Insecticide resistance caused by recombination of two genes
A novel enzyme makes cotton bollworm resistant against pyrethroids. Insecticide resistance in crop pests is a serious global problem. Scientists of the Max Planck Institute for Chemical Ecology in Jena, Germany, have now found out what causes the strong resistance of an Australian strain of cotton bollworms (Helicoverpa armigera) to fenvalerate. The larvae evolved a novel enzyme capable of detoxifying fenvalerate in one single chemical reaction from the group of so-called P450 monooxygenases. The gene encoding the enzyme is a chimera − a combination of parts of two precursor genes. (PNAS, Early Edition, September 4th, 2012. DOI: 10.1073/pnas.1202047109)
Larvae of the cotton bollworm (Helicoverpa armigera) are dreaded pests all over the world. They have a very wide host range: About 200 different plant species are known as potential food for the voracious insect. The herbivore attacks crops in Africa, South Europe, India, Central Asia, New Zealand, and Australia. Nearly 30% of all globally used insecticides − Bt toxins as well as pyrethroids − are applied to protect cotton and other crops against the bollworm.
Resistance to pyrethroids
Pyrethroids are synthetic substances based on compounds of the natural insecticide pyrethrum in Tansy flowers (Tanacetum). They have been successfully applied in fruit, vegetable and crop farming for decades. In cotton bollworms, development of resistance to the particularly effective pyrethroid fenvalerate was observed in Australia since 1983. In 1998, David Heckel, who became director at the Max Planck Institute for Chemical Ecology in 2004, mapped the location of the resistance gene in the genome of the insect for the first time. This gene locus was later found to encode a P450 monooxygenase enzyme. These so-called cytochrome P450 (CYP) enzymes are also known in human medicine, because they can render toxins or pharmaceuticals ineffective by metabolizing the active molecule.
Nicole Joußen, scientist at the institute and expert on P450 enzymes, studied the Helicoverpa armigera strain “TWB” resistant to fenvalerate. She could identify the P450 monooxygenase in this strain, suspected to mediate pyrethroid resistance. After cloning larger genome regions, DNA sequencing, and performing crossing experiments as well as biochemical analyses, she was surprised to find that of the seven P450 enzymes only one, CYP337B3, hydroxylates the fenvalerate molecule to 4‘-hydroxyfenvalerate. This chemical reaction increases the resistance to this toxin 42-fold: A LD50 value of 1.9 µg of fenvalerate was measured in resistant TWB larvae, whereas half of the non-resistant caterpillars died after intake of only 0.04 µg of the toxin.