GM Crop Database

Database Product Description

DK404SR

Host Organism: Zea mays (Maize)

Trait: Cyclohexanone herbicide tolerance, specifically sethoxydim.

Trait Introduction: Selection of somaclonal variants from embryo cultures.

Proposed Use: Production for human consumption and livestock feed.

Product Developer: BASF Inc.

Summary of Regulatory Approvals

Country	Food	Feed	Environment	Notes
Canada	1997	1996	1996

Introduction Expand

The maize hybrid DK404SR was developed to allow the use of sethoxydim, the active ingredient in the herbicide Poast®, for the control of annual and perennial grasses in maize crops. DK404SR originated from a somaclonal variant selected from embryo tissue and expresses a modified version of acetyl-CoA-carboxylase (ACCase) enzyme.

ACCase is a key enzyme in the fatty acid biosynthesis pathway, necessary for the synthesis and maintenance of membranes and for the incorporation of fatty acids into triacylglycerides. The enzyme is inhibited by cyclohexanedione herbicides, such as sethoxydim, resulting in a lethal disruption of lipid biosynthesis. The mutant ACCase gene encodes for a modified version of acetyl-CoA-carboxylase enzyme in which the sethoxydim binding site is altered such that the ACCase enzyme is not inhibited by the sethoxydim herbicide, yet still retains its normal catalytic properties in fatty acid synthesis. The modified maize plants also demonstrated some level of cross-tolerance to the related herbicides, aryloxyphenoxypropionates, including haloxyfop (not registered in Canada), fluazifop (Venture) and quizalofop (Assure). These herbicides all have the same mode of action through ACCase inhibition. Natural tolerance to sethoxydim herbicide is found in several broadleaved plants and grasses and includes annual ryegrass, green foxtail and red fescue.

Summary of Introduced Genetic Elements Expand

Code	Name	Type	Promoter, other	Terminator	Copies	Form
ACCase	acetyl-CoA-carboxylase	MUT	N/A N/A	N/A	single partially dominant allele	No introduced genetic material. Sethoxydim selection.

Characteristics of Zea mays (Maize) Expand

Center of Origin	Reproduction	Toxins	Allergenicity
Mesoamerican region, now Mexico and Central America	Cross-pollination via wind-borne pollen is limited, pollen viability is about 30 minutes. Hybridization reported with teosinte species and rarely with members of the genus Tripsacum.	No endogenous toxins or significant levels of antinutritional factors.	Although some reported cases of maize allergy, protein(s) responsible have not been identified.

Modification Method Expand: The maize hybrid DK404SR was developed from sethoxydim tolerant inbred lines. The original sethoxydim tolerant mutant line (S2) was selected as a somaclonal variant from maize embryo tissue grown under sethoxydim selective pressure. The process involved growing somatic embryos on sethoxydim enriched media. From the somatic embryos that survived, the somaclonal variant cell line S2 was selected and subsequently regenerated. This S2 line was backcrossed at least six times with both parental lines of the hybrid DK404SR to transfer the sethoxydim tolerant trait.

Characteristics of the Modification Expand

The Introduced DNA

The novel herbicide tolerant trait introduced into the maize hybrid DK404SR was achieved through a somaclonal mutation within the maize genome. There was no new genetic material introduced into the maize hybrid DK404SR as a result of the modification.

Genetic Stability of the Introduced Trait

Expression of the herbicide tolerance trait was consistently displayed by DK404SR plants that were at least six generations away from the original S2 line.

The sethoxydim tolerance trait was inherited as a single partially dominant allele, as determined from the original mutant S2 line in which regenerated S2 plants were heterozygous for the mutant allele. The heterozygous progeny expressed herbicide tolerance at lower levels compared to homozygous plants.

Expressed Material

The activity of the ACCase enzyme in unmodified maize and homozygous seedlings derived from the S2 line was shown to be similar in the absence of sethoxydim herbicide. Application of sethoxydim herbicide inhibited the activity of ACCase by 50% in modified maize plants when applied at a concentration 100 fold higher than the sethoxydim concentration required to achieve a similar level of inhibition of ACCase activity in unmodified maize plants.

