GM Crop Database

Database Product Description

MON809 (PH-MON8Ø9-2)

Host Organism: Zea mays (Maize)

Trait: Resistance to European corn borer (Ostrinia nubilalis); glyphosate herbicide tolerance.

Trait Introduction: Microparticle bombardment of plant cells or tissue

Proposed Use: Production for human consumption and livestock feed.

Product Developer: Pioneer Hi-Bred International Inc.

Summary of Regulatory Approvals

Country	Food	Feed	Environment
Canada	1996	1996	1996
Japan		1998	1997
United States	1996	1996	1996

Introduction Expand: Maize line MON809 was developed through a specific genetic modification to be resistant to attack by European corn borer (ECB; Ostrinia nubilalis), a major insect pest of maize in agriculture. The novel variety produced the insecticidal protein, Cry1Ab, derived from Bacillus thuringiensis subsp. kurstaki (B.t.k.) HD-1 strain. Delta-endotoxins, such as the Cry1Ab protein expressed in MON809, act by selectively binding to specific sites localized on the brush border midgut epithelium of susceptible insect species. Following binding, cation-specific pores are formed that disrupt midgut ion flow and thereby cause paralysis and death. Cry1Ab is insecticidal only to lepidopteran insects, and its specificity of action is directly attributable to the presence of specific binding sites in the target insects. There are no binding sites for delta-endotoxins of B. thuringiensis on the surface of mammalian intestinal cells, therefore, livestock animals and humans are not susceptible to these proteins.

Summary of Introduced Genetic Elements Expand

Code	Name	Type	Promoter, other	Terminator	Copies	Form
cry1Ab	Cry1Ab delta-endotoxin (Btk HD-1)	IR	enhanced CaMV 35S	A. tumefaciens nopaline synthase (nos) 3'-untranslated region	1	Truncated, modified; second partial copy present
CP4 epsps	5-enolpyruvyl shikimate-3-phosphate synthase	SM	enhanced CaMV 35S chloroplast transit peptide from A. thaliana EPSPS gene (CTP2)		2
goxv247	glyphosate oxidoreductase	HT	chloroplast transit peptide from A. thaliana SSU1A gene (CTP1)			partial copy, no expression detected

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.

Donor Organism Characteristics Expand

Latin Name	Gene	Pathogenicity
Bacillus thuringiensis subsp. kurstaki	EC2.4.2.19	While target insects are susceptible to oral doses of Bt proteins, no evidence of toxic effects in laboratory mammals or birds given up to 10 µg protein/g body weight.
Agrobacterium tumefaciens strain CP4	CP4 epsps	Agrobacterium tumefaciens is a common soil bacterium that is responsible for causing crown gall disease in susceptible plants. There have been no reports of adverse effects on humans or animals.

Modification Method Expand: Maize line MON809 was produced by biolistic transformation of embryogenic maize cells with a mixture of DNA from two plasmids, PV-ZMBK07 and PV-ZMGT10. Plasmid PV-ZMBK07 contained the synthetic cr1Ab gene regulated by the enhanced duplicated cauliflower mosaic virus 35S promoter (CaMV E35S) and the maize hsp70 heat shock protein intron. The polyadenylation signal was from the Agrobacterium tumefaciens nopaline synthase (nos) gene. Plasmid PV-ZMBK07 also contained sequences from lacZ operon (a partial E. coli lacI coding sequence, the promoter plac and a partial coding sequence for beta-galactosidase from pUC119), ori-pUC (replication origin for pUC plasmids), and the neo gene, which encodes neomycin phosphotransferase II (NPT II) that imparts resistance to the antibiotic kanamycin. The second plasmid, PV-AMGT10, contained genes encoding the CP4 EPSPS enzyme from the common soil bacterium, A. tumefaciens sp. CP4, and glyphosate oxidoreductase (goxv247) from Ochrobactrum anthropi. The expression of these genes was used as a selectable marker to identify transformed plants on medium containing glyphosate but was not sufficient to provide field level tolerance to glyphosate containing herbicides. Constitutive expression of these genes in plant cells was under the control of the CaMV E35S promoter, the hsp70 intron, and nos 3' terminator. Post-translational targeting of the CP4 EPSPS and glyphosate oxidoreductase enzymes to the chloroplast was accomplished by fusion of the 5'-terminal coding sequences with the chloroplast transit peptide DNA sequences from the Arabidopsis thaliana EPSPS gene (CTP2) and from A. thaliana SSU1A gene (CTP1), respectively. Plasmid PV-ZMGT10 also contained sequences from the lacZ operon, ori-pUC and the neo gene. The neo gene on both plasmids was included as a selectable marker to identify bacteria transformed with recombinant plasmid DNAs. Expression of the neo gene was regulated by a bacterial promoter and therefore is not functional in plants.

