GM Crop Database

Database Product Description

MON89034 (MON-89Ø34-3)

Host Organism: Zea mays (Maize)

Trait: Insect resistant, Lepidoptera.

Trait Introduction: Agrobacterium tumefaciens-mediated plant transformation.

Proposed Use: Production for human consumption and livestock feed.

Product Developer: Monsanto Company

Summary of Regulatory Approvals

Country	Food	Feed	Environment
Argentina	2010	2010	2010
Australia	2008
Brazil	2009	2009	2009
Canada	2008	2008	2008
Colombia	2010	2007
European Union	2009	2009
Japan	2007	2008	2008
Korea	2009	2009
Malaysia	2015	2015
Mexico	2008	2008
New Zealand	2009
Paraguay	2013	2013	2013
Philippines	2009	2009	2010
Singapore	2014
South Africa	2010	2010	2010
Taiwan	2008
Turkey		2011
United States	2007	2007	2008
Vietnam	2014	2014

Introduction Expand

Maize (Zea mays L.), otherwise known as corn, is the world’s third leading cereal crop, behind wheat and rice, and is grown in over 25 countries worldwide. The majority of grain and forage derived from maize is used as animal feed, however maize also has a long history of safe use as food for human consumption. The grain can be processed into industrial products such as ethanol (by fermentation), and highly refined starch (by wetmilling) and sweetener products. In addition to milling, the maize germ can be processed to obtain corn oil and numerous other products. The majority of maize is grown for animal feed production and human food, however an increasing percentage is being used for the production of ethanol as a biofuel alternative to petroleum products from fossil sources.

MON 89034 was developed as a second generation insect-resistant maize product to provide enhanced benefits for the control of lepidopteran insect pests. MON 89034 produces the Cry1A.105 and Cry2Ab2 proteins derived from Bacillus thuringiensis, which are active against lepidopteran insect pests. MON 89034 is intended to serve farmers' needs for controlling a wider spectrum of lepidopteran pests and help assure the durability of insect-resistant maize. MON 89034 provides control of Ostrinia species such as European corn borer (ECB) and Asian corn borer, and Diatraea species such as southwestern corn borer (SWCB) and sugarcane borer. MON 89034 also provides a high level control of fall armyworm (FAW) throughout the season and improved protection from damage caused by corn earworm (CEW). In addition to the wider spectrum of insect control, the combination of the Cry1A.105 and Cry2Ab2 insecticidal proteins in a single plant provides a much more effective insect resistance management (IRM) tool. The results of mathematical modeling indicate that biotechnology-derived plants expressing two Cry proteins will have significantly greater durability than plants producing either of the single proteins if the cross-resistance between the Cry proteins is low and the mortality of susceptible insects caused by each of the individual proteins is at least 90%. Comparative biophysical studies indicate that the Cry1A.105 and Cry2Ab2 proteins have important differences in their mode of action, specifically in the way in which they bind to the lepidopteran midgut. Therefore, the probability of cross-resistance between these two proteins is low. Furthermore, in vitro and in planta studies with Cry1A.105 and Cry2Ab2 demonstrate that both proteins are highly active against the primary lepidopteran pests of corn (ECB, SWCB, CEW, and FAW), particularly ECB, achieving close to or greater than the critical 95% level of control in all cases.

Summary of Introduced Genetic Elements Expand

Code	Name	Type	Promoter, other	Terminator	Copies	Form
cry1A.105	chimeric cry1 delta-endotoxin	IR	CaMV 35S 5' untranslated leader from wheat chlorophyll a/b-binding protein Rice actin gene intron	3' untranslated region of wheat heat shock protein 17.3	1
cry2Ab	cry2Ab delta-endotoxin	IR	FMV-35S - promoter from Figwort Mosaic Virus Hsp70 intron from maize heat shock protein gene. Transit peptide from maize RBC-small subunit.	Agrobacterium tumefaciens nopaline synthase (nos) 3'-untranslated region	1

Code

Name

Type

Promoter, other

Terminator

Copies

Form

cry1A.105

chimeric cry1 delta-endotoxin

CaMV 35S 5' untranslated leader from wheat chlorophyll a/b-binding protein Rice actin gene intron

3' untranslated region of wheat heat shock protein 17.3

cry2Ab

cry2Ab delta-endotoxin

FMV-35S - promoter from Figwort Mosaic Virus

Hsp70 intron from maize heat shock protein gene. Transit peptide from maize RBC-small subunit.

Agrobacterium tumefaciens nopaline synthase (nos) 3'-untranslated region

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	cry1Ab-Ac	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.

