DD1998-23: Determination of Environmental Safety of Dekalb Genetics Corporation's European Corn Borer (ECB) Resistant Corn (Zea mays L.) Line DBT418
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This Decision Document has been prepared to explain the regulatory decision reached under the guidelines Dir94-08 Assessment Criteria for Determining Environmental Safety of Plants with Novel Traits and its companion document Dir94-11 The Biology of Zea mays L. (Corn/Maize) and the guidelines Dir95-03 Guidelines for the Assessment of Livestock Feed from Plants with Novel Traits.
The Canadian Food Inspection Agency (CFIA), specifically the Plant Biosafety Office of the Plant Health and Production Division and the Feed Section of the Animal Health and Production Division, has evaluated information submitted by Dekalb Genetics Corporation. This information is in regard to the European Corn Borer (ECB) resistant corn line DBT418. The CFIA has determined that this plant with novel traits does not present altered environmental interactions or pose concerns for the safety of livestock consuming feed derived from the PNT, when compared to currently commercialized corn varieties in Canada.
Unconfined release into the environment and use as livestock feed of the corn line DBT418 is therefore authorized as of March 10, 1997. Any other Zea mays lines and intraspecific hybrids resulting from the same transformation event and all their descendants, are also approved, provided no inter-specific crosses are performed, provided the intended use is similar, provided it is known following thorough characterization that these plants do not display any additional novel traits and that the resulting lines are substantially equivalent to currently grown corn, in terms of their potential environmental impact and livestock feed safety and provided that pest resistance management requirements described in the present document are applied.
Please note that, while determining the environmental safety of plants with novel traits is a critical step in the commercialization of these plant types, other requirements still need to be addressed, such as the evaluation of food safety (Health Canada).
Table of Contents
- Brief Identification of Plant with Novel Traits (PNT)
- Background Information
- Description of the Novel Traits
- Resistance to the European Corn Borer (ECB)
- Glufosinate Ammonium Herbicide Tolerance
- Development Method
- Stable Integration into the Plant's Genome
- Assessment Criteria for Environmental Safety
- Potential of the PNT to Become a Weed of Agriculture or be Invasive of Natural Habitats
- Potential for Gene Flow to Wild Relatives Whose Hybrid Offspring May Become More Weedy or More Invasive
- Altered Plant Pest Potential
- Potential Impact on Non-Target Organisms
- Potential Impact on Biodiversity
- Potential for Development of ECB Resistance to the PNT
- Nutritional Assessment Criteria for Use as Livestock Feed
- Nutritional Composition of the PNT's
- Anti-Nutritional Factors
- Regulatory Decision
I. Brief Identification of the Plants with Novel Traits (PNT's)
Designation(s) of the PNT: DBT418
Applicant: Dekalb Genetics Corporation
Plant Species: Zea mays L.
Novel Traits: Resistance to European Corn Borer (Ostrinia nubilalis); tolerance to the herbicide glufosinate ammonium.
Trait Introduction Method: Microprojectile bombardment of plant cells.
Proposed Use of PNT's: Production of Zea mays for human consumption (wet mill products, dry mill products and seed oil) and seed oil, meal, whole grain and silage for livestock feed. These materials will not be grown outside the normal production area for corn in Canada.
II. Background Information
Dekalb has developed a corn line resistant to the European Corn Borer (ECB) larvae, a periodic pest of corn in Canada. This corn line, referred to in this document as DBT418, was developed to provide a method to control yield losses from insect feeding damage caused by the larval stages of ECB, without the use of conventional pesticides.
DBT418 was developed using recombinant DNA technology, resulting in the introduction of bacterial genes conferring ECB resistance and glufosinate ammonium herbicide tolerance, into elite hybrid lines. Herbicide tolerance was used to select modified plants, but is not expressed at sufficient levels to confer field tolerance to the herbicide.
Dekalb has provided data on the identity of line DBT418, a detailed description of the transformation method, data and information on the gene insertion sites, copy number and levels of expression in the plant, the role of the inserted genes and regulatory sequences in donor organisms and full nucleotide sequences. The novel proteins were identified, characterized and compared to the original bacterial proteins, including an evaluation of their potential toxicity to livestock and non-target organisms. Relevant scientific publications were supplied.