Environmental Safety Considerations Expand

Field Testing

Field testing of DK404SR maize hybrids demonstrated that agronomic characteristics such as: plant size and vigour, growth, male and female fertility, time to maturity, flowering period, and seed yield, were within the range of values displayed by currently commercialized hybrids. It was determined that the growing habit of the modified maize plants was not inadvertently altered and that DK404SR maize hybrids did not display any altered pest potential.

Outcrossing

Since pollen production and viability were unchanged by the genetic modification resulting in maize hybrid DK404SR, pollen dispersal by wind and outcrossing frequency should be no different than for other maize hybrids. Gene exchange between DK404SR and other cultivated maize hybrids will be similar to that which occurs naturally between cultivated maize varieties at the present time. In Canada, where there are no plant species closely-related to maize in the wild, the risk of gene flow to other species is remote. Cultivated maize, Zea mays L. subsp. mays, is sexually compatible with other members of the genus Zea, and to a much lesser degree with members of the genus Tripsacum. However, none of the sexually compatible relatives of maize in Canada are considered to be weeds in Canada, and it is therefore unlikely that introgression of the modified ACCase gene would provide a selective advantage to these populations as they would not be routinely subject to herbicide treatments.

Weediness Potential

No competitive advantage was conferred to maize line DK404SR, other than that conferred by resistance to sethoxydim herbicide. Resistance to Poast® herbicide will not, in itself, render maize weedy or invasive of natural habitats since none of the reproductive or growth characteristics were modified.

Cultivated maize is unlikely to establish in non-cropped habitats and there have been no reports of maize surviving as a weed. In agriculture, maize volunteers are not uncommon but are easily controlled by mechanical or by using herbicides that are not based on sethoxydim as appropriate. Zea mays is not invasive and is a weak competitor with very limited seed dispersal.

Secondary and Non-Target Adverse Effects

The novel trait present in the maize hybrid DK404SR was a mutation within a single corn enzyme, that altered the sethoxydim binding site without modification of metabolic abilities. Therefore, the potential toxicity of the ACCase enzyme was not a concern. It was determined that the maize hybrid DK404SR would not result in altered impacts on interacting organisms, including humans, compared with currently commercialized counterparts.

Impact on Biodiversity

Maize hybrid DK404SR has no novel phenotypic characteristics that would extend its use beyond the current geographic range of maize production. Since the risk of outcrossing with wild relatives in Canada is remote, it was determined that the risk of transferring genetic traits from DK404SR to species in unmanaged environments was insignificant.

Food and/or Feed Safety Considerations Expand

Dietary Exposure

Maize hybrid DK404SR is a yellow dent type used primarily for animal feeding. However, such field corn may be dry- or wet-milled into various processed maize products, such as fructose maize syrups, starch, oil, grits and flour for human food use. The human food uses of grain from DK404SR maize hybrid are not expected to be different from the uses of unmodified maize hybrids. As such, the dietary exposure of consumers to grain from DK404SR maize hybrids will not be different from that for other commercially available field corn varieties.

Nutritional Data

Nutritional composition for the modified and unmodified maize was compared. The selection of the DK404SR maize hybrid through somaclonal variation had no meaningful effect on the corn plant nutrient levels. The use of maize products derived from DK404SR maize hybrid would therefore have no significant impact on the nutritional quality of the food supply.

Toxicity

The novel trait in DK404SR maize hybrid results from modifications of a single maize enzyme, thus altering the sethoxydim binding site without modifying the metabolic abilities of ACCase enzyme. Potential toxicity of this enzyme was therefore not a concern.

Abstract Collapse

As the development of DK404SR did not employ recombinant DNA technologies, this product was not subject to regulation in any jurisdiction except Canada, where it was regulated as a plant with novel trait under the Seeds Act, a novel feed under the Feeds Act, and as a novel food under the Food and Drug Regulations. Specific environmental, livestock feed, and food safety assessment of this product was only required for Canada.