Modification Method Expand

MON89034 was produced by Agrobacterium-mediated transformation of corn with PVZMIR245, which is a binary vector containing 2 separate T-DNA regions. The first T-DNA, designated as T-DNA I, contained the cry1A.105 and the cry2Ab2 expression cassettes which express the insecticidal proteins Cry1A.105 and Cry2Ab2. The cry1A.105 gene, is a chimeric gene comprising of 4 domains from other cry genes previously used in transgenic plants. The amino acid sequences of Domains I and II are identical with the respective domains from Cry1Ab and Cry1Ac proteins, Domain III is almost identical to the Cry1F protein, and the C-terminal Domain is identical to Cry1Ac protein. Overall, the amino acid sequence homology of Cry1A.105 is 93.6%, 90.0%, and 76.7% respectively with the Cry1Ac, Cry1Ab and Cry1F proteins. The expression cassette for the coding sequence of the Cry1A.105 protein consists of the promoter (P-e35S) and leader for the cauliflower mosaic virus (CaMV) 35S RNA with a duplicated enhancer region. The cassette also contains the 5' untranslated leader of the wheat chlorophyll a/b/ binding protein (L-Cab), the intron from the rice actin gene (I-Ract1), the cry1A.105 coding sequence that was optimized for expression in monocots, and the 3’ nontranslated region of the coding sequence for wheat heat shock protein 17.3 (T-Hsp17), which terminates transcription and provides the signal for mRNA polyadenylation

The Cry2Ab2 protein is a member of the Cry2Ab class of proteins that share >95% amino sequence homology. Like the Cry2Ab2 protein produced in the transgenic cotton event 15985, the Cry2Ab2 protein produced in MON89034 is a slight variant of the wild-type Cry2Ab2 protein isolated from Bacillus thuringiensis subsp. kurstaki. The Cry2Ab2 produced in MON 89034 is identical to the Cry2Ab2 produced in 15985 cotton, and they differ from the wild-type Cry2Ab2 of Bt by only one amino acid. The promoter region of the cry2Ab2 gene expression cassette consists of the 35S promoter from figwort mosaic virus (P-FMV) and the first intron from the corn heat shock protein 70 gene (I-Hsp 70). The cassette also contains a cry2Ab2 coding sequence with a modified codon usage fused to a chloroplast transit peptide region of corn ribulose 1,5-biphosphate carboxylase small subunit including the first intron (TSSSU-CTP). The 3’ nontranslated region of the nopaline synthase (T-nos) coding region from Agrobacterium tumefaciens T-DNA terminates transcriptionand directs polyadenylation.

The second T-DNA, designated as T-DNA II, contains the nptII (neomycin phosphotransferase II) expression cassette. The nptII gene cassette that produces the NPTII protein consists of the promoter (P-e35S) from the cauliflower mosaic virus (CaMV) 35S RNA. The cassette also contains the coding sequence for the NPTII protein, followed by the 3’ nontranslated region of the nopaline synthase (T-nos) sequence from Agrobacterium tumefaciens which terminates the transcription and directs polyadenylation.

During transformation, both T-DNAs were inserted into the genome. The nptII gene confers resistance to kanamycin and similar antibiotics and was used as the selectable marker for isolation of transformed cells and regeneration of transgenic plants. Once transgenic plants had been regenerated, the selectable marker gene was no longer needed and conventional breeding was used to isolate plants that only contain the cry1A.105 and the cry2Ab2 expression cassettes (T-DNA I) and did not contain the nptII expression cassette (T-DNA II), thereby, producing marker-free transgenic lines, one of which was selected and designated as MON89034.

Characteristics of the Modification Expand

The Introduced DNA

Molecular characterization of MON89034 by Southern blot analyses demonstrated that the DNA inserted into the genome is present at a single locus and contains one functional copy of the cry1A.105 and the cry2Ab2 expression cassettes. Furthermore, no backbone plasmid DNA or nptII sequences were detected in these analyses. The organization of the elements within the insert in MON89034 was confirmed by DNA sequencing analyses. PCR primers were designed to amplify seven overlapping regions of DNA that span the entire length of the insert (9317 bp), and the amplified DNA fragments were subjected to DNA sequencing analyses. The results confirmed that the sequence of the DNA insert in MON89034 matched the designed, corresponding sequences in PV-ZMIR245 with one exception.