These materials have been field tested in Canada under confined conditions in Ontario in 1995 and 1996.
Agronomic characteristics of corn hybrids derived from DBT 418 such as seed yield, seedling vigour, pollen GDU, silk GDU, final stand count, stalk lodging, ear height, plant vigour, susceptibilities to various Z. mays pests and pathogens, were compared to those of unmodified Z. mays counterparts.
The Plant Biosafety Office of the Plant Health and Production Division, CFIA, has reviewed the above information, in light of the assessment criteria for determining environmental safety of plants with novel traits, as described in the regulatory directive Dir94-08:
- potential of the PNT's to become weeds of agriculture or be invasive of natural habitats,
- potential for gene flow to wild relatives whose hybrid offspring may become more weedy or more invasive,
- potential for the PNT's to become plant pests,
- potential impact of the PNT's or their gene products on non-target species, including humans, and
- potential impact on biodiversity.
The CFIA has consulted with the Pest Management Regulatory Agency of Health Canada on issues related to potential development of ECB populations resistant to the insecticidal protein produced by the PNT's.
The Feed section of the Animal Health and Production Division, CFIA, has also reviewed the above information with respect to the assessment criteria for determining the safety and efficacy of livestock feed, as described in Dir95-03 the following have been considered:
- potential impact to livestock and
- potential impact on livestock nutrition.
III. Description of the Novel Traits
1. Resistance to the European Corn Borer (ECB)
- Bacillus thuringiensis var. kurstaki strain HD-73 (B.t.k.) is a common gram-positive soil-borne bacterium. In its spore forming stage, it produces several insecticidal protein crystals, including the delta-endotoxin Cry1A(c), which is active against certain lepidopteran insects, such as ECB. This protein has been shown to be non-toxic to humans, other vertebrates and beneficial insects. B.t.k. based foliar applied insecticides have been registered for over 30 years in Canada and have a long history of safe use.
- A synthetic gene coding the first 613 amino acids of cryIA(c) was developed to provide maximum expression in corn, and introduced into line DBT418. The gene codes for a protein similar to the Bacillus thuringiensis var. kurstaki HD-73 insecticidal crystal protein tryptic core fragment. The protein is insecticidal to lepidopteran larvae. Insecticidal delta-endotoxin activity is believed to depend on the binding of the active fragment to specific receptors present in susceptible insects on midgut epithelial cells, forming pores which disrupt osmotic balance and eventually results in cell lysis. Specific Lepidopteran pests of corn sensitive to the protein are ECB and corn earworm.
- The cry1A(c) gene is linked to a strong constitutive promoter. Average protein level ranges in various tissues is as follows:
- harvest leaves: 459.6 to 1194.4 ng Cry1A(c) per gram tissue (dry weight)
- harvest kernels: 36.0 to 42.8 ng Cry1A(c) per gram tissue (dry weight)
- harvest root: 58.0 to 125.4 ng Cry1A(c) per gram tissue (dry weight)
- harvest stalk: 40.9 to 123.6 ng Cry1A(c) per gram tissue (dry weight)
- The Cry1A(c) protein was not detectable in pollen.
- The Cry1A(c) protein degrades readily in the environment. Monsanto studies with Cry1A(b) a similar protein indicated a half life of 1.6 days when the protein was a component of corn tissue. In Cotton tissue, Cry1A(c) has a half life of 41 days.
- Protein allergens are normally resistant to digestion unlike the Cry1A(c) protein which was shown to degrade readily in simulated gastric fluid. Unlike many known allergens the insecticidal protein is not glycosylated. A search for amino acid sequence similarity between the Cry1A(c) protein and known allergens, using the GENINFO-BLAST network server, revealed no significant amino acid sequence homologies. A search of a similarly constructed database of known toxins indicated no amino acid sequence homologies between known toxins and the Cry1A(c) protein, with the exception of homologies to other Bt insecticidal proteins.
