Information on
the Ballot Initiative in Humboldt County to Ban Genetically Modified Organisms
Mark S. Wilson, Humboldt State University, Department of Biology
Non-GMO Plant Breeding Techniques
By Kalei Colridge
Fall 2004, Genetics Lab
Introduction
In 1997 genetically modified foods were introduced to commercial agriculture in the form of herbicide resistant soybean seed (Farnham, Wang, and Wisner 2000). The seven years since have marked a major change in the way people worldwide look at food and its production. It has become an important issue for farmers, consumers, the government and world economies, as the safety and ethics of GMOs are debated. In response to the use of GMOs, and the overall distrust many consumers have toward them, there has been an explosion in the marketing of organic foods. All this talk about genetically modified foods and the increasing popularity of organic foods brings to the forefront an important question. How much do we know about the production of non-GMO/organic crops and can it be considered safer and more ethically sound in comparison to GMO plant production?
Throughout history crop production has been an ongoing process of altering the genotype of plants to improve their yield. It has been traditional for farmers after every season to harvest seeds from the plants that appear phenotypically superior, saving them to be planted the following season. After thousands of years of doing this food crops today are a far cry from the wild lineages they were derived from (Chrispeels and Sadava pg331). While wild lineages have undergone centuries upon centuries of natural selection producing successive generations of offspring adapted to the environment, domesticated species have undergone the pressures of hand selection. This results in observable differences between domesticated plants and their wild relatives. Todayıs crop plants have no natural seed dispersal mechanisms, nor seed dormancy periods to overcome seasonal weather conditions. Crop plants have been bred for similar growth habits so that at the time of harvest they are of uniform shape and size (Kimball, 2000). This is the reason for example that we do not see shrubby corn or viney wheat varieties. Congruency of this type among crop species has made it easier for farmers to develop universal harvest mechanisms. Gigantism is the term given to the huge difference in the size of the fruiting bodies of crop plants versus wild relatives, which is due to the selection of the seed from parent plants that produce large fruits (Chrispeels and Sadava pg342).
In the 1700ıs people began to cross plants with the intention of making crop plant varieties superior to those in existence, as opposed to the traditional and more passive method of hand picking seeds from superior plants. One example of this is the origin of the modern strawberry. In the 18th century French botanist Antoine Duchesne hand crossed in a green house two North American species of strawberry, Fragaria chiloensis, from the Pacific coast and Fragaria virginiana, from eastern North America (Chrispeels and Sadava pg351). The offspring of his cross possessed an increased fecundity and larger fruits. This kind of intentional plant breeding, called hybridization, became the mainstay for producing varieties of plants that combine the beneficial traits of both parents.
Over the years hybridization has evolved into the technique that is still used by some plant breeders today. Modern hybridizing technique consists of performing an initial cross, done by hand, followed by the selection of desirable first generation (F1) progeny. A series of backcrosses are then performed so that subsequent generations can be self-crossed in order to have offspring that are homozygous for the desired traits (King 2004). Interestingly most genes significant for plant breeding only have a few alleles, making hybridization a fairly straightforward process (Kimball 2004).
Todayıs grocery store varieties of plants, including those labeled organic, have been created using a combination of hybridization and some prominent laboratory techniques, including: induced polyploidy, tissue culture, embryo rescue, and mutagenesis. These methods are outlined below.
Polyploidy is an increase in the number of chromosomes of a plant. There are many types of polyploidy but two types are most capitalized on in plant breeding, allopolyploidy and autopolyploidy (King 2004). Autopolyploid individuals have multiple structurally identical copies of the same genome, and thus the ability for the chromosomes to recombine in an endless number of ways. Autopolyploids are also known to produce lusher plants with larger cell size, which contributes to larger fruiting bodies (University of Wisc. 2003).
Allopolyploids are plants that have multiple copies of different genomes. The different genotypes of allopolyploids maintain the plants as hybrids because the chromosomes of the genomes infrequently recombine, thus increasing and maintaining the hybrid vigor of these plants (University of Wisc. 2003). Polyploidy can happen naturally by spontaneous chromosome doubling or the union of diploid gametes. Contemporary cotton varieties are derived from one of two naturally occurring allopolyploids. In crop production polyploidy is often induced in the lab with the use of colchicines, a chemical that inhibits the formation of the mitotic spindle during replication, thus chromosomes double but do not separate (Hartl and Jones, pg 199). Doing this produces crop varieties that possess the positive traits associated with polyploidy, by doubling the number of chromosomes. This doubling of chromosomes tends to create new species, as plants cannot be backcrossed to their parents due to the difference in chromosome number.
Tissue culture is the technique used whereby tissue taken from a plant is sterilized and placed in a medium containing growth hormones that induce shoots to grow from the isolated tissue. These shoots can be constantly divided by hand to produce a large quantity of clones from the original plant, a method termed micropropagation. There are two other major innovations for applying tissue culture to breeding: embryo rescue, and anther or pollen culture (Linberger 2001).
