The WX-F strain of Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae) was derived from a field-collected strain (WX strain) after 77 generations of continuous selection with fipronil. The selection resulted in 86.6-fold resistance to fipronil; WX-F showed no cross-resistance to dieldrin (1.8-fold) and low-level cross-resistance to endosulfan (3.7-fold) compared with the primary WX strain. Bioassay with synergists and analysis of detoxification enzyme activities indicated that metabolic detoxification is not significantly associated with fipronil resistance in the WX-F strain. Further molecular analysis indicated that the fipronil resistance was most likely attributed to target site insensitivity. Sequencing a cloned single cDNA fragment of the Rdl γ-aminobutyric acid (GABA) receptor gene showed a high frequency (80%) of the A2′N mutation in the fipronil-selected strain (WX-F). It carried a mutant LsRdl allele with the amino acid replacement of Ala (GCC) to Asn (AAC). This mutation was not detected in the 23 individuals in the WX strain. The high frequency of the LsRdl-Asn allele in the WX-F strain indicates that the A2′N mutation of the γ-aminobutyric acid (GABA) receptor gene may be associated with fipronil resistance in L. striatellus. In addition, a novel R305Q or R305W mutation was found in the cytoplasmic loop between M3 and M4 of the L. striatellus Rdl γ-aminobutyric acid (GABA) receptor subunit (Ls-Rdl). These were only found in cDNA clones carrying the A2′N mutation.
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