- The full nucleotide sequence and corresponding amino acid sequence were provided.
- The 66 kDa protein expressed in plants was compared to the bacterial protein core fragment and shown to be of similar molecular weight and immunological reactivity. The protein was not glycosylated and showed similar bioactivity and host range specificity to the native protein.
2. Glufosinate Ammonium Herbicide Tolerance
- Phosphinothricin (L-PPT), the active ingredient of the herbicide glufosinate ammonium, inhibits glutamine synthetase, which results in the accumulation of lethal levels of ammonia in susceptible plants within hours of application.
- The phosphinothricin tolerance gene (bar) engineered into DBT418 was isolated from a common aerobic soil streptomycete and codes for PPT-acetyltransferase (PAT). This enzyme detoxifies phosphinothricin by acetylation into an inactive compound. It has extremely high substrate specificity; experimental data clearly showed that neither L-PPT's analog L-glutamic acid, D-PPT, nor any protein amino acid can be acetylated by the PAT enzyme.
- The gene, modified for enhanced gene expression, is linked to a constitutive promoter. Data showed average protein level ranges in various
tissues as follows:
- pollen stage leaves: 501.8 to 1099.4 ug PAT per g tissue (dry weight)
- pollen stage stalk 66 to 77 ug PAT per g tissue (dry weight)
- pollen stage root 27.5 to 69.5 ug PAT per g tissue (dry weight)
- kernel 3.1 to 6.0 ug PAT per g tissue (dry weight)
The PAT protein was not detected in pollen.
- The expressed PAT enzyme was compared to the bacterial protein: molecular weights were similar, indicating that the protein had not been glycosylated nor had it undergone post transcriptional modifications.The amino terminal sequence of DBT 418 derived PAT is the same as the native protein.
- Studies showed that the enzyme was completely degraded within five minutes when subjected to typical mammalian gastric conditions and is thus digested as a conventional dietary protein.
- The gene nucleotide sequence and the enzyme amino acid sequence were provided. The nucleotide sequence showed no significant homology with the toxins or allergens entered in the GENINFO-BLAST network server database.
3. Development Method
- DBT418 was co-transformed with three vectors, one carrying a synthetic cry1A(c) gene and the second bearing the herbicide resistance gene. The third carried the potato proteinase inhibitor (pin II) gene. An ampicillin resistance gene, regulated by a bacterial promoter, also present in the vectors used to transform was not expressed. The three plasmid vectors were introduced by microprojectile bombardment into cultured plant cells. Glufosinate Ammonium tolerant transformed cells were selected, then cultured in tissue culture medium for plant regeneration.
4. Stable Integration into the Plant's Genome
Analysis indicated integration of two intact copies of the cry1A(c) gene, one intact copy of the bar gene and one rearranged copy of the bar gene. Partial copies of the pin II gene and alcohol dehydrogenase (adh) intron 1 were also integrated. There are four intact and one partial copy of the beta lactamase gene and four intact copies of the Col E1 origin of replication. The beta lactamase genes are not expressed and the pin II gene is non-functional and is not expressed.
IV. Assessment Criteria for Environmental Safety
1. Potential of the PNT to Become a Weed of Agriculture or be Invasive of Natural Habitats
The biology of corn (Zea mays), described in Dir94-11, shows that unmodified plants of this species are not invasive of unmanaged habitats in Canada. Corn does not possess the potential to become weedy due to traits such as lack of seed dormancy, the non-shattering aspect of corn cobs, and poor competitive ability of seedlings. According to the information provided by Dekalb, DBT418 and derived corn hybrids were determined not to be different from their counterparts in this respect.
CFIA evaluated data submitted by Dekalb on the reproductive and survival biology of corn hybrids derived from DBT418, and determined that vegetative vigour, time to maturity and seed production were within the normal range of expression of these traits currently displayed by commercial corn hybrids. No genes were inserted for cold tolerance or winter survival.
