The genetic basis of resistance to dieldrin
The genetic basis of resistance to dieldrin has been described for several insect species as a mutation in the transmembrane domain II of the GABA-Cl or rdl (resistance to dieldrin) gene (ffrench-Constant et al., 2004; Bass et al., 2004; Du et al., 2005). A mutation in the GABA-Cl gene was found at nucleotide positions 868 and 869 (AC to CT) in dieldrin-resistant Rhipicephalus australis Fuller, 1899 [former R. microplus, see Estrada-Peña et al. (2012)]. The mutation codes for an amino Nilvadipine change from threonine (T) to leucine (L) at position 290, located four amino acids from the rdl loci, 302 of the Drosophila melanogaster ortholog (Hope et al., 2010). It can be hypothesized that if there were cross-resistance between fipronil and cyclodienes, the same mutation would confer resistance to both products as demonstrated in some insects. In this study, we describe the existence of mutations in the GABA-Cl gene of fipronil- and lindane-resistant strains and field populations of R. microplus from Uruguay and Brazil.
Materials and methods
Discussion The T290 L mutation observed in ticks resistant to dieldrin in Australian cattle tick populations (Hope et al., 2010) was not present in fipronil- and lindane-resistant R. microplus ticks from Uruguay and Brazil. Therefore, several amino acid substitutions in the RDL region were registered. The substitutions in the domain TM2 (A286S/L) were observed only in fipronil-resistant populations (Jaguar, Juarez, RFSan, U-MOR, SGF1, 1071 and 1073) and they corresponded to the same alanine residue in the position 302 substituted by a serine or glicine (A302S/G) identified in D. melanogaster and other insects resistant to cyclodienes and fipronil (ffrench-Constant et al., 2004). It should be noted that the mutations associated with lindane- and cyclodiene-resistant planthoppers in Japan (Nakao, 2017) corresponding to A302S/G would produce low levels of cross-resistance to fipronil. The same author found an association of the A302 N with high levels of resistance to fipronil. Therefore, we can hypothesize that the mutations found in the present study (A286S or L) can be associated to cross-resistance pre-selected by cyclodienes, as this mutation was not found in lindane-susceptible populations but in populations resistant to fipronil. In the case of populations resistant to both chemicals, both substitutions where found, A to S and A to L. According to Remnant et al. (2014), phenylpyrazole resistance levels in D. melanogaster depended on the altered amino acid in the position 301 of the Gaba-Cl gene. These authors demonstrated that the replacement of alanine (301) by glycine resulted in more survivors to phenylpyrazole treatment than the replacement by a serine residue. In the present study, the A286 L substitution was detected as a single mutation in the TM2 and the A286S mutation was in combination with other four mutations. Maybe this fact could be related to different levels of resistance. In Nilaparvata lugens (Hemiptera: Delphacidae), a pest of rice crops, a single point mutation (A301S) in the RDL confers low levels of resistance to fipronil; however, a combination with a Q359E mutation was associated with higher levels of resistance (Garrood et al., 2017). Le Goff et al. (2005), working with fipronil-resistant Drosophila simulans, detected an A301 G replacement that, when found in combination with a T350M substitution, conferred a higher level of fipronil resistance than when found alone. In D. melanogaster, the combination of A301S with Q359E resulted in higher levels of resistance to ethiprole, a phenylpyrazole insecticide (Zhang et al., 2016). The same was observed in Sogatella furcifera (Hemiptera: Delphacidae) when presenting A301N and R340Q (between TM3 and TM4; Nakao et al., 2012). It is possible that when there is an association of mutations, one compensates for the deleterious effects of the other, as was observed in Nilaparvata lugens (Zhang et al., 2016). Low levels of fipronil resistance are associated with the A302S substitution. When combined with R300Q (TM2), the resistance level increased. The R300Q mutation is never found alone and it increases the resistance to fipronil. In this case, the A302S mutations would have a compensatory effect over the deleterious effect on the function of the GABA receptor caused by the R300Q mutation. Perhaps the S281 T substitution detected in the present study could be associated with deleterious effects on GABA receptor function due to its very low frequency and it was never detected alone in field populations.This mutation does not appear to have the same impact that R300Q has on N. lugens because other mutations are present.