Only a small amount of whole maize kernel is consumed by humans. Maize oil is extracted from the germ of the maize kernel and maize is also a raw material in the manufacture of starch. A complex refining process converts the majority of this starch into sweeteners, syrups and fermentation products, including ethanol. Refined maize products, sweeteners, starch, and oil are abundant in processed foods such as breakfast cereals, dairy goods, and chewing gum.
In the United States and Canada maize is typically used as animal feed, with roughly 70% of the crop fed to livestock, although an increasing amount is being used for the production of ethanol. The entire maize plant, the kernels, and several refined products such as glutens and steep liquor, are used in animal feeds. Silage made from the whole maize plant makes up 10-12% of the annual corn acreage, and is a major ruminant feedstuff. Livestock that feed on maize include cattle, pigs, poultry, sheep, goats, fish and companion animals.

Industrial uses for maize products include recycled paper, paints, cosmetics, pharmaceuticals and car parts.

The maize hybrid DK404SR was developed to allow the use of sethoxydim, the active ingredient in the herbicide Poast®, for the control of annual and perennial grasses in maize crops. The original sethoxydim-tolerant mutant line (S2), which was the source of the sethoxydim tolerance trait in hybrid DK404SR, was selected as a somaclonal variant from maize embryo tissue grown under sethoxydim-selective pressure. The process involved growing somatic embryos on sethoxydim-enriched media in the presence of mutagenic agents. The somaclonal variant cell line S2 was selected from the resulting mutant cells and subsequently regenerated. This S2 line was backcrossed at least six times with both parental lines of the hybrid DK404SR to transfer the herbicide tolerance trait.

Sethoxydim and other cyclohexanedione herbicides act by inhibiting the enzyme acetyl-CoA-carboxylase (ACCase), a key enzyme in the fatty acid biosynthesis pathway necessary for the synthesis and maintenance of membranes and for the incorporation of fatty acids into triacylglycerides. Inhibition of this enzyme results in a lethal disruption of lipid biosynthesis. Cyclohexanedione herbicides applied at rates recommended for effective weed control are toxic to conventional maize varieties. The ACCase present in DK404SR contains a mutation within the sethoxydim binding site that reduces the herbicide’s affinity for the enzyme, and thus its inhibitory activity, while retaining the normal catalytic properties of the enzyme. The modified maize plants also demonstrated some level of cross-tolerance to the related herbicides, aryloxyphenoxypropionates, including haloxyfop (not registered in Canada), fluazifop (Venture®) and quizalofop (Assure®). These herbicides all have the same mode of action through ACCase inhibition. Natural tolerance to sethoxydim herbicide is found in several broad-leaved plants and grasses, including annual ryegrass, green foxtail and red fescue.

As the development of DK404SR did not employ recombinant DNA technologies, this product was not subject to regulation in any jurisdiction except Canada, where it was regulated as a plant with novel trait under the Seeds Act, a novel feed under the Feeds Act, and as a novel food under the Food and Drug Regulations. Specific environmental, livestock feed, and food safety assessment of this product was only required for Canada.

Field testing of DK404SR maize hybrids demonstrated that agronomic characteristics such as plant size and vigour, growth, male and female fertility, time to maturity, flowering period, and seed yield were within the range of values displayed by currently commercialized hybrids. It was determined that the growing habit of the modified maize plants was not inadvertently altered and that they did not exhibit weedy characteristics, or negatively affect beneficial or non-target organisms. DK404SR was not expected to impact on threatened or endangered species.

Maize does not have any closely related species growing in the wild in Canada. Cultivated maize can naturally cross with annual teosinte (Zea mays ssp. mexicana) when grown in close proximity, however, these wild maize relatives are native to Central America and are not naturalized in North America. Additionally, reproductive and growth characteristics were unchanged in DK404SR. Gene exchange between DK404SR and maize relatives was determined to be negligible in managed ecosystems, with no potential for transfer to wild species in Canada.

A food and livestock feed safety assessment of DK404SR compared the nutritional composition of the modified maize line to that of its unmodified counterparts, revealing no significant differences. The selection of the DK404SR maize hybrid through somaclonal variation had no meaningful effect on the maize plant nutrient levels. It was therefore determined that the use of maize products derived from the DK404SR maize hybrid would have no significant impact on the nutritional quality of the food supply in Canada.