No competitive advantage was conferred to these plants, other than that conferred by resistance to European Corn Borer and tolerance to glufosinate. Resistance to ECB will not, in itself, render corn weedy or invasive of natural habitats since none of the reproductive or growth characteristics were modified. Any glufosinate tolerant corn volunteer plants can be easily managed by either mechanical means or by the use of other available herbicides with differing modes of action.
NOTE: A longer term concern, if there is general adoption of several different crop and specific herbicide weed management systems, is the development of crop volunteers with novel tolerances to specific herbicides. This could result in the loss of the use of these herbicides in some crop rotation cycles. Some canola (Brassica napus) varieties modified to express glufosinate ammonium tolerance (see DD95-02) were recently registered in Canada. Should glufosinate tolerant corn and canola be grown in rotation, volunteers would not be controlled with this herbicide. Agricultural extension personnel, in both the private an public sectors, should therefore promote careful management practices for growers who use any of these herbicide tolerant crops.
The above considerations, together with the fact that the novel traits have no intended effects on weediness or invasiveness characteristics, led CFIA to conclude that DBT418 has no altered weed or invasiveness potential compared to currently commercialized corn.
2. Potential for Gene Flow to Wild Relatives Whose Hybrid Offspring May Become More Weedy or More Invasive
The biology of corn, as described in Dir94-11, indicates that there are no wild relatives in Canada that can hybridize with Zea mays.
CFIA therefore concludes that gene flow from DBT418 to wild corn relatives is not possible in Canada.
3. Altered Plant Pest Potential
The intended effects of both novel traits are unrelated to plant pest potential, and corn is not a plant pest in Canada (Dir94-11). In addition, agronomic characteristics of the modified corn hybrids were shown to be within the range of values displayed by currently commercialized corn hybrids, and indicate that the growing habit of corn was not inadvertently altered. Field observations did not indicate modifications of disease and pest susceptibilities, other than to ECB.
CFIA has therefore determined that DBT418 does not display any altered pest potential.
4. Potential Impact on Non-Target Organisms
The PAT enzyme responsible for glufosinate ammonium tolerance has very specific enzymatic activity, does not possess proteolytic or heat stability, and does not affect plant metabolism. Acetyltransferases are common enzymes in bacteria, plants and animals. A search of the GENEBANK DNA sequence database revealed no significant homology with the toxins or allergens entered in that database.
The history of use and literature suggest that the bacterial B.t.k. proteins are not toxic to humans, other vertebrates, and beneficial insects and the B.t.k. proteins produced in corn were shown to be equivalent to the original microbial proteins. These proteins are active only against specific lepidopteran insects.
Based on the above, CFIA has determined that the unconfined release of DBT418, when compared with currently commercialized corn, will not result in altered impacts on interacting organisms, including humans, with the exception of specific lepidopteran insect species.
5. Potential Impact on Biodiversity
DBT418 has no novel phenotypic characteristics which would extend its use beyond the current geographic range of corn production in Canada. Since corn does not outcross to wild relatives in Canada, there will be no transfer of novel traits to unmanaged environments.
At present, the use of chemical insecticides to control ECB is not recommended for grain corn produced in Canada, as they are only effective when the larvae first emerge and before these begin to move to the whorl of the plant and into the stalk. It is not considered cost-effective to use insecticides to control ECB in grain corn, but treatment is recommended in sweet corn given the higher cash value of the crop and lower consumer tolerance for insect-damaged cobs. Current recommendations by provincial extension services for the control of ECB in grain corn are: to grow corn hybrids which have been traditionally bred for natural resistance to ECB, to avoid planting at densities that are too high for the chosen hybrid, to plant early to reduce infestations by the second generation of ECB, to harvest early in order to decrease losses from dropped ears and broken stalks, and to practice crop rotation. Despite the promotion of no-till practices in corn for soil conservation, it is still recommended to plow-down corn residues in the fall, in order to kill over-wintering larvae. Thus, the cultivation of DBT418 will not result in any overall change in the use of chemical insecticidesFootnote 1.
CFIA has therefore concluded that the potential impact on biodiversity of DBT418 will be neutral.