Characteristics of the Modification Expand

The Introduced DNA

Southern blot analysis of MON809 genomic DNA indicated one integrated DNA segment, of approximately 23Kb DNA, which included a complete and a partial copy of the cry1Ab gene, two complete copies of the CP4 EPSPS encoding gene, a partial gox gene and the NPT II encoding antibiotic resistance marker gene. Additional Southern blot analyses were performed to demonstrate that additional plasmid backbone DNA sequences from PV-ZMBK07 were not integrated into the host genome.

Genetic Stability of the Introduced Trait

Segregation analysis over five generations of crossing indicated that the cry1Ab gene was stably integrated and segregated according to Mendelian rules of inheritance.

Expressed Material

Western blot analysis of protein extracts from MON809 demonstrated the presence of an immunoreactive protein of the predicted size. Lower molecular weight species corresponding to the predicted size of the truncated cry1Ab gene were not detected, indicating that this partial gene sequence is not expressed. The expression of Cry1Ab protein was evaluated using enzyme linked immunosorbent assay (ELISA) of tissue samples obtained from plants grown at 6 locations. The average levels of expressed Cry1Ab protein were: 1.63 ± 0.75 µg/g fresh weight (fwt) in leaves, 0.55 ± 0.23 µg/g fwt in grain and 1.23 ± 0.5 µg/g fwt in the whole plant. Cry1Ab protein was not detected in pollen and the levels expressed in leaves declined over the growing season. Similarly, CP4 EPSPS gene activity, used as a selectable marker conferring tolerance to glyphosate, was quantified by testing for EPSPS protein levels and averaged results were as follows: 21.68 µg/g fwt in leaves, 9.41 µg/g fwt in grain and 1.6 µg/g fwt in the whole plant. Data were provided showing that the level of expression of CP4 EPSPS was too low to confer significant glyphosate resistance in the field. Expression of the second gyphosate tolerance gene, gox, which encodes the enzyme glyphosate oxidase, could not be detected by Western blot analysis. The Cry1Ab protein was shown to degrade readily in the environment. The plant expressed protein had DT50 and DT90 values (time to degrade to 50% and 90 % of the original bioactivity) of 2 and 15 days respectively.

Environmental Safety Considerations Expand

Field Testing

MON809 maize was field tested under confined conditions in Canada from 1994-1996. Field reports on MON809 and corn hybrids derived from MON809 determined that early stand establishment, vegetative vigour, time to maturity and seed production were within the normal range of expression of these traits in commercial corn hybrids. Hybrids derived from MON809 produced similar yields in the absence of ECB, but under heavy infestations of ECB, MON809 hybrids out-yielded their non-transgenic counterparts by 10-15%. Hybrids derived from MON809 were not tolerant to the concentrations of glyphosate herbicide that are typically used for weed control. No significant differences were observed in resistance to a number of other significant insect pests or diseases compared to non-transgenic controls. Overall the field data reports and data on agronomic traits showed that MON809 lines have no potential to pose a plant pest risk.

Outcrossing

Since pollen production and viability were unchanged by the genetic modification resulting in maize line MON809, pollen dispersal by wind and outcrossing frequency should be no different than for other maize varieties. Gene exchange between MON809 and other cultivated maize varieties will be similar to that which occurs naturally between cultivated maize varieties at the present time. In Canada and the United States, where there are few 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.

Weediness Potential

No competitive advantage was conferred to MON809, other than that conferred by resistance to ECB, which 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 means or by using herbicides. Zea mays is not invasive and is a weak competitor with very limited seed dispersal.