The novel trait in the DK404SR maize hybrid results from modifications of a single maize enzyme, altering the sethoxydim binding site without modifying the metabolic abilities of the ACCase enzyme. Potential toxicity of this enzyme was therefore not a concern. No new genetic material was introduced into the maize genome during the modification process, minimizing potential toxicity and allergenicity concerns.

Links to Further Information Expand

Canadian Food Inspection Agency, Plant Biotechnology Office

Decision Document 96-13: Determination of Environmental Safety of BASF Canada Inc.'s Sethoxydim Tolerant Corn (Zea mays L.) Hybrid DK404SR
[PDF Size: 138.41K bytes]
Supplement to Decision Document DD96-13:Determination of Livestock Feed Safetyof BASF Canada Inc.'s SethoxydimTolerant Corn (Zea mays L.) Hybrid DK404SR
[PDF Size: 53.43K bytes]

Office of Food Biotechnology, Health Canada

NOVEL FOOD INFORMATION - FOOD BIOTECHNOLOGYSETHOXYDIM TOLERANT CORN (DK412SR AND DK404SR)
[PDF Size: 8.05K bytes]

This record was last modified on Friday, March 26, 2010

Product Related Info

Biology Documents

Maize biology document
Biology of Maize L.
General Description

Maize is a tall, monoecious, annual grass varying in height from 1 to 4 meters (Watson & Dallwitz 1992). The main stem is made up of clearly defined nodes and internodes. Internodes are wide at the base and gradually taper to the terminal inflorescence at the top of the plant. Leaf blades are found in an alternating pattern along the stem. Maize is a unique grass as both male and female flowers are borne on the same plant but are located separately. The tassels or staminate (male) inflorescence form large spreading terminal panicles that resemble spike-like racemes. Pollen is shed from the tassel and is viable for approximately 10 to 30 minutes as it is rapidly desiccated in the air (Kiesselbach 1980). Maize plants shed pollen for up to 14 days. The reproductive phase begins when one or two auxiliary buds, present in the leaf axils, develop and form the pistillate inflorescence or female flower (Purseglove 1972). The auxiliary bud starts the transformation to form a long ‘cob’ on which the flowers will be borne. From each flower a style begins to elongate towards the tip of the cob in preparation for fertilization. These styles form long threads, known as silks. The base of the silk is unique, as it elongates continuously until fertilization occurs (Purseglove 1972). Styles may reach a length of 30 cm, the longest known in the plant kingdom. Individual maize kernels, or fruit, are unique in that mature seed is not covered by floral bracts (glumes, lemmas, and paleas) as in most other grasses, but rather the entire structure is enclosed and protected by large modified leaf bracts, collectively referred to as the ear (Hitchcock and Chase 1951). The mature female flowers will remain ready for fertilization for up to two weeks, at which point if fertilization has not occurred, the nucleus will de-organize and fertilization will no longer be possible (Hitchcock & Chase 1951). The pollen of maize, a protandrous plant, matures before the female flower is receptive (Purseglove 1972). This may have been an ancient mechanism to ensure cross-pollination, but is no longer considered conducive to modern agricultural practices. However, decades of conventional selection and improvement have produced many maize varieties with similar maturities for both male and female flowers, to ensure seed set for agricultural proposes.

Reproduction

Under natural conditions, maize reproduces only by seed production. Pollination occurs with the transfer of pollen from the tassels to the silks of the ear. About 95% of the ovules are cross-pollinated and about 5% are self-pollinated (Poehlman 1959), although plants are completely self-compatible. Maize, as a thoroughly domesticated plant, has lost all ability to disseminate its seeds and relies entirely on the aid of man for its distribution (Stoskopf 1985). The kernels are tightly held on the cobs and if ears fall to the ground, so many competing seedlings emerge that the likelihood that any will grow to maturity is extremely low. Maize cannot reproduce asexually by natural means. It is possible to reproduce maize using tissue culture techniques, however, it has proven extremely difficult with a low rate of success (Hoisington et al. 1998).

