hominivorax. However, the high similarities found between this AChE and the other dipteran AChEs suggest that this gene is an ortholog of the AChEs of D. melanogaster, M. domestica, H. irritans and

L. cuprina and, therefore, a member of the Ace2 group ( Weill et al., 2002). A survey of the geographical distribution of mutations in these genes in NWS revealed a high frequency of G137D mutation in several Brazilian populations and Uruguay. The low frequency of the G137D mutation in Pará (Brazil) could be correlated with lower selective pressure since the livestock activity in this region is more recent. Absence of mutant alleles (D137) in Colombia, Venezuela and Cuba could be due to low OP pressure in these localities or associated with a historical event in which emergence of Amazon forest divided NWS into two geographical populations, BVD-523 research buy restricting gene flow (Fresia PC, personal communication). A MDV3100 cost recent investigation

of the W251S mutation in the NWS E3 gene, involved in dimethyl-OP and pyrethroid resistance, showed a considerable frequency of this mutation in most of the populations analyzed (Silva and Azeredo-Espin, 2009). Alterations in the frequencies of both mutations in the E3 gene seem to be associated with the use of insecticides for NWS control, as shown in Uruguay (Carvalho et al., 2010). In addition to the possibility of fitness cost caused by altered AChE, the low frequency of AChE mutants found in natural populations of NWS could be explained by the fact that the mutant forms of NWS E3 may possess a higher affinity for OPs than the AChE target site itself, which may serve to protect AChE, a process seen in L. cuprina

( Campbell et al., 1997 and Newcomb et al., 1997). Although there are no studies reporting phenotypic OP resistance in the NWS fly, this report documents a high frequency of E3 mutants and the E3-based resistance mechanism may have been selected by OP pressure in this species. Molecular assays provide information as to the presence and distribution of resistance-associated alleles in populations, even when occurring at a low frequency, allowing resistance to be detected earlier than by traditional insecticide exposure assays. In this regard, this study provides useful information that can facilitate the monitoring and management of resistance to improve the effectiveness of NWS control programs. The authors thank R.A. Rodrigues, next S.M. Couto and A.S. Oliveira for valuable technical assistance and the International Atomic Energy Agency (IAEA) for providing the samples from the Caribbean region. This work was supported by a grant from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (grant 578231/2008-5) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) to A.M.L.A.E. (grant nos. 03/01458-9 and 07/54431-1) and R.A.C. (grant no. 04/12532-8), and N.M.S. was supported by a fellowship from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (grant no.