RESUMO
Pyrethroid insensitivity in resistant (kdr) insects has been correlated with a leucine to phenylalanine replacement in the S6 transmembrane segment of domain II of the axonal sodium channel alpha(para)-subunit. An alpha-subunit of rat brain type II sodium channel containing this mutation has been expressed and its sensitivity to permethrin compared with that of the wild-type channel. The steady-state activation curve of the mutant was shifted 14 mV in the depolarizing direction. We propose that an equivalent shift of the sodium current activation curve in kdr insects could account for their low sensitivity to permethrin toxicity.
Assuntos
Resistência a Inseticidas/fisiologia , Inseticidas/farmacologia , Piretrinas/farmacologia , Canais de Sódio/fisiologia , Animais , Proteínas de Drosophila , Condutividade Elétrica , Resistência a Inseticidas/genética , Potenciais da Membrana/efeitos dos fármacos , Mutagênese Sítio-Dirigida , Mutação , Oócitos , Permetrina , RNA Mensageiro , Ratos , Canais de Sódio/genética , Xenopus laevisRESUMO
We report the isolation of cDNA clones containing the full 6.3-kb coding sequence of the para-type sodium channel gene of the housefly, Musca domestica. This gene has been implicated as the site of knockdown resistance (kdr), an important resistance mechanism that confers nerve insensitivity to DDT and pyrethroid insecticides. The cDNAs predict a polypeptide of 2108 amino acids with close sequence homology (92% identity) to the Drosophila para sodium channel, and around 50% homology to vertebrate sodium channels, Only one major splice form of the housefly sodium channel was detected, in contrast to the Drosophila para transcript which has been reported to undergo extensive alternative splicing. Comparative sequence analysis of housefly strains carrying kdr or the more potent super-kdr factor revealed two amino acid mutations that correlate with these resistance phenotypes. Both mutations are located in domain II of the sodium channel. A leucine to phenylalanine replacement in the hydro-phobic IIS6 transmembrane segment was found in two independent kdr strains and six super-kdr strains of diverse geographic origin, while an additional methionine to threonine replacement within the intracellular IIS4-S5 loop was found only in the super-kdr strains. Neither mutation was present in five pyrethroid-sensitive strains. The mutations suggest a binding site for pyrethroids at the intracellular mouth of the channel pore in a region known to be important for channel inactivation.
Assuntos
Genes de Insetos , Moscas Domésticas/genética , Inseticidas/farmacologia , Piretrinas/farmacologia , Canais de Sódio/genética , Sequência de Aminoácidos , Animais , Sítios de Ligação , Clonagem Molecular , DDT/farmacologia , Moscas Domésticas/metabolismo , Resistência a Inseticidas/genética , Inseticidas/metabolismo , Dados de Sequência Molecular , Mutação , Reação em Cadeia da Polimerase , Piretrinas/metabolismo , Splicing de RNA , Alinhamento de Sequência , Análise de Sequência , Canais de Sódio/química , Canais de Sódio/metabolismoRESUMO
Insecticide resistance in peach-potato aphids, Myzus persicae, results from the amplification of genes encoding an esterase that hydrolyses and sequesters insecticides. Resistance is normally stable, but highly resistant aphid clones sometimes lose resistance when insecticidal selection pressure is removed. This loss of resistance, termed reversion, arises from a loss of elevated esterase enzyme through transcriptional control, i.e. without loss of the amplified esterase DNA sequences. We have shown that loss of the elevated enzyme occurred simultaneously with loss of methylation at CCGG sites in the amplified DNA sequences. During reselection of resistance in these revertant clones, enzyme levels increased, but there was no corresponding return of methylation to DNA sequences. Thus, although DNA methylation is closely correlated with expression of the amplified esterase genes during reversion, it may not be a factor in the reverse process.