Mutagenesis meets DNA transfer
Mutagenesis, the most basic form of DNA mutation, can be created by simply leaving a seed in direct sunlight for 48 hours or more. The heat and light break the DNA strand and the genes naturally recombine. Unlike in genetic engineering, these genes are not removed from another DNA strand and forced into a segment of the strand where they do not naturally “want” to go, and mutagenesis is a random approach. Ten seeds from the same tomato can mutate in 10 different ways. Early researchers would take several seeds from the same plant, expose them to the sun, plant each one, see the results at harvesttime and select the seeds from the plants with the most desirable new traits.
Similar to hybrid seeds, mutated seeds need several generations (eight years) to mature and stabilize into open pollinators. Some open-pollinated seeds on the market today may be the result of mutagenesis, but are not marked or marketed as such.
Today, seed mutation is created in two ways: via radiation or via chemical application. Radiation mutagenesis is done in a lab with microwaves, gamma rays, UV rays or other forms of radiation treatment. Chemical mutagenesis on seeds is also done in the lab, but with an alkaloid such as ethyl methane sulfonate (EMS), diethyl sulfate (DES) or dimethyl sulfate. The seed is soaked in the agent for a period of time to induce chemical modification of nucleotides (by breaking firm links within the genetic structure), resulting in changes.
Although the 11th Report on Carcinogens, put out by the U.S. National Toxicology Program at the Department of Health & Human Services in 2005, lists EMS, DES and dimethyl sulfate as potential carcinogens, Dr. Jeff Parker of Genome Alberta believes plants that are the result of chemical mutagenesis are safe and appropriate for organic growing. “They are appropriate in my estimation, as their use is many generations removed from the grower; as with all chemicals it is a matter of degree.”
Parker explains that because these chemicals are both mutagenic and probably carcinogenic, they should be handled with care in their pure form. “Many chemicals and environmental factors can increase the rate of mutation in living organisms: UV light, heavy metals, some fungicides and pesticides and even other plant-based, natural molecules,” he says. “It is all a matter of exposure and concentration. Don’t forget water, CO2, salt and other compounds are all toxic to people at high concentrations and exposures.”
Joel Reiten, seed production manager at Seeds of Change, says using toxic chemicals to create mutagenesis is less stressful and obtrusive to a seed than genetic engineering because it doesn’t force unnatural recombinations of genes. Once DNA is changed it’s inheritable, and the chemical used to induce mutagenesis is gone.
Certain types of seeds respond better to mutagenesis. Canola has been a popular plant for alteration. Polyploid seeds, such as today’s common strawberry, are difficult for researchers to work with using mutagenesis because scientists must make sure to alter each set of chromosomes within the seed to achieve the desired results. Diploid vegetables, such as broccoli and carrots, are generally easier to mutate using this process.
Reiten isn’t convinced that seeds altered via mutagenesis offer any value for farmers. Attempts to mutate vegetable seeds with chemicals or radiation have created such minor changes in the genetic structure that he believes the process ultimately has little value in vegetables. Still, he feels discussion among the organic community is warranted. “Mutagenics have been used in a lot of different cases over the years, and a lot of them haven’t been very successful. The only successes I am aware of were agronomic seeds (canola, barley, oats and flax), but I’m not aware of a single instance [in vegetable seeds] in which they found anything worth pursuing.”
Mutations naturally occur in seeds over time; essentially, mutagenesis is a way of accelerating nature’s process. Parker says the process is beneficial for its potential to create a larger array of new traits than a commercial grower or plant breeder may find in nature. However, because mutations are largely either neutral or negative, for every 10 positive traits created through mutagenesis, 100 undesirable traits may be created. Such negative traits are usually not threatening to a consumer or grower, as the plant breeder removes them before they end up in a commercial variety.
Despite advances in mutagenic technology, Parker sees mutagenesis as less efficient than other methods. “If I shoot, whether with radiation or with chemicals, at the 10 or 20,000 genes inside the plant and mutate 100 of them or more, a lot of those plants will just die. Of those that survive, a very small number may be useful or many may not be useful. It’s a lot of work to go through that process.”
In contrast to the “hit or miss” approach of mutagenesis is a method recently developed by San Diego-based Cibus Corporation: Rapid Trait Development System (RTDS). RTDS is considered mutagenesis, but is similar to genetic engineering. Unlike transgenic engineering, which takes genetic material from one species and inserts it into another, RTDS derives its genetic traits from the very same plant species being altered.
As a result, seeds altered via RTDS do not produce plants with surprise traits. “You can’t get there unless you know you’re gonna get there,” claims Keith Walker, president of Cibus, who cites one of the great benefits of RTDS as that lack of uncertainty. “That differentiates us from the guys who are doing random mutagenesis. There you have to sort everything out and throw away everything except the trait you want to have. On the same token, the transgenic guys moving hunks of DNA can’t scientifically guarantee the result. In our case, we know the change we’re putting in; it’s exact, it’s precise.”
To date, Cibus has focused its technology on canola seed. However, Walker says that RTDS can apply to all plants. He is excited about the possibilities that result from canola’s relationship to the vegetable brassica and the mustard family. Walker anticipates a transition in Cibus’ core activities from canola vegetable brassica by 2013.
The author is a freelance writer based in Massachusetts.