Classification and Phylogeny of Maize

Zea is a genus belonging to the grass family, Poaceae, in the Andropogoneae tribe, but is commonly cited in literature to belong to the tribe Maydeae. Zea (zeia) was derived from an old Greek name for a food grass. The genus Zea consists of four species of which only Zea mays ssp. mays L. is economically important. The other Zea species, referred to as teosintes, are largely wild grasses native to Mexico and Central America (Doebley 1990). The number of chromosomes in Zea mays is 2n=20, 21, 22, 24 (FAO 2000c). The tribe Maydeae consists of seven genera: two of American origin, Zea and Tripsacum; and five of Eastern origin which extend from India through Southeastern Asia to Australia and include Coix, Sclerachne, Polytoca, Chionachne and Trilobachne (Watson & Dallwitz 1992). It is generally agreed that maize phylogeny was largely determined by the American genera Zea and Tripsacum, however it is accepted that the genus Coix contributed to the phylogenetic development of the species Z. mays (Radu et al. 1997). The genus Manisuris, from the tribe Andropogoneae, may also have contributed to the evolution of Zea mays (Eubanks 1997a; Radu et al. 1997).

Center of Origin and Progenitors of Maize

The center of origin for Zea mays ssp. mays has been established as the Mesoamerican region, now Mexico and Central America (Watson & Dallwitz 1992). The exact period of domestication and the ancestors from which maize arose are unclear. Archaeological records suggest that domestication of maize began at least 6000 years ago, occurring independently in regions of the southwestern United States, Mexico, and Central America (Mangelsdorf 1974). The origins of domesticated maize have been difficult to trace as hybridization events in its evolution are thought to have involved a now extinct wild maize ancestor of which little evidence has been found (Eubanks 1997a).

Germplasm Diversity

A study in 1992 concluded that maize landraces occupied 42% of the land under production in less developed countries (México 1994). Mexico and Central America are the only regions where ancient maize lines, such as pod corn, are found (Mink & Dorosh 1985). The survival of maize landraces has been attributed to the integral role maize sustains in small communities where it has very specific uses for food and other special functions. These landraces have been well characterized in Latin America, and germplasm not yet evaluated is known to be present in the region. There is a growing trend in developing countries to adopt improved maize varieties, primarily to meet market demand. In Mexico, only 20% of the corn varieties grown 50 years ago remain in cultivation (World Watch Institute 2000). The narrowing of genetic diversity in modern maize varieties emphasizes the importance of conserving genetic traits for future plant breeding. Leading the way in preserving maize germplasm is CIMMYT (International Maize and Wheat Improvement Centre), which has the world’s largest collection of maize accessions, with over 17,000 lines (CIMMYT 2000). Collections of distant maize relatives native to Asia have been recommended by conservation experts to ensure their preservation for future research (Sharma 1998).

Relatives of Maize

The closest wild relatives of maize are the teosintes which all belong to the genus Zea. The teosintes are wild grasses native to Mexico and Central America and have limited distribution (Mangelsdorf et al. 1981). Teosinte species show little tendency to spread beyond their natural range and distribution is restricted to North, Central and South America (Watson & Dallwitz 1992). The nearest teosinte relative to Z. mays ssp. mays is Z. mays ssp. mexicana (Schrader) Iltis (previously classified as Euchlaena mexicana, Zea mexicana) (2n = 20). This Central Mexican annual teosinte is a large flowered, mostly weedy grass with a broad distribution across the central highlands of Mexico. It does not spread readily. It has limited use as a forage and green fodder crop, but can be problematic due to weedy tendencies (Doebley 1990, Watson & Dallwitz 1992). The closest wild relatives to maize outside the Zea genus are from the genus Tripsacum. The genus Tripsacum is comprised of about 12 species that are mostly native to Mexico and Guatemala but are widely distributed throughout warm regions in the USA and South America, with some species present in Asia and Southeast Asia (Watson & Dallwitz 1992). All species are perennial, warm season grasses.

