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AChE

Acetylcholine receptors

FIGURE 2 Schematic diagram of a nerve synapse showing examples of insecticide resistance mechanisms: (1) changes in the structure of the sodium channel confer kdr or super-kdr target-site resistance to pyrethroids; (2) modified AChE is no longer bound by organophosphates and remains available to break down acetylcholine molecules after neurotransmission across the synapse; (3) detoxifying enzymes degrade or sequester insecticides before they reach their targets in the nervous system.

spread through migration and/or human agency? Although there is molecular evidence for some resistance genes having several independent origins in the same species (e.g., for target-site resistance to cyclodienes in the red flour beetle, Tribolium castaneum), other examples suggest that some mechanisms have arisen only once.

Organophosphate resistance in the mosquito Culex pipiens is primarily conferred by allozymes at two closely linked loci (esterases A and B), coding for insecticide-detoxifying carboxylesterases. Overproduced allozymes (resulting from amplification of A or B genes) tend to recur in geographically disjunct areas. This situation could be explained by recurrent mutations generating each amplification event de novo, or by a nonrecurrent mutation that has spread within and between populations. Restriction mapping of DNA around the esterase genes suggest the latter, with large-scale gene flow attributable to passive migration of mosquitoes on ships and/or airplanes. A new resistance allele in southern France is known to have originated in the vicinity of the international airport and seaport at Marseilles.

Resistance to organophosphates in the aphid M. persicae, is also attributable to the amplification of a gene encoding an insecticide-detoxifying carboxylesterase. Despite the often widespread dispersion of these amplified genes in the aphid genome, restriction analyses have indicated that all copies are in the same immediate genetic background. This suggests that amplification occurred only once, whereupon the amplified DNA was moved intact around the genome through chromosomal rearrangements.

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