This exception is that the e35S promoter that regulates expression of the cry1A.105 gene has been modified and that the Right Border sequence present in PV-ZMIR245 was replaced by a Left Border sequence in MON89034. This molecular rearrangement can be explained by a recombination event which occurred, either prior to or during the process of T-DNA transfer to the plant cell, between the DNA sequences near the 35S promoters in T-DNA I and T-DNA II. Due to this recombination event, the reconstituted e35S promoter in MON89034 no longer has the duplicated enhancer elements compared to the original e35S promoter in PV-ZMIR245. Despite the deletion of the enhancer elements, the Cry1A.105 protein expression levels in MON89034 are still sufficiently high under the regulation of the modified e35S promoter to deliver the required efficacy against target insect pests.

Genetic Stability of the Trait

Analysis of the stability of the integrated DNA demonstrated that the unique Southern blot fingerprint of MON89034 was maintained through seven generations of conventional breeding, thereby confirming the stability of the insert. Additionally, the Southern blot analysis of multiple generations in the MON89034 breeding history indicated that there were no detectable T-DNA II elements other than those which are common to T-DNA I, i.e., 35S promoter, nos 3´ end sequence and the Left Border sequence. Furthermore, these generations were shown not to contain any detectable backbone sequence from plasmid PVZMIR245. The stability was further confirmed by the fact that the inheritance of the lepidopteran protection trait in MON89034 follows Mendelian segregation principle.

Expressed Material

The expression levels of the Cry1A.105 and Cry2Ab2 proteins in different tissues of MON89034 are relatively low. Therefore, it was necessary to produce the proteins in a high-expressing, recombinant microorganism in order to obtain sufficient quantities of the protein for safety studies. Recombinant Cry1A.105 and Cry2Ab2 proteins were produced in Escherichia coli, with the sequences engineered to match the Cry1A.105 and Cry2Ab2 proteins produced in MON89034. The equivalence of the physicochemical characteristics and functional activity between the MON89034-produced and E. coli-produced proteins was confirmed by a panel of analytical techniques, including sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), Western blot analysis, matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), glycosylation analysis, and assays of biological activity.

The levels of the Cry1A.105 and Cry2Ab2 proteins in various tissues of MON89034 were determined using enzyme-linked immunosorbent assays (ELISA). To produce the tissues for analysis, MON89034 and conventional maize were planted at five field locations during the 2005 growing season. In tissues harvested throughout the growing season, mean Cry1A.105 protein levels across all sites varied from 72-520 µg/g dwt in leaf, 11-79 µg/g dwt in root, 5.9 µg/g dwt in grain and 42-380 µg/g dwt in whole plant. In tissues harvested throughout the growing season, mean Cry2Ab2 protein levels across all sites varied from 130-180 µg/g dwt in leaf, 21-58 µg/g dwt in root, 1.3 µg/g dwt in grain and 38-130 µg/g dwt in whole plant. In general, levels of both proteins declined in vegetative tissues over the growing season.

Food and/or Feed Safety Considerations Expand

Dietary Exposure

Humans consume relatively little whole kernel or processed maize, compared to maize-based food ingredients. Maize is a raw material for the manufacture of starch, the majority of which is converted to a variety of sweetener and fermentation products, including high fructose syrup and ethanol. Maize oil is commercially processed from the germ. These materials are components of many foods including bakery and dairy goods, and the human food uses of grain from MON89034 are not expected to be different from the uses of non-transgenic field maize varieties. As such, the dietary exposure to humans of grain from insect resistant hybrids will not be different from that for other commercially available field maize varieties. Maize grain and by-products of wet and dry milling are used as animal feed. The whole maize plant can also be harvested at an appropriate stage and fed to animals or stored as silage. As for food use, the dietary exposure of animals to grain or forage from MON89034 will not be different to exposure from other commercially available maize.

Potential toxicity and allergenicity

Cry1A.105 is a chimeric Cry1A protein of Bacillus thuringiensis that is comprised of three N-terminal domains (I, II, and III) and the C-terminal region. The domains I and II of Cry1A.105 are 100% identical to the respective domains of Cry1Ab or Cry1Ac. The domain III of Cry1A.105 is 99% identical to the domain III of Cry1F. The C-terminal region of Cry1A.105 is 100% identical to that of Cry1Ac. The overall amino acid sequence identity of 93.6%, 90.0%, and 76.7% to Cry1Ac, Cry1Ab, and Cry1F proteins, respectively. Cry2Ab2 is a Bacillus thuringiensis (subsp. kurstaki) protein. Bacillus thuringiensis strains producing Cry1Ac, Cry1Ab, Cry1F, and Cry2A proteins have been used for decades as biopesticides, and pesticides that contain Cry1Ac/Cry1F chimeric protein have been used since 1997. Transgenic maize and cotton expressing Cry1Ac, Cry1Ab, Cry1F, or Cry2Ab2 proteins have been cultivated in large areas in the U.S. and other countries for up to a decade. There are no known reports of allergy or toxicity to Bt or to these Cry proteins.

