isolated. For example, to make human insulin, DNA is obtained from an insulin-producing pancreatic cell (see the following figure). The insulin gene is isolated and inserted into a bacterial plasmid. The plasmid containing the human gene is then introduced into
E. coli, which then acts as a factory for making the insulin protein. Transgenic bacteria are used to produce large quantities of insulin, which can then be purified and used to treat patients with diabetes.
Transgenic plants
Plants are really different from animals, but not in the way you may think. Plant cells are
totipotent, meaning that practically any plant cell can eventually give rise to every sort of plant tissue: roots, leaves, and seeds. When animal cells differentiate during embryo development, they lose their totipotency forever (but the DNA in every cell retains the potential to be totipotent). For genetic engineers, the totipotency of plant cells reveals vast possibilities for genetic manipulation.
Much of the transgenic revolution in plants has focused on moving genes from one plant to another, from bacteria to plants, or even from animals to plants. Like all transgenic organisms, transgenic plants are created to achieve various ends, including nutritionally enhancing certain foods (such as rice) or altering crops to resist either herbicides used against unwanted competitor plants or the attack of plant-eating insects.
Getting new genes into the plant
To put new genes into plants, genetic engineers can either:
- Use a vector system from a common soil bacterium called Agrobacterium. Agrobacterium is a plant pathogen that causes galls — big, ugly, tumor-like growths — to form on infected plants. In the figure, you can see what a gall looks like. Gall formation results from integration of bacterial genes directly into the infected plant’s chromosomes. The bacteria enters the plant from a wound such as a break in the plant’s stem that allows bacteria to get past the woody defense cells that protect the plant from pathogens (just as your skin protects you). The bacterial cells move into the plant cells, and once inside, DNA from the bacteria’s plasmids — the circular DNAs that are separate from the bacterial chromosome — integrate into the host plant’s DNA. The bacterial DNA pops itself in more or less randomly and then hijacks the plant cell to allow it to replicate.
Like the geneticists using virus vectors for gene therapy, genetic engineers snip out gall-forming genes from the Agrobacterium plasmids and replace them with transgenes. Host plant cells are grown in the lab and infected with the Agrobacterium. Because these cells are totipotent, they can be used to grow an entire plant — roots, leaves, and all — and every cell contains the transgene. When the plant forms seeds, those contain the transgene, too, ensuring that the transgene is passed to the offspring.
- Shoot plants with a gene gun so that microscopic particles of gold or other metals carry the transgene unit into the plant nucleus by brute force. Gene guns are a bit less dependable than Agrobacterium as a method for getting transgenes into plant cells. However, some plants are resistant to Agrobacterium, thus making the gene gun a viable alternative. With gene guns, the idea is to coat microscopic pellets with many copies of a transgene and by brute force (provided by compressed air) shove the pellets directly into the cell nuclei. By chance, some of the transgenes are inserted into the plant chromosomes.
Commercial applications
Transgenic plants have made quite a splash in the world of agriculture. So far, the main applications of this technology have addressed two primary threats to crops:
- Weeds: The addition of herbicide-resistant genes make crop plants immune to the effects of weed-killing chemicals, allowing farmers to spread herbicides over their entire fields without worrying about killing their crops. Weeds compete with crop plants for water and nutrients, reducing yields considerably. Soybeans, cotton, and canola (a seed that produces cooking oil) are only a few of the crop plants that have been genetically altered to tolerate certain herbicides.
- Bugs: The addition of transgenes that confer pest-killing properties to plants effectively reduces crop losses to plant-eating bugs. Geneticists provide pest-protection traits using the genes from Bacillus thuringiensis (otherwise known as Bt). Organic gardeners discovered the pesticide qualities of Bt, a soil bacterium, years ago. Bt produces a protein called Cry. When an insect eats the soil bacteria, digestion of Cry releases a toxin that kills the insect shortly after its meal. However, the Cry toxin is not toxic to animals and has been deemed safe for consumption by the FDA. Transgenic corn and cotton were the first to carry the Bt Cry Others now include potatoes, eggplants, soybeans, and tomatoes, although not all are commercially available yet.
Points of contention about GMOs
Few genetic issues have excited the almost frenzied response met by transgenic crop plants. Opposition to transgenic plants generally falls into four basic categories, including food safety issues, transgene escape, the development of resistance, and harming unintended targets.
Food safety issues
Normally, gene expression is highly regulated and tissue-specific, meaning that proteins produced in a plant’s leaves, for example, don’t necessarily show up in its fruits. Because of the way transgenes are inserted, however, their expression isn’t under tight control. Opponents to transgenics worry that proteins produced by transgenes may prove toxic, making foods produced by those crop plants unsafe to eat. Safety evaluations of transgenic crops rely on a concept called substantial equivalence.
Substantial equivalence is a detailed comparison of transgenic crop products with their nontransgenic equivalents. This comparison involves chemical and nutritional analyses, including tests for toxic substances. If the transgenic product has some detectable difference, that trait is targeted for further evaluation. Thus, substantial equivalence is based on the assumptions that any ingredient or component of the nontransgenic product is already deemed safe and that only new differences found in the transgenic version are worth investigating. For example, in the case of transgenic potatoes, unmodified potatoes are thought to be safe, so only the Bt that had been introduced was slated for further tests.
Escaped transgenes
The escape of transgenes into other hosts is a widely reported fear of transgenics opponents. Canola, a common oil-seed crop, provides one good example of how quickly transgenes can get around. Herbicide-resistant canola was marketed in Canada in 1996 or so. By 1998, wild canola plants in fields where no transgenic crop had ever been grown already had not one but
two different transgenes for herbicide resistance. This finding was quite a surprise because no commercially available transgenic canola came equipped with both transgenes. It’s likely that the accidental transgenic acquired its new genes via pollination. In 2002, several companies in the United States failed to take adequate precautions mandated by law to prevent the escape of corn transgenes via pollination or the accidental germination of untended transgenic seeds. These lapses resulted in fines — and the release of transgenes into unintended crops.
Developing resistance
The third major point of opposition to transgenics — the development of resistance to transgene effects — is connected to the widespread movement of transgenes. The point of developing most of these transgenic crops is to make controlling weeds or insect pests easier. Additionally, transgenic crops (particularly transgenic cotton) have the potential to significantly reduce chemical use, which is a huge environmental plus. However, when weeds or insects acquire resistance to transgene effects, the chemicals that transgenics are designed to replace are rendered obsolete. Full-blown resistance development depends on artificial selection supplied by the herbicide or the plant itself. Resistance develops and spreads when insects that are susceptible to the pesticide transgene being used are all killed. The only insects that survive and reproduce are, you guessed it, able to tolerate the pesticide transgene. Insects produce hundreds of thousands of offspring, so it doesn’t take long to replace susceptible populations with resistant ones.
Damaging unintended targets
The argument against transgenic plants is that nontarget organisms may suffer ill effects. For example, when Bt corn was introduced, controversy arose surrounding the corn’s toxicity to beneficial insects (that is, bugs that eat other bugs) and desirable creatures like butterflies. Indeed, Bt is toxic to some of these insects, but it’s unclear how much damage these natural populations sustain from Bt plants. The biggest threat to migratory monarch butterflies is likely habitat destruction in their overwintering sites in Mexico, not Bt corn.