Embryo rescue is the removal of the embryo from a plant followed by the tissue culture method previously outlined. Tissue from the embryo is removed and propagated. Embryo rescue is important in continuing strains of hybridized plants with low fertility rates or those that cannot produce viable seeds. Unless removed, embryos from these plants will die.
Pollen culture involves collecting pollen from the anther, which by default is male and thus haploid, and then culturing it. Any plants from these cultures showing desirable features are exposed to colchicine in order to make them diploid, allowing them to reproduce on their own.
Mutagenesis, the last method to be discussed is used to produce new crop plant varieties. It may be the most widely used and it is often combined with a number of the above techniques (Chrispeele and Sadava pg377). Instead of waiting for novel mutations of a gene to appear naturally, plants are exposed to a mutagen (either irradiation or a carcinogen). If the right amount of mutagen is administered then approximately half of the plants will die, while the others undergo primarily point mutations (King 2004). It is infrequent that surviving plants do not possess any mutations at all (King 2004). Plants are then screened to select for those that may have beneficial characteristics resulting from random mitations (approx 1:800) (King 2004). After screening, any possible positives are then cloned using tissue culture methods. While this is time consuming, in comparison to the amount of time it takes for novel mutations to occur naturally it is very rapid, and many mutations can be experimented with in a short period of time.
For the last fifty years the above techniques have been used to improve crops' agronomic traits (plant architecture, disease resistance, yield, size of fruit, time of flowering, growth rate, etc.), chemical composition (amount of oil, protein, amino acids, and starches), and reproductive factors (male sterility, self-incompatibility, increased reproductive success) (Chrispeele and Sadava pg379). Most widely used modern crop plants have been developed with these techniques. As of 2003 the International Atomic Energy Agency listed 2,252 new crop varieties now in use that were produced by mutagenesis, which is almost always combined with some type of micropropagation (Chrispeele and Sadava pg 382). Two examples of how some of the modern plant breeding techniques are used in conjunction with one another are higher yielding Japanese rice varieties and tritical, a grain species containing both wheat and rye chromosome. Dr. Toshiro Kinoshita and colleagues recently made major changes to rice crops in Japan. They exposed anther cultures to gamma irradiation. The resulting plantlets showed several new mutations for early maturation and dwarfness. Cells from the plantlets were extracted, micropropagated, and exposed to colchicine making the haploid cells diploid. Thus the plants regenerated form this experiment, were identical, homozygous for the new traits, and able to reproduce.
Tritical is a species derived from a wheat/rye hybrid bred created in the 1960s. The initial cross between the two produced sterile progeny because of an irregular number of chromosomes, attributed to lack of pairing of chromosomes during meiosis. Polyploidy was induced, by exposing embryos to colchicines, the increase in chromosome number resulted in some of the embryos being fertile. Since the first successful plants were produced breeders have made further laboratory crosses with tritical to yield the varieties used today. Tritical combines the high yield of wheat with the winter hardiness, adaptation to acidic soils and high protein of rye, and is widely grown throughout Eastern Europe (Chrispeele and Sadava pg 399).
By definition a GMO is an organism in which foreign DNA has been intentionally inserted into its genome, usually with the purpose of modifying the organism's phenotype in a way that will improve its economic value. The introduction of genetic modification by the insertion of specific genes into the genomes of plants has started to shift the industry away from the above practices, though they are still widely used. GMO plant breeding has come to be preferred by crop breeders and crop engineers because it avoids the use of the techniques outlined in this paper which are time consuming, and often use mutagenesis and colchicine which are harmful to people. Mutagens can be cancer causing, while colchicine according to the Department of Health is known to cause mutations, heart arrhythmia, vomiting, weakness, coma, pulmonary edema, shortness of breath and death. In contrast GMO engineering uses human safe techniques. Changes to a plant undergoing genetic modification are known and the location of the gene insert and the procedure is premeditated. In comparison, plant cultures exposed to high levels of carcinogenic chemicals and irradiation result in individual plants with unknown amounts of genotypic changes.
It is argued that GMOs are unnatural by many who oppose their use. In comparison to what, I ask? Historical plant hybridization practices have long been abandoned by large-scale plant breeders because it is very labor intensive and can take many years to obtain desired results. The foodstuffs that people consume whether organic or non organic were bred using one or many of the techniques outlined in this paper, which are no more or less unnatural than GMO production. Potentially harmful side effects of current plant production practices are no more or less known than are those of GMOs. Unlike the products from plant varieties produced by mutagenesis GMO products have to go through a long list of health inspections. An example of the many tests GMOs undergo is the test for allergenicity. Plant varieties that are created by mutagenesis do not have to go through such testing though they could possess a mutation due to the process that causes it to produce a human allergen.
Bad-mouthing GMOs seems to be more of an act of ignorance and lack of knowledge of the positive aspects of this type of crop production. More importantly it shows lack of knowledge of non-GMO crop production by the general public. It is hard to say the effect that current food production practices have on the consumer. It may be found that in fact GMO products are less of a threat to human health and our environment than other techniques, while at the same time improving methods of crop production that will continue to feed the world in the future.