A dietary safety assessment was performed to evaluate the risks to humans and animals from the Cry1A.105 and Cry2Ab2 proteins present in the foods and feeds derived from MON89034. Risks are quantified as a margin of exposure (MOE), which is defined as the ratio of the No Observable Effect Level (NOEL) from an acute mouse gavage study to estimates of the dietary intake of the respective Cry protein. Mice acute oral toxicity studies demonstrated that the two proteins are not acutely toxic and do not cause any adverse effects even at the higest dose levels tested, which are 2072 and 2198 mg/kg body weight for Cry1A.105 and Cry2Ab2 proteins, respectively. The dietary safety assessment showed that the MOEs for the overall U.S. population were greater than or equal to 199,000 and 981,000 for the Cry1A.105 and Cry2Ab2 proteins, respectively. For children aged 3-5 years old, an age group with the highest corn consumption, the MOEs were greater than or equal to 79,400 and 390,000 for the Cry1A.105 and Cry2Ab2 proteins, respectively. For poultry and livestock, the MOEs ranged between 1,930 – 13,500 and 2,160 – 47,600 for the Cry1A.105 and Cry2Ab2 proteins, respectively.

Cry1A.105 and Cry2Ab2 proteins do not share any amino acid sequence similarities with known allergens, gliadins, glutenins, or protein toxins which have adverse effects to mammals. This has been shown by extensive assessments with bioinformatic tools, such as FASTA sequence alignment tool and an eight amino acid sliding window search. Furthermore, Cry1A.105 and Cry2Ab2 proteins are rapidly digestible in simulated gastric fluids. Greater than 95% to 99% of the proteins were digested in simulated gastric fluids in less than 30 seconds. Proteins that are rapidly digestible in mammalian gastrointestinal systems are unlikely to be allergens when consumed.

Nutritional and Compositional Data

The composition of forage and grain from MON89034 and a non-transgenic control was compared to assess whether the transgenic corn differs from non-transgenic corn in nutritional characteristics. The non-transgenic control has a genetic background similar to that of MON89034 but does not contain the cry1A.105 and cry2Ab2 genes. In addition, grain and forage from 15 different conventional maize hybrids produced in the same field trials with MON89034 and the non-transgenic control was evaluated, with three different hybrids planted at each of the five sites. Data derived from these 15 conventional hybrids as references to generate a 99% tolerance interval for each analyzed component.
The levels of proximates (ash, fat, protein and moisture), fiber (acid detergent fiber-ADF and neutral detergent fiber-NDF) and minerals (calcium and phosphorous) were compared in forage from MON89034 and the non-transgenic control. No statistically significant differences were found in the levels of all analyzed components between forage from MON89034 and forage from the conventional control, with the exception of phosphorus. MON89034 had statistically significantly higher levels of phosphorus than its control, but these levels were within the 99% tolerance intervals for the commercial varieties and within the ranges reported in the literature and the ILSI Crop Composition Database.

The levels of proximates, minerals, amino acids, fatty acids, anti-nutrients, secondary metabolites and vitamins were compared in mature grain from MON89034 and control maize. A statistical analysis of the analytical results combined from all field trials found no statistically significant differences between MON89034 and the conventional non-transgenic control, except for stearic acid and arachidic acid (calculated as a percentage of total fatty acids). However, differences in the levels of these fatty acids were small. The levels of all measured components were within the ranges reported in the literature and the ILSI Crop Composition Database and within the 99% tolerance interval established from the 15 control varieties. There were several statistically significant differences in the level of nutrients within the individual locations, but these differences were not consistent across all locations and it was concluded that these differences were not biologically significant.

The results of these compositional analyses led to the conclusion that MON89034 forage and grain are not different in nutritional and anti-nutritional composition compared to maize hybrids currently marketed, grown and consumed.

Abstract Collapse

Maize, or corn (Zea mays L.) is grown commercially in over 100 countries with a combined harvest of nearly 700 million metric tonnes in 2006. The top five producers of maize in 2005 were the United States, China, Brazil, Argentina, and Mexico, accounting for 70% of world production. Maize is grown primarily for its kernel (grain), the majority of which is used for animal feed, but with significant amounts refined into products used in a wide range of food, medical, and industrial goods.

In the United States maize is typically used as animal feed, with roughly 70% of the crop fed to livestock, although an increasing share is now being used to produce ethanol for fuel. 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 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, car parts.

