HYBRID SEEDS
A hybrid is created when two genetically different parent plants of the same species, are cross-pollinated. During pollination, pollen from the male fertilizes gametes from the female ovaries to produce offspring seeds. Genetic material from the male and female plants combine to form what is known as first generation (F1) hybrid seeds.
In nature:
Flowering plants have evolved various mechanisms in order to produce offspring with varied genetic traits for greater chance of survival in changing environments.
Dicliny is the occurrence of unisexual (as opposed to hermaphrodite) flowers. Dioecious plants carry male and female flowers on separate plants (as opposed to monoecious, which carry both on the same plant). This forces cross-pollination to take place.
Dichogamy is the temporal difference in anther and stigma maturity (male and female reproductive plant organs respectively), again encouraging cross-pollination. Protandry refers to dehiscence (maturing) of the anther before the stigma becomes receptive, while protogyny may be seen as the opposite scenario.
Self-incompatibility (rejection of pollen from the same plant) and herkogamy (spatial separation of anthers and stigma) ensures that self-fertilization is avoided.
Self-incompatibility is divided into heteromorphic and homomorphic types. Plants with distyle (2 types of flowers) or tristyle (3 types) heteromorphic flowers, exhibit visible differences in reproductive structures between each type. Only flowers of different types are compatible for pollination due to stigma and style heights. Homomorphic flowers, although morphologically the same (in appearance), have compatibilities controlled by genes. The more genetic similarity between pollen and ovules (female gametes), the more likely they are to be incompatible for fertilization.[i]
Commercial use:
Although hybridization occurs naturally in nature, it can be controlled by plant breeders to develop plants with a commercially desirable combination of traits. Examples are resistance to pests, diseases, spoilage, chemicals and environmental stresses such as drought and frost, as well as improvement of yield, appearance and nutrient profile.
Hybrids are produced in low-tech environments such as covered crop fields or greenhouses. Examples of new crops that exist only as hybrids include Canola, grapefruit, sweet corn, cantaloupes, seedless watermelons, tangelos, clementines, apriums and pluots. [ii]Hybrid crops were researched in the U.S. in the 1920s and by the 1930s, hybrid maize had become widely used.[iii]
Hybridization originated from the theories of Charles Darwin and Gregor Mendel in the mid-1800s. The very first method employed by farmers is known as corn detasseling, where mother corn plants’ pollen is removed and planted between rows of father plants, ensuring pollination only from father pollen. Thus the seeds harvested from the mother plants are hybrids. ii The manual removal of the plant’s male organ structures, is known as hand emasculation.
Sex modification is another method adopted by farmers in order to direct plant breeding. Sex expression can be controlled by changing factors such as plant nutrition, light and temperature exposure and phytohormones. Plant hormones such as auxins, etherl, erthephon, cytokinins and brassinosteroids, as well as low temperatures, cause a shift toward female sex expression. Hormone treatments of gibberellins, silver nitrate and pthalimide, as well as high temperatures, tend to favour maleness. i
Patenting and economic concerns
The F1 generation is a unique variety that, when crossed with its own generation to produce the F2 series, will result in plants with new, random genetic combinations of parent DNA. For this reason, the F1 seeds give their producers patenting rights, since the same seed has to be bought each year for planting.
Although beneficial, hybrid seeds are too expensive for use in developing countries, since the cost of seeds is coupled with the requirement of pricey machinery for fertigation and application of pesticides. The Green Revolution, a campaign aimed at spreading the use of hybrid seeds for increased food production, was actually economically detrimental in rural farming communities. The high maintenance costs involved, forced farmers to sell their land to agribusinesses, widening the gap between the rich and poor even more.
