GMO

Question 1

Why do we need transgenic plants?

Answer 1

Because transgenic plants can:

• Increase crop yield;
• Improve product quality
• Improve tolerance to unfavorable abiotic factors such as drought, heat, cold, etc;
• Improve tolerance to diseases, pesticide, insect pests, etc.

Question 2

List three major traits that have been genetically modified in crop protection.

Answer 2

• Herbicide resistance (RR and LL crops)
• Bt genes for insect resistance (Bt crops)
• Drought tolerant corn (DroughtGard ™ corn)

Question 3

List the major GMO crops (ranking in area planted in the word).

Answer 3

1st - Soybeans (94.1 MM hectares)
2nd – Corn (59.7 MM hectares)
3rd – Cotton (24.1 MM hectares)
4th – Canola (10.2 MM hectares)

Question 4

What is Bt?

Answer 4

Bt is a rod-shaped gram positive bacterium commonly found in the soil. It grows when nutrients are abundant, but forms a dormant spore and tiny crystals when food supply runs short. Crystals are composed of insecticide proteins known as δ-endotoxins, which account for more than 30% of the total protein made by Bt. Also, Bt produces insecticidal proteins during vegetative stages (VIP – vegetative insecticidal protein).

Question 5

General mode of action of Bt toxins.

Answer 5

Insect ingests Bt-Crystalline inclusion, which is solubilized in the midgut, realeasing proteins called δ-endotoxins(prototoxins).Prototoxins are activated by the midgut proteinases to become real toxins. Activated toxins interact with Bt binding receptors in the larval midgut membrane. This interactions causes a disruption in the membrane integrity, which causes cell swelling and lysis leading the insect to death.

Question 6

Give a short summary of the overall performance of Bt crops.

Answer 6

Overall, Bt crops have high resistance to target insect pests and some level of resistance to secondary pests. Additionally, Bt crops are excellent in yield protection.

Question 7

Give a short summary of Hutchison et al. 2010. Science 330: 222-225.

Answer 7

Using statistical analysis of per capita growth rate estimates, this study found that areawide suppression of European corn borer is associated with Bt maize use. Cumulative benefits over 14 years are an estimated $3.2 billion for maize growers in Illinois, Minnesota, and Wisconsin, with more than $2.4 billion of this total accruing to non-Bt maize growers. Comparable estimates for Iowa and Nebraska are $3.6 billion in total, with $1.9 billion for non-Bt maize growers. These results affirm theoretical predictions of pest population suppression and highlight economic incentives for growers to maintain non-Bt maize refuge for sustainable insect resistance management.

Question 8

What benefits have Louisiana crop growers gained from planting Bt corn?

Answer 8

SCB is the dominant corn borer species across LA. Net benefits from planting Bt corn for LA corn growers alone are more than $20 million annually. In addition, there are benefits for sorghum and rice growers.

Question 9

Where are the U.S. Corn Belt region and U.S. Cotton Belt region?

Answer 9

According to the USDA, the U.S. corn belt is centered in Iowa, Indiana, and Illinois, extending to W Ohio, S Minnesota, SE South Dakota, N Missouri, E Nebraska, and NE Kansas, in which corn and soybeans are the dominant crops. The U.S. cotton belt mainly include Noth Carolina, South Carolina, Georgia, Alabama, Mississippi, W Tennessee, E Arkansas, Louisiana, E Texas, and S Oklahoma.

Question 10

Tell the current situation of Bt crops planted in the world.

Answer 10

Last season, the total area of Bt crops planted in the world is 100.9 million hectares. The mains Bt crops are soybeans, maize, cotton, and canola. Brazil and USA are the countries that plant the most (approximately 35.7 and 33.6 million ha, respectively), followed by India (11 million ha), Argentina (6 million ha), and Pakistan (3 million ha). These five countries represented approximately 90% of the total area planted in the world. In total, 32 countries plant Bt crops worldwide.

Question 11

What are your major concerns in the use of GMO?

The major concerns are:

Answer 11

• Potential for allergic reaction (StarLink issue 1999);
• Possibility of gene flow to close relative plants;
• Possible introduction or increase in production of toxic compounds;
• The use of antibiotic resistance as markers;
• Impact on non-target organisms;
• Resurgence of secondary pests;
• Development of resistance in target pests.

Question 12

Give a short summary about the impact of Bt crops on non-target organisms.

Answer 12

Many studied have been conducted to address the impact of Bt on non-target organisms. Direct impact of Bt plants on non-target , if existed, are negligible. The current commercial Bt plants are relatively safe to their non-target pests. Special attention should be paid to the indirect effects of Bt plants on the secondary arthropod pests.

Question 13

How does resistance occur in field insect populations?

Answer 13

Field resistance occurs when there is a repeated field control failure or reduced efficacy for pest control in field conditions.

Question 14

Factors related to resistance development to Bt crop in the field.

Answer 14

A high degree of selection pressure favors individuals with resistance allele, which increases the population of resistant insects along the crop seasons causing the development of resistance (when more than 50% of individuals in a population are resistant, according to Tabashnik et al. 2014).

Question 15

What are the three groups of Bt proteins that have been used in transgenic crops to control lepidopteran pests?

Answer 15

Cry1, Cry2, and VIP proteins.

Question 16

General cross-resistance patterns among the three Bt groups for controlling lepidopteran pests.