Secondary and Non-Target Adverse Effects

MON809 maize and the Cry1Ab protein, in particular, should not have a significant potential to harm organisms beneficial to agricultural ecosystems. The history of use and literature suggest that the bacterial Bt protein is not toxic to humans, other vertebrates, and beneficial insects. Most insecticidal protein toxins from B. thuringiensis subspecies kurstaki, including Cry1Ab, have been shown to be specifically toxic to Lepidopteran larvae on ingestion and appear non-toxic to other species of insects, either directly or through secondary ingestion (predation). Laboratory studies of honeybee larvae and adults, predaceous insects (green lacewing larvae, ladybird beetles), and parasitic hymenoptera exposed to the Cry1Ab protein recorded no discernible effects at approximately 10 times the usual LC50 dose for a target insect. Field examination of beneficial arthropods (the Anthocorid Orius insidiosus and spiders) in genetically modified MON809 maize, concluded that any potential impact on non-target arthropods would be less than that from the use of conventional insecticides. Results from feeding studies of young quail fed with modified maize meal in their diet showed no adverse effects. Studies were conducted to evaluate the impact of residual MON809 maize on soil organisms and soil function. It was shown that the Bt endotoxin may become adsorbed to some soil fractions and that degradation was by microbial action. However, since very little of the crop material remains after harvest for incorporation it was concluded based on evidence available that the risks to organisms and soil function were very low. In summary, it was determined that genetically modified maize MON809 did not present an increased risk to, or impact on interacting organisms, including humans, with the exception of specific lepidopteran insect species. Furthermore, MON809 will not impact on threatened or endangered lepidopteran species, as none are listed which feed on maize plants in Canada or the United States. In addition, these organisms are not expected to be affected by the expressed CP4 EPSPS.

Impact on Biodiversity

MON809 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 and the United States is remote, it was determined that the risk of transferring genetic traits from MON809 maize to species in unmanaged environments was insignificant.

Other Considerations

In order to prolong the effectiveness of plant-expressed Bt toxins, and the microbial spray formulations of these same toxins, regulatory authorities in Canada and U.S. have required developers to implement specific Insect Resistant Management (IRM) Programs. These programs are mandatory for all transgenic Bt-expressing plants, including MON809 maize, and require that growers plant a certain percentage of their acreage to non-transgenic varieties in order to reduce the potential for selecting Bt-resistant insect populations. Details on the specific design and requirements of individual IRM programs are published by the relevant regulatory authority. MON809 maize plants are not likely to eliminate the use of chemical insecticides, which are traditionally applied to about 25 to 35% of the total planted maize acreage. MON809 maize may positively impact current agricultural practices used for insect control by 1) offering an alternative method for control of European corn borer (and potentially other Cry1Ab-susceptible pests of maize); 2) reducing the use of insecticides to control European corn borer and the resulting potential adverse effects of such insecticides on beneficial insects, farm worker safety, and ground water contamination; and 3) offering a new tool for managing insects that have become resistant to other insecticides currently used or expressed in maize, including other Bt-based insecticides.

Food and/or Feed Safety Considerations Expand

Dietary Exposure

MON809 maize was intended mainly for use in animal feed. The major human food uses for maize are extensively processed starch and oil fractions prepared by wet or dry milling procedures and products include corn syrup and corn oil, neither containing protein. The potential human exposure to the modified protein from whole grain corn in the diet was considered to be very low due both to its low abundance in the protein fraction of the grain and to the proportionately low percentage of protein in the kernel, compared with the major starch component. Overall, the dietary exposure of consumers in the United States and Canada to insect resistant hybrids of MON809 maize was anticipated to be the same as for other lines of commercially available field maize.

Nutritional Data

The nutritional quality of grain from MON809 maize was similar to that of grain from an equivalent non-transgenic line and to grain currently available in commerce. Grain was subjected to proximate analysis (e.g., ash, moisture content, crude protein, crude fat), amino acid and fatty acid composition. Some minor differences were observed between MON809 and an equivalent non-transgenic line, but the levels were within the range of variation normally observed in the commercial grain supply and were attributed to variations in the plant genotypes rather than to the presence of the introduced genes.