GM SEEDS
Recombinant DNA technology involves the splicing together of genes of organisms, even from different species (which could never breed in nature), to result in a ‘’transgenic’’ organism. Rather than sexual reproduction, expensive lab techniques are used to create the genetically modified organism, or ‘’GMO’’. ii
Methods:
Gene guns are the most common method of introducing foreign genetic material into the genomes of monocot crops such as wheat or maize. DNA is bound to gold or tungsten particles, which are accelerated at high energy levels and penetrate the cell wall and membranes, where the DNA integrates into the nucleus. A disadvantage is that cellular tissue damage may occur.[iv]
Agrobacteria are plant parasites that have the natural ability to transform plant cells by inserting their genes into plant hosts. This genetic information, carried on a ring of separate DNA known as a plasmid, code for tumour growth in the plant. This adaptation allows the bacterium to obtain nutrients from the tumour. Scientists use Agrobacterium tumefaciens as a vector to transfer desirable genes via the Ti (tumour-inducing) plasmid into dicotyledonous plant varieties, such as potatoes, tomatoes and tobacco. The T DNA (transforming DNA) integrates into the plant DNA and these genes are then expressed by the plant.[v]
Microinjection and electroporation are other methods of transferring genes into the DNA, the first directly and the second via pores. Recently CRISPR-CAS9 and TALEN technologies have emerged as even more precise methods of editing genomes.
DNA transfers also occur in nature, mainly in bacteria via mechanisms such as activity of transposons (genetic elements) and viruses. This is how many pathogens evolve to become antibiotic resistant. iv
Plant genomes are modified to include traits that cannot occur naturally in the species. These organisms are patented for use in the food and medicine industries, amongst other biotechnological applications, such as production of pharmaceuticals and other industrial products, biofuels and waste management. ii
Commercial use:
The first “GM” (genetically modified) crop was an antibiotic-resistant tobacco plant, produced in 1982. Field trials for herbicide-resistant tobacco plants in France and the USA followed in 1986 and a year later a Belgian company genetically engineered insect-resistant tobacco. The first GM food sold commercially was a virus-resistant tobacco that entered the People’s Republic of China’s market in 1992. iv The ‘’Flavr Savr’’ was the first GM crop sold commercially in the U.S. in 1994: a rot-resistant tomato developed by Calgene, a company which was later bought by Monsanto. The same year, Europe approved its first genetically engineered crop for commercial sales, a herbicide-resistant tobacco. ii
Tobacco, corn, rice and cotton plants have been modified by adding genetic material from the bacterium Bt (Bacillus thuringiensis) to incorporate the bacterium’s insect-resistant properties. Resistance to the cucumber mosaic virus, amongst other pathogens, has been introduced to papaya, potato and squash crops. ‘’Round-up Ready’’ crops such as soybeans, are able to survive exposure to the glyphosate-containing herbicide known as Round-up. Glyphosate kills plants by disrupting their amino acid-synthesizing metabolic pathways. iv
Plant nutrient profiles have been enhanced for human health benefits as well as improved livestock feed. Countries that rely seed and legume crops naturally lacking amino acids, produce GM seeds with higher levels of amino acids lysine, methionine and cysteine. Beta-carotene-enriched rice has been introduced in Asian countries where vitamin A deficiency is a common cause of eyesight problems in young children.
Plant pharming is another aspect of genetic engineering. This is the use of mass-grown modified plants for production of pharmaceutical products such as vaccines. Plants such as thale cress, tobacco, potato, cabbage and carrot are the most commonly used plants for genetic research and harvesting of useful compounds, since individual cells can be removed, altered and grown in tissue cultures to become a mass of undifferentiated cells called a callus. These callus cells have not yet specialized in function and may thus form an entire plant (a phenomena known as totipotency). Since the plant developed from a single genetically altered cell, the whole plant will consist of cells with the new genome and some of its seeds will produce offspring with the same introduced trait. v
Ethical debates and economic effects
By 1999, two-thirds of all U.S. processed foods contained GM ingredients. Since 1996, the total land surface area cultivating GMOs has increased 100-fold. GM technology has resulted in large increases in crop yields and farmer profits, as well as reduction in pesticide use, especially in developing countries. ii The founders of crop genetic engineering, namely Robert Fraley, Marc Van Montagu and Mary-Dell Chilton, were awarded the World Food Prize in 2013 for improving the “quality, quantity or availability” of food internationally. iv
The production of GMOs is still a controversial topic and countries differ in their regulation of patenting and marketing aspects. Concerns raised include safety for human consumption and the environment and the question of living organisms becoming intellectual property. The Cartagena Protocol on Biosafety is an international agreement on safety standards concerning the production, transfer and use of GMOs. ii