The Resistance to dieldrin (Rdl) gene encodes a subunit of the insect gamma-aminobutyric acid (GABA) receptor. Cyclodiene resistance in many insects is associated with replacement of a single amino acid (alanine at position 302) with either a serine or a glycine in the Rdl gene. Two Rdl-orthologous genes of GABA receptors (PxGABARalpha1 and PxGABARalpha2) were cloned and sequenced from a susceptible strain (Roth) of Plutella xylostella. PxGABARalpha1 and PxGABARalpha2 showed 84% and 77% identity with the Rdl gene of Drosophila melanogaster at an amino acid level, respectively. The coding regions of PxGABARalpha1 and PxGABARalpha2 both comprise ten exons, with two alternative RNA-splicing forms in exon 3 of both genes. At the orthologous position of alanine-302 in D. melanogaster Rdl, PxGABARalpha1 has a conserved alanine at position 282. PxGABARalpha2 has a serine instead of an alanine at the equivalent position. With two informative DNA markers, both PxGABARalpha1 and PxGABARalpha2 were mapped onto the Z chromosome of P. xylostella.
A field-collected strain (HF) of Plutella xylostella (L.) showed 420-fold resistance to fipronil compared with a susceptible laboratory strain (Roth). The HF-R strain, derived from the HF strain by 25 generations of successive selection with fipronil in the laboratory, developed 2,200-fold resistance to fipronil relative to the Roth strain. The F(1) progeny of the reciprocal crosses between HF-R and Roth showed 640-fold (R♀ × S♂) and 1,380-fold (R♂ × S♀) resistance to fipronil, indicating resistance is inherited as an incompletely dominant trait. Analysis of progeny from a backcross (F1♂ × S♀) suggests that resistance is controlled by one major locus. The LC(50) of the R♂ × S♀ cross F(1) progeny is slightly but significantly higher than that of the R♀ × S♂ cross F(1) progeny, suggesting a minor resistance gene on the Z chromosome. Sequence analysis of PxGABARα1 (an Rdl-homologous GABA receptor gene of P. xylostella) from the HF-R strain identified two mutations A282S and A282G (corresponding to the A302S mutation of the Drosophila melanogaster Rdl gene), which have been previously implicated in fipronil resistance in several insect species including P. xylostella. PxGABARα1 was previously mapped to the Z chromosome of P. xylostella. In conclusion, fipronil resistance in the HF-R strain of P. xylostella was incompletely dominant, and controlled by a major autosomal locus and a sex-linked minor gene (PxGABARα1) on the Z chromosome.
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