Non-blood-fed An. gambiae female mosquitoes were preserved in RNAlater® 3–5 days post-eclosion. The following insecticide resistant strains were obtained through BEI Resources, NIAID, NIH: strain AKRON, bulk frozen, MRA-913B, contributed by Martin Akogbeto; strain RSP, bulk frozen, MRA-334 and the strain ZANU MRA-594, both contributed by Hilary Ranson and Frank H. Collins. The VK7 and Tiassalé strains were kindly provided by the Liverpool Insect Testing Establishment (LITE). The AKRON (MRA-913) strain carries the L1014F kdr and G119S Ace-1 target site mutations which leads to phenotypic resistance to carbamate [23 ]. The RSP (MRA-334) strain’s name stands for reduced susceptibility to permethrin, which is caused by the L1014S kdr mutation and increased cytochrome P450 and beta-esterase activity [24 (link), 25 (link)]. The metabolic resistance to DDT of the ZANU (MRA-594) strain is conferred by elevated glutathionine-S-transferase and beta-esterase activity [26 (link)]. The An. gambiae strain from Tiassalé in Côte d’Ivoire exhibits the L1014F kdr and G119S Ace-1 mutations as well as upregulation of several P450s [3 , 27 (link)]. The combination of these different resistance mechanisms leads to multiple-insecticide resistance to pyrethroids (documented for permethrin and deltamethrin), organochlorides (DDT), carbamates (bendiocarb) and organophosphates (fenitrothion) in the Tiassalé strain. The VK7 strain’s high resistance to pyrethroids and DDT is a consequence of mutations in the common target site of these insecticides. The effect of the fixed L1014F kdr mutation is enhanced by the N1575Y super-kdr mutation in a substantial proportion of the VK7 colony [28 (link)]. The Kisumu and Ngusso laboratory colonies are susceptible to all above mentioned insecticides and were used as control comparator strains in this study. The insectary at the Swiss Tropical and Public Health Institute provided specimens of the Kisumu colony that originates from an MRA-762 egg batch provided by BEI Resources [29 ]. The Ngusso specimens used in this study were reared in the insectary of the Institute of Molecular Biology and Biotechnology, Foundation for Research and Technology-Hellas. The description of the characteristics of the laboratory colonies included in the study is summarised in Additional file 1: Table S1. Specimens from a field-caught population from Bioko Island that was recently characterised [10 (link)] were also included in the validation of the assays.
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