RESUMO
The resistance of Rhipicephalus microplus to pyrethroids is widely dispersed worldwide and has been associated with several nucleotide substitutions in its target site, the para-sodium ion channel (Na-channel) gene. The resistance of the tick to fipronil has been increasing in South America, and mutations in the GABA-gated chloride channel (GABA-Cl) have been described in fipronil-resistant tick strains. We developed a multiplex allele-specific PCR (mAS-PCR) to screen for single-nucleotide polymorphisms (SNPs) associated with the resistance to pyrethroids (knockdown resistance or kdr) and fipronil (resistance to dieldrin or rdl) in susceptible tick populations from Uruguay (n = 11) and the Rio Grande do Sul state in Southern Brazil (n = 15). Toxicological in vitro assays with larvae and adults were used to confirm the resistance to cypermethrin, flumethrin, and fipronil. Three SNPs in the Na-channel gene were investigated (C190A, G215 T, and T2134A), and the mAS-PCR included the detection of an SNP (G858 T) coding a non-synonymous mutation in the GABA-Cl gene. C190A was present in all pyrethroid-resistant populations from Uruguay and Brazil, most frequently homozygous. The SNPs G215 T and T2134A were not found. Of the seventeen fipronil-resistant populations, fourteen presented at least one mutant GABA-Cl gene allele, more frequently in heterozygosis. Other mechanisms apart from target site insensitivity may be involved in fipronil resistance since in some resistant populations, the SNP G858 T was not detected. Sixteen (61,5%) of the populations presented individuals with simultaneous mutations in the Na-channel and GABA-Cl genes. This could be a significant problem for the future control of R. microplus. This study shows the wide dispersion of a pyrethroid resistance-associated SNP in high frequency in the region. Fipronil resistance mutations are also dispersed across the region and increasing.