Answer 16

Cross-resistance among Bt toxins is common and patterns are likely associated with specific binding sites in the midgut. The proteins of different groups of Bt are similar among themselves, thus, cross-resistance occurs in many cases.

Question 17

Give a short summary about the dominance levels of the resistance to Bt crops.

Answer 17

There are two dominance levels of resistance to Bt – Dominant and Recessive resistance. Dominant resistance is the inheritance of resistance in which the phenotype is resistant for individuals with either RS or RR resistance alleles at a genetic locus that determines susceptibility. In contrast, recessive resistance is the inheritance of resistance in which individuals have a resistant phenotype only if they have two resistance alleles (RR) at a genetic locus that determines susceptibility. Phenotype of RS is susceptible as SS.

Question 18

Give a short summary about the fitness costs of the resistance to Bt crops.

Answer 18

Fitness cost is the decrease in ability of a resistant population to survive and reproduce in absence of selection. Some Bt resistance is associated with recessive costs. In some cases, fitness costs can interact with environmental factors.

Question 19

What are the two resistance management strategies for planting Bt crops currently used in the US and several other countries?

Answer 19

“high dose/refuge” strategy and gene-pyramiding.

Question 20

Major assumptions of high dose/refuge IRM strategy.

insect occurs.

Answer 20

The key assumptions are: a high dose will kill more than 95% of the RS population; Resistance is functionally recessive or at least partially recessive; Initial resistance allele frequency is lower than 0.001; and random mating between susceptible and resistant

Question 21

How to determine if a Bt plant is qualified as a high dose plant.

Answer 21

A Bt plant will qualify as a high dose plant if it has 25 times the toxin concentration to kill both RS and SS and if it is able to kill more than 95% of the RS population.

Question 22

Why we need monitoring Bt resistance.

Answer 22

We need to monitor resistance continually because it can happen very quickly and there are many cases of resistance worldwide that teach us lessons about Bt crop use. Models have shown that once at least 2 RR insects exist, there is a potential for entire population to become resistant in six years. Therefore, monitoring can help to preserve Bt susceptibility.

Question 23

Major resistance monitoring methods for Bt resistance.

Answer 23

• Dose response bioassay
• Diagnostic/discriminating dose bioassay
• DNA-marker method
• F1 screen
• F2 screen
• Field survey plus laboratory bioassays
• Screening against known laboratory resistant insects (F1 screen)
• Sentinel plots

Question 24

Advantages and disadvantages of each of the resistance monitoring methods.

Answer 24

?

Question 25

If the allele frequency for a Bt resistance is 0.001 in an insect population, what is the frequency for RR, RS, SS in the population?

Answer 25

RR = 0.000001; RS = 0.001; SS = 0.998999

Question 26

If the allele frequencies for two Bt resistance gene A and B is 0.001 and 0.01, respectively, in an insect population, what are the frequencies for AABB, AABb, AAbb, AaBb, aabB, aaBB, aabb in the population?

Answer 26

AA = 1e-6
Aa = 1e-3 - 1e-6
aa = 1 - 1e-3 - 1e-6 = 0.998999
BB = 1e-4
Bb = 1e-2 - 1e-4
bb = 1 - 1e-2 - 1e-4 = 0.9899

Question 27

What are the refuge sizes required for Bt corn in the Northern U.S. Corn Belt and in the southern U.S. region.

Answer 27

For the structured refuge with a single gene, 20% non-Bt corn in corn-growing areas (the U.S. Corn Belt) and 50% non-Bt in cotton-growing areas (Southern U.S. Cotton Belt). For pyramided Bt corn, 5% in the U.S. corn belt and 20% in the U.S. cotton belt.

Question 28

Do crop growers follow the requirements of the refuge planting?

Answer 28

A compliance studied conducted between 2001 and 2007 revealed that approximately 80% of Bt corn growers follow both requirements of refuge planting (percentage and proximity). In 2008, this number dropped to 74%. In Canada, these number were similar for the same period, but decreased to 61% in 2009.

Question 29

What are the major problems in the use of seed mixture for planting Bt corn for corn earworm?

Answer 29

Pollen contamination may reduce the refuge populations in the RIB planting, especially for ear feeder targets such as the corn earworm.

Question 30

Do you think seed mixture is a good strategy for the U.S. southern region?

Answer 30

It might be for FAW but definitely not for CEW.

Question 31

What are major challenges in the continued success of the Bt crop technology?

Answer 31

Conservation of Bt susceptibility will continue to be a great challenge. Remedial action plans for managing Bt resistance should be developed quickly once it occurs. Finally, more alternative management strategies should be developed.

Question 32

In the class, we discussed several factors that might have contributed to the recently wide development of resistance to Bt crops in the world. List these factors and discus why these factors might relate to the resistance development.

Answer 32

‘High dose/refuge” strategy did not function as a non-recessive resistance.
- Based on previous studies, High levels of Bt resistance were thought to be typically controlled by largely recessive genes. However, the results of recent studies do not appear to agree with this general conclusion. Dominance levels of major Bt resistant genes have been examined in 13 cases associated with seven species and six Bt proteins. 6 were recessive to incompletely recessive, and the rest were functionally non-recessive with 4 cases being dominant or incompletely dominant.
Limited mode of action:
Cross-resistance:
Similar Bt proteins among groups:
Non-fitness costs:
Multiple selection of insect populations each year:
Cross-crop resistance: