Conservation of the voltage‐sensitive sodium channel protein within the Insecta

Abstract

The voltage‐sensitive sodium channel (VSSC) is essential for the generation and propagation of action potentials. VSSC kinetics can be modified by producing different splice variants. The functionality of VSSC depends on features such as the voltage sensors, the selectivity filter and the inactivation loop. Mutations in Vssc conferring resistance to pyrethroid insecticides are known as knockdown resistance (kdr). We analysed the conservation of VSSC in both a broad scope and a narrow scope by three approaches: (1) we compared conservation of sequences and of differential exon use across orders of the Insecta; (2) we determined which kdr mutations were possible with a single nucleotide mutation in nine populations of Aedes aegypti; and (3) we examined the individual VSSC variation that exists within a population of Drosophila melanogaster. There is an increasing amount of transcript diversity possible from Diplura towards Diptera. The residues of the voltage sensors, selectivity filter and inactivation loop are highly conserved. The majority of exon sequences were >88.6% similar. Strain‐specific differences in codon constraints exist for kdr mutations in nine strains of A. aegypti. Three Vssc mutations were found in one population of D. melanogaster. This study shows that, overall, Vssc is highly conserved across Insecta and within a population of an insect, but that important differences do exist.

The F1534C voltage‐sensitive sodium channel mutation confers 7‐ to 16‐fold resistance to pyrethroid insecticides in Aedes aegypti

“…Insects have multiple voltage‐sensitive sodium channel ( Vssc ) transcripts, and these different VSSCs (produced by differential exon use and RNA editing) can have different electrophysiological and pharmacological properties . A total of four mutually exclusive exons (c/d and k/l), two optional exons (j and a), five 5′/3′ splice site variants (i, b, e, f and h) are known . In A. aegypti , three other splice variants (called m, n and f´) have been detected .…”

“…The voltage sensitive sodium channel (VSSC) is the target site of pyrethroid insecticides. There is a single copy of Vssc in most insect species, and mutations in the Vssc , collectively referred to as knockdown resistance ( kdr ), are a major mechanism of pyrethroid resistance in A. aegypti …”

“… ‡ This mutation has been studied in oocytes, but is only found to co‐occur with other mutations in field populations of A. aegypti …”

Fitness costs of individual and combined pyrethroid resistance mechanisms, kdr and CYP-mediated detoxification, in Aedes aegypti

“…An additional genotype selection for homozygous kdr individuals was done on 630 females and 180 males prior to the fitness component studies (see Fitness component studies section) to ensure the following generations of KR were homozygous for kdr. Both kdr mutations in KR occur in the same domain (Domain II) of Vssc and recombination (sites are~310 bp apart) has not been observed [53,54].…”

Evidence for both sequential mutations and recombination in the evolution of kdr alleles in Aedes aegypti

“…Vssc in A. aegypti is >450 kb in length and contains >30 exons [25]. To investigate the evolution of kdr mutations in Vssc, we needed to explore the three regions containing mutations (V410L, S989P ± V1016G/I and F1534C) that are common in A. aegypti (Table 1).…”