MON 89034 was produced by Agrobacterium-mediated transformation of corn with PVZMIR245, which is a binary vector containing 2 T-DNAs. The first T-DNA, designated as T-DNA I, contained the cry1A.105 and the cry2Ab2 expression cassettes. The second T-DNA, designated as T-DNA II, contained the nptII (neomycin phosphotransferase II) expression cassette. During transformation, both T-DNAs were inserted into the genome. The nptII gene was used as the selectable marker which was needed for selection of the transformed cells. Once the transgenic cells were identified, the selectable marker gene was no longer needed and traditional breeding was used to isolate plants that only contained the cry1A.105 and cry2Ab2 expression cassettes (T-DNA I). Molecular characterization of MON 89034 by Southern blot analyses demonstrated that the DNA inserted into the corn genome is present at a single locus and contains one functional copy of the cry1A.105 and the cry2Ab2 expression cassettes. All genetic elements are present in the inserted DNA as expected with the exception that the e35S promoter, which regulates expression of the cry1A.105 gene, has been modified and that the Right Border sequence present in PV-ZMIR245 was replaced by a Left Border sequence in MON 89034. No backbone plasmid DNA or nptII sequences were detected.

As the selectable marker gene employed in the transformation process was removed during development of the final event, the two insecticidal proteins are the only new proteins expressed in this plant. This was confirmed by a molecular analysis that identified only the genes coding for the insecticidal proteins as being present in the final event, with no evidence for presence of the selecable marker gene. A review of the potential toxicity and allergenicity of these two proteins was made, in addition to a comparison of the composition of the grain and forage from this transgenic event with non-transgenic maize. Agronomic characters were evaluated in field trials, also in comparison to non-transgenic maize. No evidence was found to suggest that this product would pose a greater risk to human or animal health, or to the environment.

The food and livestock feed safety of MON89034 maize grain and forage was established based on several standard criteria. As part of the safety assessment, the nutritional composition of MON89034 grain was found to be equivalent to conventional maize as shown by the analyses of key nutrients including proximates (i.e., moisture, crude protein, crude fat, ash and carbohydrates), acid detergent fibre, neutral detergent fibre, total dietary fibre, amino acid composition, fatty acid profiles, minerals (e.g., calcium, phosphorus, magnesium), and vitamins (i.e., folic acid, niacin, vitamin E), as well as antinutrient compounds. Similar compositional analyses were conducted on MON89034 forage harvested at the late dough to early dent stages.

Links to Further Information Expand

Canadian Food Inspection Agency, Plant Biotechnology Office

Decision Document DD2008-74: Determination of the Safety of Monsanto Canada Inc.'s Corn (Zea mays L.) Event MON 89034
[PDF Size: 86.34K bytes]

Comissão Técnica Nacional de Biossegurança - CTNBio (Brazil)

Technical Opinion No. 2052/2009
[PDF Size: 61.40K bytes]

European Food Safety Authority

Scientific Opinion: Application (Reference EFSA-GMO-NL-2007-37) for the placing on the market of the insect-resistant genetically modified maize MON89034, for food and feed uses, import and processing under Regulation (EC) No 1829/2003 from Monsanto
[PDF Size: 230.29K bytes]

European Food Safety Authority (EFSA)

COMMISSION DECISION of 30 October 2009 authorising the placing on the market of products containing, consisting of, or produced from genetically modified maize MON 89034 (MON-89Ø34-3) pursuant to Regulation (EC) No 1829/2003 of the European Parliament and of the Council
[PDF Size: 720.90K bytes]

Food Standards Australia New Zealand

Final Assessment Report - Application A595: Food Derived From Insect-Protected Corn Line MON 89034
[PDF Size: 451.67K bytes]

Japanese Biosafety Clearing House, Ministry of Environment

Outline of the biological diversity risk assessment report: Type 1 use approval for MON89034
[PDF Size: 474.47K bytes]

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

Determination of Nonregulated Status for Corn Genetically Engineered for Insect Resistance
[PDF Size: 58.74K bytes]
Petition for Non-Regulated Status for Corn Event MON89034 (APHIS 06-289-01p)
[PDF Size: 4.97M bytes]
Petition for Non-Regulated Status for Corn Event MON89034 (APHIS 06-289-01p): Finding of No Significant Impact
[PDF Size: 545.33K bytes]

United States Food and Drug Administration

Biotechnology Consultation Note to the File BNF No. 000107
[PDF Size: 40.13K bytes]

This record was last modified on Thursday, September 22, 2016

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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