Abstract: Resistance of Anopheles gambiae to organophosphate and carbamate insecticides was first reported in Côte d'Ivoire, West Africa. Subsequent studies revealed that it resulted from a single point mutation in the oxyanion hole of the acetycholinesterase enzyme (ace-1(R) mutation). We investigated the distribution and prevalence of the ace-1(R) mutation in An. gambiae s.l. populations from seven locations in south-western Burkina Faso. The ace-1(R) mutation was found in both M and S molecular forms of An. gambiae s.s., but it was absent in An. arabiensis. Its frequency ranged from 0.25 to 0.5 in S form and 0.04 to 0.13 in M form, though they were sympatric. The lack of homozygous resistance indicated a strong genetic cost associated with the mutation. These data suggest that organophosphate and carbamate resistance conferred by target site insensitivity is spreading in populations of An. gambiae s.s. from West Africa.
Abstract: Selection of insensitive acetycholinesterase 1 (AChE1) has occurred in several mosquito species controlled with carbamate (CX) and organophosphate (OP) insecticides. In case of pyrethroid resistance, these insecticides represent an alternative for disease vector control program. Their heavy use in agriculture has selected resistant populations of Anopheles gambiae in West Africa. The evolution of resistance has to be studied to prevent, or at least slow down, the spread of resistant mosquito in wild populations. An. gambiae shares the same resistance mechanism to CX and OP insecticides as Culex pipiens, which was attributed to the G119S substitution in the AChE1 enzyme. By comparing resistant AChE1 from both species, we show here that similar resistance levels are obtained toward 10 insecticides of both classes. Moreover, similar AChE1 activity levels are recorded between either susceptible or resistant mosquitoes of both species. Enzymes belonging to both species seem thus to share identical properties. Consequently, we hypothesize that fitness cost associated with AChE1 insensitivity in C. pipiens mosquitoes should be similar in An. gambiae and thus be used in strategies to control resistant populations where malaria is prevalent.
Abstract: Resistance to organophosphates and carbamates was investigated in three samples of Culex pipiens quinquefasciatus Say, from Cotonou and Parakou in the Republic of Benin. Each population of larvae was selected with propoxur to eliminate susceptible individuals and more easily analyse resistant mosquitoes if the resistance allele is present in the field sample. The mass-selection showed that there are resistant mosquitoes in natural populations of C. p. quinquefasciatus in Benin and that nowadays they are still at a low frequency. The result of the PCR-based assay revealed the presence of the G119S mutation in all the C. p. quinquefasciatus mosquitoes that survived exposure to mass-selection. This mutation is the same than reported in a large range of insects, including Anopheles gambiae. Thus, the G119S mutation is spreading in natural populations of C. p. quinquefasciatus in Africa. Therefore, good mosquito control strategies and agricultural use of insecticides are critical, and should be carefully applied to limit the spread of this resistance gene.
Abstract: BACKGROUND: The role of inter-specific hybridisation is of particular importance in mosquito disease vectors for predicting the evolution of insecticide resistance. Two molecular forms of Anopheles gambiae s.s., currently recognized as S and M taxa, are considered to be incipient sibling species. Hybrid scarcity in the field was suggested that differentiation of M and S taxa is maintained by limited or absent gene flow. However, recent studies have revealed shared polymorphisms within the M and S forms, and a better understanding of the occurrence of gene flow is needed. One such shared polymorphism is the G119S mutation in the ace-1 gene (which is responsible for insecticide resistance); this mutation has been described in both the M and S forms of A. gambiae s.s. METHODS AND RESULTS: To establish whether the G119S mutation has arisen independently in each form or by genetic introgression, we analysed coding and non-coding sequences of ace-1 alleles in M and S mosquitoes from representative field populations. Our data revealed many polymorphic sites shared by S and M forms, but no diversity was associated with the G119S mutation. These results indicate that the G119S mutation was a unique event and that genetic introgression explains the observed distribution of the G119S mutation within the two forms. However, it was impossible to determine from our data whether the mutation occurred first in the S form or in the M form. Unexpectedly, sequence analysis of some resistant individuals revealed a duplication of the ace-1 gene that was observed in both A. gambiae s.s. M and S forms. Again, the distribution of this duplication in the two forms most likely occurred through introgression. CONCLUSIONS: These results highlight the need for more research to understand the forces driving the evolution of insecticide resistance in malaria vectors and to regularly monitor resistance in mosquito populations of Africa.
Abstract: Characterization of insecticide resistance provides data on the evolutionary processes involved in the adaptation of insects to environmental changes. Studying the dominance status and resistance level represents a great interest, in terms of understanding resistance evolution in the field to eventually adapt vector control. Resistance and dominance levels conferred by the G119S mutation of acetylcholinesterase (ace-1R) of the mosquito Anopheles gambiae s.s. (Diptera: Culicidae) were studied for various insecticides belonging to different classes, using strains sharing the same genetic background. Our survey shows that the homozygote resistant strain AcerKis displayed a very high resistance level to various carbamates (range 3,000- to 5,000-fold) compared with that of various organophosphates (range 12- to 30-fold). Furthermore, the dominance status varied between semi-recessivity with fenitrothion and chlorpyrifos methyl insecticides to semidominance with temephos, carbosulfan, and propoxur. These results indicate that this resistance mechanism could spread rapidly in the field and then compromise the use of organophosphate and carbamate compounds in public health. This study underlines the necessity to monitor the ace-1R mutation in natural populations before planning and implementing malaria control programs based on the use of these insecticides.