Abstract
In some insect species, knockdown resistance (kdr) to pyrethroids and DDT is linked to point mutations in the sequence of the para-type voltage-dependent sodium channel gene, The effects of pyrethroids were assayed on six Culex pipiens strains: two were susceptible to pyrethroids and the four others displayed various levels of resistance, but, in each case, a hdr-type mechanism 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 super-kdr mutations have been found in Musca domestica. As expected, the amplified fragment was highly homologous to the para sequences. The super-kdr-like mutation (methionine to threonine at position 918 of the M domestica para sequence) was never detected in any strain. In contrast, the same hdr mutation (leucine to phenylalanine at position 1014) was present in some Culex pyrethroid-resistant samples. An alternative substitution of the same leucine to a serine was detected in one strain slightly resistant to pyrethroids but highly resistant to DDT, These data have allowed us to design a PCR-based diagnostic test on genomic DNA to determine the presence or the absence 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 resistance allele and the toxicological data.