Outcrossing

Gene flow from maize can occur by two means: pollen transfer and seed dispersal. Seed dispersal can be readily controlled in maize as domestication has all but eliminated any seed dispersal mechanisms that ancestral maize may have previously used (Purseglove 1972). The kernels are held tightly on the cobs and if the ear falls to the ground, competing seedlings limit growth to maturity (Gould 1968). Pollen movement is the only effective means of gene escape from maize plants. Pollen is released from the tassels at the top of the plant and transported by wind to the female flowers located on the stalk. Pollen shed occurs over a 14 day period, with a peak during the first 5 days of shed (Sears 2000). The stigmas are receptive for a large part of this two week interval (Kiesselbach 1980). Insects, such as bees, have been observed to collect pollen from maize tassels, but they do not play a significant role in cross-pollination as there is no incentive to visit the female flowers (Rayor et al. 1972).

Wind speed and direction affect pollen distribution

Differences in flowering dates among commercial maize varieties are small. Cross-pollination between varieties may occur if grown in adjacent fields. The limited viability of maize pollen reduces the risk of cross-pollination, as a receptive host must be found within the 30 minutes that the pollen remains biologically active. Cross-pollination is also affected by the concentration of maize pollen released; pollen produced by a maize crop will successfully compete with foreign pollen sources when present in higher concentrations (Rayor et al. 1972). The isolation of crops using separation distances and physical barriers are common techniques for restricting gene flow and ensuring seed purity for maize seed production. Cross-pollination is controlled in seed lots by separating different lines. The OECD standards require a minimum 200 m (660 ft) isolation distance for maize seed production. This distance has been estimated to maintain inbred lines at 99.9% purity. Physical barriers, such as trees or barrier rows, are also effective in reducing cross-pollination. Detasseling or bagging the tassel effectively prevents pollen movement and limits gene flow.

Intraspecific hybidization

All forms of Zea mays spp mays, such as dent, sweet, and pop corn, freely cross pollinate forming fertile hybrids (Pursglove 1972).

Interspecific hybridization

All teosintes can be crossed to maize and form fertile hybrids, except for the tetraploid Z. perennis. Maize-teosinte hybrids exhibit low fitness and have little impact on gene introgression in subsequent generations (Galinat 1988). The tendency to form natural hybrids differs among teosintes: Z. luxurians rarely hybridizes with maize, whereas Z. mays ssp. mexicana frequently forms hybrids (Wilkes 1977). Molecular data confirms that there is gene flow between maize and teosinte and suggests that introgression of maize and teosinte occurs in both directions but at low levels (Doebley 1990).

Intergeneric hybridization Tripsacum species (T. dactyloides, T. floridanum, T. lanceolatum, and T. pilosum) have been successfully hand crossed with maize to form hybrids. The hybrids generally have 28 chromosomes, 10 from maize and 18 from Tripsacum, and are pollen sterile with limited female fertility (Mangelsdorf & Reeves 1939, Mangelsdorf 1974). This infertility is common in such wide crosses because of differences in chromosome number and lack of pairing between chromosomes (Eubanks 1997b). Maize readily crosses with hexaploid wheat (Triticum aestivum) with high frequencies of fertilization and embryo formation (plant breeders use maize pollen to develop double haploids of hexaploid wheat). However, maize chromosomes are eliminated from the genome during the initial stages of meiosis and result in haploid embryos. There is little evidence to suggest fertile hybrids between maize and hexaploid wheat could be produced in nature. There have been unsubstantiated reports of hybridization between maize and sugar cane (Saccaharum sp.).