6. Potential for Development of ECB Resistance to the PNT
The potential for lepidopteran insects to develop resistance to conventional chemical insecticides is well documented. A B.t.k. foliar insecticide is currently registered for control of ECB on hybrid seed corn production. Another B.t.k. insecticide is registered to control cabbage looper (but not ECB) on pepper, a crop also visited and attacked by ECB. Resistance may develop as a result of increased use of these B.t.k. foliar sprays; resistance to the B.t.k. proteins could also develop following continued exposure to ECB-resistant hybrid corn.
The development of such a resistance would result in the loss of valuable B.t.k. tools for the control of ECB infestations in corn, and to a lesser extent, pepper.
DBT418 steadily produces high levels of B.t.k. delta endotoxin in leaves resulting in mortality of ECB feeding on this plant. Target insects will thus be exposed to significantly higher levels of B.t.k. than through the current foliar spray treatments, leading to high selection pressures for resistant ECB individuals. It is currently accepted that ECB has one or more generations a year in Canada. The number of ECB generations that will develop in any one season will be influenced by the environmental conditions in a given area, particularly temperature and day length. The potential for development of resistant ECB populations may also increase in areas with multi-generations.
A component of a possible resistance management strategy linked to the use of DBT 418 is the presence of non-selecting refugia (unmodified corn) in close proximity, where susceptible insect populations are maintained. Should resistant insects occur, they would then be able to mate with susceptible insects to produce heterozygotes, which are expected to be susceptible to the ECB-resistant corn hybrids. The behaviour of ECB during mating is such that individuals migrate to grassy areas adjacent to corn fields to mate, hence increasing the likelihood that any resistant ECB individuals mate with susceptible ones. Initially there will exist sufficient unstructured non-Bt corn refugia and this may delay the development of resistance. Should the acreage of ECB- resistant corn become greater than the non-Bt hybrids, careful management resulting in the maintenance of non-Bt corn (structured refugia) may be necessary to provide the required non-Bt refugia. Even though the majority of the scientific community agrees that this approach sounds effective in theory, it is very difficult at this point to predict the extent and rapidity of resistance development without field validation of the proposed strategy. These corn plants should therefore be responsibly managed and ECB populations monitored for development of resistant individuals in a regular and consistent manner.
Consideration must be given to the possibility that ECB populations developing resistance to the corn produced B.t.k. protein could also develop cross-resistance to other Bt delta-endotoxins, resulting in the loss of other Bt protein types that may be used for the control of ECB infestations.
The development of resistance to the Cry1A(c) protein by non-target insect pests, that may then cause further problems in other crops, is another consideration. Armyworm (Lepidoptera: Pseudaletia unipuncta) is a sporadic pest in Canada. It feeds on corn and other crops such as forage grasses (e.g. timothy), wheat, oats and barley. Corn earworm (Lepidoptera: Helicoverpa zea) feeds on the silks and developing ears (pre-dough stage of kernels) of corn. Presently, B.t.k. foliar sprays are not registered for control of these insects in Canada, so even should resistance occur, control of these insects would not be compromised. CFIA has therefore concluded that development of resistance in non-target insect pests is unlikely to have an impact on the conventional control of these pests.
CFIA believes that sound management practices can reduce and delay the development of resistant ECB populations, and that ECB populations must be monitored for the development of resistance in a regular and consistent manner. CFIA understands that Dekalb has developed and will implement a pest resistance management plan that includes the following key components:
- The early detection of ECB populations resistant to the corn-expressed insecticidal protein is extremely important. Close monitoring for the presence of such populations, in ECB-resistant corn fields and surrounding areas, is therefore warranted. Monitoring includes the development of appropriate detection tools such as visual field observations and laboratory bioassays, education of growers, reporting schedules, and enforcement procedures in case of resistance development.
- Education tools will be developed and provided to all growers, district managers and field managers. These will include information on product performance, resistance management, monitoring procedures and timetables, detection protocols for resistant ECB individuals, instructions to contact Dekalb and strategies to be followed if unexpected levels of ECB damage occur.
