Voltage-dependent Na+ channels in pyrethroid-resistant Culex pipiens L mosquitoes

In some insect species, knockdown resistance (kdr) to pyrethroids and DDT i s linked to point mutations in the sequence of the para-type voltage-depend ent sodium channel gene, The effects of pyrethroids were assayed on six Cul ex pipiens strains: two were susceptible to pyrethroids and the four others displayed various levels of resistance, but, in each case, a hdr-type mech anism was strongly suggested. Degenerate primers were designed on the basis of the corresponding sequences of the para orthologous gene reported from several orders of insects. These primers were used to amplify the region of the sodium channel gene which includes the positions where the kdr and sup er-kdr mutations have been found in Musca domestica. As expected, the ampli fied fragment was highly homologous to the para sequences. The super-kdr-li ke mutation (methionine to threonine at position 918 of the M domestica par a sequence) was never detected in any strain. In contrast, the same hdr mut ation (leucine to phenylalanine at position 1014) was present in some Culex pyrethroid-resistant samples. An alternative substitution of the same leuc ine to a serine was detected in one strain slightly resistant to pyrethroid s but highly resistant to DDT, These data have allowed us to design a PCR-b ased diagnostic test on genomic DNA to determine the presence or the absenc e of the hdr allele in single C pipiens collected in several countries. The validity of this test was checked by comparing the frequency of the resist ance allele and the toxicological data. (C) 1999 Society of Chemical Indust ry.