Toxicity

Data were submitted that demonstrated that the active Cry1Ab protein product of the inserted cry1Ab gene was equivalent to the protein contained in microbial Bt spray formulations that have been safely used in agriculture for more than 40 years. Studies on the Cry1Ab protein expressed in plants and the resulting proteolytic fragments were compared to the bacterial proteins and shown to be of similar molecular weight, amino acid sequence, immunological reactivity and trypsin resistance. The protein was not glycosylated and showed similar bioactivity and host range specificity to the native protein. The Cry1Ab and CP4 EPSPS proteins should pose no risks for consumption of products produced from insect resistant maize. The introduced proteins were compared to databases of known protein toxins and neither showed any significant amino acid sequence similarity to known protein toxins. Neither protein was toxic when fed to mice.

Allergenicity

The Cry1Ab and CP4 EPSPS proteins are extremely unlikely to be allergens. The deduced amino acid sequences of the introduced proteins were compared to sequences of known allergens contained in the public domain databases GenBank, EMBL, Pir and SwissProt. No significant sequence homologies were detected. Neither the Cry1Ab and CP4 EPSPS protein originated from an allergenic source, or were structurally similar to known allergens. EPSPS is ubiquitous in bacteria, plants and animals and no toxic or allergenic effects have ever been reported. In addition, the potential for allergenicity was assessed based upon the characteristics of known food allergens (stability to digestion, stability to processing, abundance in foods). Protein allergens are normally resistant to digestion unlike the Cry1Ab protein that was shown to degrade readily in simulated gastric fluid. Similarly, data were presented showing that the CP4 EPSPS enzyme was rapidly inactivated by heat and by digestion in simulated mammalian gastric fluid.

Maize products are an important alternative to wheat flour for individuals afflicted with coeliac disease, an immune mediated food intolerance for which wheat gliadins have been implicated as the causal agent. In light of the importance of corn products to these individuals, a sequence similarity search was conducted and no amino acid sequence homologies between the Cry1Ab protein and gliadins were detected.

