Question

Which class of Bt-endotoxins are Coleopteran -specific

• A. Cry -I
• B. Cry -II
• C. Cry- III
• D. Cry- IV

Hint:

To easily remember target orders: Cry I = Lepidoptera, Cry II = Lepidoptera/Diptera, Cry III = Coleoptera, and Cry IV = Diptera.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The bacterium Bacillus thuringiensis (Bt) produces crystalline parasporal inclusions containing delta-endotoxins (Cry proteins) during sporulation.
These proteins are highly toxic to specific insect orders upon ingestion, which has been leveraged to develop transgenic insect-resistant crops.

Step 2: Detailed Explanation:
Under the classical classification system of Bt delta-endotoxins, Cry proteins are grouped based on their insecticidal host specificity:
1. Cry I: Highly toxic and specific to pests of the order Lepidoptera (such as bollworms, stem borers, and leaf folders).
2. Cry II: Shows toxicity toward both Lepidoptera and Diptera (mosquitoes and flies).
3. Cry III: Specifically toxic to insects of the order Coleoptera (beetles and weevils, such as the Colorado potato beetle).
4. Cry IV: Exclusively toxic and specific to the order Diptera.
This order-specific toxicity is mediated by the binding of activated Cry toxins to specific midgut receptors present only in target insect groups.

Step 3: Final Answer:
Therefore, Cry-III endotoxins are Coleopteran-specific.