Résumé des résultats (Abstract)
(Anglais)
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Our task in this project was to develop molecular markers for pesticide resistance genes in the Western Flower Thrips Frankliniella occidentalis. This greenhouse pest was introduced only about ten years ago from Western USA, yet it already shows high levels of resistance to a broad range of insecticides and especially pyrethroids. We therefore chose to focus on a genetic marker for pyrethroid resistance in this arrhenotokous species.
Pyrethroid resistance has been well described at the molecular level for several insect species. We thus could develop consensus PCR primers that amplify a segment of the para-like sodium channel gene, which is the molecular target of pyrethroids. This segment is known to carry pyrethroid resistance conferring point mutations (kdr and superkdr) in diptera and other insects. We then studied the genetic variation in this gene segment within a model greenhouse population of F. occidentalis, before and after treatment with different doses of a pyrethroid insecticide to search for potential resistance markers.
The amplified 760bp gene segment, although homologous, differed from published genomic sequences (Blatella germanica and Musca domestica) for intron positions and intron size. The kdr mutation was found at a high frequency in our sample and could be associated to a low level of pyrethroid resistance. In contrast, the superkdr mutation, that confers high levels of insecticide resistance in Dipteres, was not found in F. occidentalis. However, another point mutation close to the superkdr position was significantly linked to pyrethroid resistance in F. occidentalis. After treatment with a high dose of insecticide, the proportion of insects at least heterozygous for this mutation increased to nearly 90%. Other factors are probably involved in pyrethroid resistance, as this mutation is still quite frequent in insects that die after exposure to a low dose of pyrethroids. The combined occurrence of Kdr and 'thrips-superkdr' allows for a correct diagnosis of pyrethroid resistance in 94% of all tested individuals. Therefore, this marker system provides a good diagnostic tool to screen populations for pyrethroid resistance.
Nucleotide diversity within our model population was very high, comparable to levels that are known, for example, for the entire species of Drosphila simulans. This may be explained by the specific ecological conditions encountered in international plant trade networks. Live animals can travel on traded plants over long distances, and local populations can receive high percentages of immigrants with very different pesticide exposure histories. As the travellers likely are resistant to one or several pesticides, plant trade could lead to the dissemination of independently selected resistance mutations. Resistance monitoring and co-ordinated treatments adapted to local resistance profiles seem thus necessary for resistance management in this and probably other greenhouse pest species.
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