Abstract Collapse: Maize (Zea mays L.), or corn, is grown primarily for its kernel, which is largely refined into products used in a wide range of food, medical, and industrial goods. 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 ethanol production. 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 European corn borer (ECB), Ostrinia nubilalis, is the most damaging insect pest of maize in the United States and Canada; losses resulting from ECB damage and control costs exceed $1 billion each year. An average of one ECB cavity per maize stalk across an entire field can reduce yield by as much as 5% when caused by first generation larvae, and 2.5% when caused by second generation larvae, with annual yield losses estimated at 5 to 10 %. Despite consistent losses to ECB, chemical insecticides are utilized on a relatively small acreage (less than 20%). Historically, this reluctance stems from the difficulties in identifying and managing ECB in maize crops: ECB larval damage is hidden, heavy infestations are unpredictable, insecticides are costly, timing of insecticide application is difficult and multiple applications may be required to guarantee ECB control. The transgenic maize line MON809 was genetically engineered to resist ECB by producing its own insecticide. This line was developed by introducing the cry1Ab gene, isolated from the common soil bacterium Bacillus thuringiensis (Bt), into the maize line by particle acceleration (biolistic) transformation. The cry1Ab gene produces the insect control protein Cry1Ab, a delta-endotoxin. The Cry1Ab protein produced by the Bt maize is identical to that found in nature and in commercial Bt spray formulations. Cry proteins, of which Cry1Ab is only one, act by selectively binding to specific sites localized on the lining of the midgut of susceptible insect species. Following binding, pores are formed that disrupt midgut ion flow, causing gut paralysis and eventual death due to bacterial sepsis. Cry1Ab is lethal only when eaten by the larvae of lepidopteran insects (moths and butterflies), and its specificity of action is directly attributable to the presence of specific binding sites in the target insects. There are no binding sites for the delta-endotoxins of B. thuringiensis on the surface of mammalian intestinal cells, therefore, livestock animals and humans are not susceptible to these proteins. MON809 expressed the Cry1Ab protein at an effective dosage over the growing season, as indicated by its efficacy in controlling both first and second generation infestations of ECB. Protein expression was found to decrease over the growing season, as evidenced by declining Cry1Ab protein concentrations in assayed leaves. MON809 was tested in field trials in Canada from 1994 to 1996. Data collected from these trials demonstrated that MON809 was not different from conventional maize lines; agronomic characteristics, including vegetative vigour, time to maturity, seed production, and disease and pest susceptibilities, were within the normal range reported for conventional maize lines. It was demonstrated that the transformed maize line did not exhibit weedy characteristics, or negatively affect beneficial or nontarget organisms. MON809 was not expected to impact on threatened or endangered species. Maize does not have any closely related species growing in the wild in the continental United States and 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 MON809. Gene exchange between MON809 and maize relatives was determined to be negligible in managed ecosystems, with no potential for transfer to wild species in Canada and the United States. Regulatory authorities in Canada and the United States have mandatory requirements for developers of Bt maize to implement specific Insect Resistant Management (IRM) Programs. The potential for ECB populations to develop tolerance or become resistant to the Bt toxin is expected to increase as more maize acreage is planted with Bt hybrids. These IRM programs are designed to reduce the potential development of Bt-resistant insect populations, as well as prolonging the effectiveness of plant-expressed Bt toxins, and the microbial Bt spray formulations of these same toxins. The food and livestock feed safety of MON809 maize was established based on several standard criteria. Analyses determined that MON809 grain was nutritionally equivalent to non-transgenic grain, posing no health risks to either humans or livestock. Proximate analysis (ash, crude fat, crude protein, and moisture content), and fatty acid and amino acid composition of MON809 grain revealed only minor differences from the levels reported for non-transgenic maize. These differences were within the normal range of variation for maize and were attributed to normal genotypic variation rather than to the presence of the introduced genes. The toxicity and allergenicity potential of the Cry1Ab protein in MON809 was assessed by examining its physiochemical characteristics, degree of amino acid sequence homology to known protein allergens, and digestibility. The Cry1Ab protein has a history of safe use, demonstrated by its use in microbial Bt spray formulations in agriculture for more than 40 years with no evidence of adverse effects. This fact, combined with the lack of amino acid sequence homology between the Cry1Ab protein and known allergens and toxins, and the rapid degradation of the Cry1Ab protein in acidic gastric fluids, was sufficient to provide with reasonable certainty a lack of toxicity and allergenic potential.

Links to Further Information Expand

Canadian Food Inspection Agency, Plant Biotechnology Office

Decision Document 97-18: Determination of the Safety of Pioneer Hi-Bred International Inc.'s European Corn Borer (ECB) Resistant Corn (Zea mays L.) Line MON809
[PDF Size: 171.69K bytes]

European Commission Scientific Committee on Plants

Opinion of the Scientific Committee on Plants regarding the submission for placing on the market of genetically modified, insect-resistant maize lines notified by the pioneer genetique S.A.R.L. Company (notification No C/F/95/12-01/B) (Submitted by the Scientific Committee on Plants, 19 May 1998)
[PDF Size: 143.27K bytes]

Impact of Bt corn pollen on monarch butterfly populations: A risk assessment

Mark K. Sears, Richard L. Hellmich, Diane E. Stanley-Horn, Karen S. Oberhauser, John M. Pleasants, Heather R. Mattila, Blair D. Siegfried, and Galen P. Dively (2001). Proc. Natl. Acad. Sci. USA Early Edition
[PDF Size: 162.67K bytes]

Office of Food Biotechnology, Health Canada

Novel Food Information: Insect resistant maize (corn), MON809
[PDF Size: 12.13K bytes]

U.S.Department of Agriculture, Animal and Plant Health Inspection Service

Monsanto Co. Petition for Determination of Non-regulated Status of Additional Yieldgard Corn Lines MON 809 and 810
[PDF Size: 1.44M bytes]

UK Register C/F/95/12-01/B

UK Department of the Environment,Transport and the Regions: Registry entry for MON809.
[PDF Size: 10.53K bytes]

This record was last modified on Wednesday, June 28, 2017

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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