Deok Ho Kwon

The molecular mechanisms and genetics of abamectin resistance mediated by target site insensitivity in the two-spotted spider mite, Tetranychus urticae, were investigated by comparing two isogenic abamectin-susceptible (AbaS) and abamectin-resistant (AbaR) strains. Cloning and sequencing of full-length cDNA fragments of gamma-amino butyric acid (GABA)-gated chloride channel genes revealed no polymorphisms between the two strains. However, sequence comparison of the full-length cDNA fragment of a T. urticae glutamate-gated chloride channel gene (TuGluCl) identified a G323D point mutation as being tentatively related with abamectin resistance. In individual F(2) progenies obtained by backcrossing, the G323D genotype was confirmed to correlate with abamectin resistance. Bioassays using progeny from reciprocal crossings revealed that the abamectin resistance trait resulting from TuGluCl insensitivity is incompletely recessive.

This study establishes deltamethrin resistance in a common bed bug, Cimex lectularius L., population collected from New York City (NY-BB). The NY-BB population was 264-fold more resistant to 1% deltamethrin in contact bioassay compared with an insecticide-susceptible population collected in Florida (FL-BB). General esterase, glutathione S-transferase, and 7-ethoxycoumarin O-deethylase activities of NY-BB were not statistically different from those of FL-BB. cDNA fragments that encoded the open reading frame of voltage-sensitive sodium channel alpha-subunit genes from the FL-BB and NY-BB populations, respectively, were obtained by homology probing polymerase chain reaction (PCR) and sequenced. Sequence alignment of the internal and 5' and 3' rapid amplification of cDNA ends (RACE) fragments generated a 6500-bp cDNA sequence contig, which was composed of a 6084-bp open reading frame (ORF) encoding 2027 amino acid residues and 186-bp 5' and 230-bp 3' untranslated regions (5' and 3' UTRs, respectively). Sequence comparisons of the open reading frames of the alpha-subunit genes identified two point mutations (V419L and L925I) that were presented only in the NY-BB population. L925I, located the intracellular loop between IIS4 and IIS5, has been previously found in a highly pyrethroid-resistant populations of whitefly (Bemisia tabaci). V419L, located in the IS6 transmembrane segment, is a novel mutation. A Val to Met mutation at the corresponding position of the bed bug V419, however, has been identified in the tobacco budworm as a kdr-type mutation. This evidence suggests that the two mutations are likely the major resistance-causing mutations in the deltamethrin-resistant NY-BB through a knockdown-type nerve insensitivity mechanism.

Monocrotophos-resistant two-spotted spider mites (TSSMs), Tetranychus urticae, are known to possess three mutations on the acetylcholinesterase (AChE) gene (Tuace) that are involved in target site insensitivity. Cross-strain comparison of three strains (highly resistant AD, moderately resistant PyriF and susceptible UD strains) revealed that resistant strains have relatively more Tuace copies than the UD strain and that the levels of transcript were directly proportional to copy numbers. AChEs from the AD and PyriF strains had similar V(max) values to those of AChE from the UD strain but increased K(m) and reduced k(cat) constants, suggesting that the mutated, resistant form of AChE may carry a fitness cost. Relative copy numbers of Tuace in field populations varied from 2.4 to 6.1, correlating well with their levels of resistance (r(2)= 0.895). These results are suggestive of the involvement of Tuace gene duplication in resistance. Thus, monocrotophos resistance in TSSMs appears to have evolved through a combination of mutation accumulation and extensive gene duplication.