- Dekalb will have procedures in place for responding to these reported instances of unexpected ECB damage. These procedures will include, where warranted, the collection of plant tissue and ECB and use of appropriate bioassays to evaluate suspected Cry1A(c) resistant individuals, and a protocol for immediate action to control resistant individuals.
- Detection of confirmed resistant ECB populations and following action plan will immediately be reported to the CFIA.
- Integrated Pest Management practices will be promoted, such as destruction of overwintering habitat and crop rotation.
- The strategy for resistance management of ECB when using plants that continually produce high concentrations of a B.t.k. delta-endotoxin and refugia has not been previously tested in the field on a large scale. Continued research in this area using sound science will be conducted.
- The developed plans, information and data from the above are available to the CFIA.
The CFIA also strongly encourages Dekalb to develop novel ECB control systems with different modes of action that would offer additional or alternative management practices to growers.
If at any time, Dekalb becomes aware of any information regarding risk to the environment, including risk to agriculture such as development of ECB resistance, or risk to animal or human health, that could result from release of these materials in Canada, or elsewhere, Dekalb will immediately provide such information to the CFIA. On the basis of such new information,the CFIA will re-evaluate the potential impact of the proposed release, and will re-evaluate its decision with respect to the unconfined release of these corn hybrids.
V. Nutritional Assessment Criteria for Use as Livestock Feed
1. Nutritional Composition of the PNT
Comparisons of protein, fat and fibre of corn grain and whole plant material from each PNT line and its respective parent line were made. In both the grain and the whole plant, there were occasional significant differences between the PNT and its respective control but there was no consistent pattern of differences from the control for any nutrient for either grain or the whole plant. Such differences are commonplace when making comparisons among the numerous varieties of hybrid grain corn in Canada.
Protein, fat and fibre concentration were within the published range for corn, in both the grain and the whole plant in the PNT. The observed variations in nutritional composition was judged to arise from normal variability rather than as a result of the inserted novel traits. The CFIA has determined that line DBT418 is substantially equivalent to traditional corn varieties.
2. Anti-Nutritional Factors
The parent plant Zea mays is not known for the production of anti-nutritional factors and the transformation event which produced DBT418, would not be expected to induce their synthesis.
VI. Regulatory Decision
Based on the review of data and information submitted by Dekalb, and through comparisons of DBT418 corn with unmodified corn counterparts, the Plant Biosafety Office of the Plant Health and Production Division, CFIA, has concluded that the novel genes and their corresponding traits do not confer to these plants any characteristic that would result in intended or unintended environmental effects following unconfined release. Dekalb has developed and will implement a resistance management plan.
Based on the review of submitted data and information, the Feed Section of the Animal Health and Production Division has concluded that the novel traits do not in themselves raise any concerns regarding the safety or nutritional composition of DBT418. Grain corn, its byproducts and corn oil are currently listed in Schedule iv of the Feeds Regulations and are, therefore approved for use in livestock feeds in Canada. The ECB resistant corn hybrids have been assessed and found to be substantially equivalent to traditional corn varieties, DBT418 and its byproducts are considered to meet present ingredient definitions and are approved for use as livestock feed ingredients in Canada.
Unconfined release into the environment and use as livestock feed of the corn line DBT418 is therefore authorized. Any other Zea mays lines and intraspecific hybrids resulting from the same transformation event, and all their descendants, are also approved, provided no inter-specific crosses are performed, provided the intended use is similar, provided it is known following thorough characterization that these plants do not display any additional novel traits and that the resulting lines are substantially equivalent to currently grown corn, in terms of their potential environmental impact and livestock feed safety and provided that pest resistance management requirements described in the present document are applied.
This bulletin is published by the Plant Health and Production Division. For further information, please contact the Plant Biosafety Office or the Feed Section at:
Plant Biosafety Office
Plant Health and Production Division
Plant Products Directorate
59 Camelot Drive, Nepean
Ontario, K1A 0Y9
Animal Health and Production Division
Animal Products Directorate
59 Camelot Drive, Nepean
Ontario, K1A 0Y9
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