Biology References
1. CIMMYT (International Maize and Wheat Improvement Centre) (2000). CGIAR Research, Areas of Research: Maize (Zea mays L.) http://www.cgiar.org/areas/maize.htm
2. Doebley, J. F. (1990). Molecular Evidence for Gene Flow among Zea species. BioScience 40, 443- 448.
3. Eubanks, M.W. (1997a). Further evidence for two progenitors in the origin of maize. Maize Newsletter 71, 35-35.
4. FAO (2000c). Species Description. Zea mays L. http://www.fao.org/WAICENT/faoinfo/agricult/agp/agpc/doc/gbase/data/pf000342.htm
5. Galinat, W.C. (1988). The origin of corn. In: Corn and Corn Improvement. Agronomy Monographs No.18. American Society of Agronomy, G.F Sprague and J.W. Dudley, (eds.). Madison, Wisconsin, pp. 1-31.
6. Gould, F.W. (1968). Grass Systematics. McGraw Hill, N.Y., USA.
7. Hitchcock, A.S. & A. Chase. (1951). Manual of the Grasses of the United States (Volume 2). Dover Publications: N.Y. p. 790-796.
8. Hoisington, D., Listman, G.M. & Morris, M.L. (1998). Varietal development: applied biotechnology. In: Maize Seed Industries in Developing Countries. M. L. Morris (ed). Lynne Rienner Publishers, Inc. pp. 77-102.
9. Kiesselbach, T.A. (1980). The structure and reproduction of corn. Reprint of: Research Bulletin No. 161. 1949. Agricultural Experiment Station, Lincoln, Nebraska. University of Nebraska Press. p. 93.
10. Laurie, D.A. & Bennett, M.D. (1986). Wheat x maize hybridization. Can. J. Genet. Cytol. 28, 313-316.
11. Mangelsdorf, P. C. & Reeves, R.G. (1939). The origin of Indian corn and its relatives. Texas Agric. Expt. Sta. Bull. 574.
12. Mangelsdorf, P. C. (1974). Corn: its Origin, Evolution and Improvement. Harvard Univ. Press, Cambridge, Mass.
13. Mangelsdorf, P. C., Roberts, L.M. & Rogers, J.S. (1981). The probable origin of annual teosintes. Bussey Inst., Harvard Univ. Publ. 10, 1- 69.
14. México, D.F. (1994). Maize seed industries revisited: emerging roles of the public and private sectors. World Maize Facts and Trends1993/94. CIMMYT.
15. Mink, S. D. & Dorosh, P.A. (1987). An overview of corn production. In: The Corn Economy of Indonesia. Cornell University Press, London.
16. Poehlman, J.M. (1959). Breeding Field Crops. Holt, New York, USA.
17. Purseglove, J.W. (1972). Tropical Crops: Monocotyledons 1. Longman Group Limited., London.
18. Radu, A, Urechean, V., Naidin, C. & Motorga, V. (1997). The theoretical significance of a mutant in the phylogeny of the species Zea mays L. Maize Newsletter 71, 77-78.
19. Rayor, G.S., Ogden, E.C. & Hayes, J.V. (1972). Dispersion and deposition of corn pollen from experimental sources. Agronomy Journal 64, 420-427.
20. Sears, M.K., Stanley-Horn, D.E. & Matilla, H.R. (2000). Ecological impact of Bt corn pollen on Monarch butterfly in Ontario. Canadian Food Inspection Agency (http://www.cfia-acia.agr.ca/english/ plaveg/pbo/btmone.shtml)
21. Sharma, A.B. (1998). Experts for conservation of wild crop varieties. The Financial Express. Indian Express Group.
22. Stoskopf, N.C. (1985). Cereal Grain Crops. Reston, Virginia: Reston Publishing Company, Inc., Prentice-Hall Company.
23. Watson, L. & Dallwitz, M.J. (1992). Grass Genera of the World: Descriptions, Illustrations, Identification, and Information Retrieval; including Synonyms, Morphology, Anatomy, Physiology, Phytochemistry, Cytology, Classification, Pathogens, World and Local Distribution, and References. Version:18th August 1999. http://biodiversity.uno.edu/delta
24. World Watch Institute (2000). Crop gene diversity declining. Trends in Plant Science 5, 7.
25. Wilkes, H.G. (1977). Hybridization of maize and teosinte, in Mexico and Guatemala and the improvement of maize. Economic Botany 31, 254-293.
LAST UPDATED: SEPTEMBER 2002

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Introduction Expand

Summary of Introduced Genetic Elements Expand

Characteristics of Zea mays (Maize) Expand

Modification Method Expand

Characteristics of the Modification Expand

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Introduction Expand

Summary of Introduced Genetic Elements Expand

Characteristics of Zea mays (Maize) Expand

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Characteristics of the Modification Expand

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Biology of Maize L.

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