Dominant Inheritance of Field-Evolved Resistance to Fipronil in Plutella xylostella (Lepidoptera: Plutellidae)

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(sic) x S(sic)) and 1,380 -fold (R,_; x S'T) resistance to fipronil, indicating resistance is inherited as an incompletely dominant trait. Analysis of progeny from a backcross (F1 x S?) suggests that resistance is controlled by one major locus. The LC50 of the R(sic) x cross F-1 progeny is slightly but significantly higher than that of the R(sic) x S(sic) cross F-1 progeny, suggesting a minor resistance gene on the Z chromosome. Sequence analysis of PxGABARy.1 (an Rth-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. PxGABAR alpha 1 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 (PxGABAR alpha 1) on the Z chromosome.