An effective strategy for malaria vector control should neither be based only on the application of insecticide components nor focused on the existing resistance mechanisms developed by these vectors. Insecticide pressure contributes to the selection of mosquitoes carrying resistance alleles which are vertically transferred to the offspring . It was demonstrated that alleles of genes conferring resistance in mosquito populations are usually associated with the disruption of some vector history traits of life [30, 31, 50]. The present study investigated the fitness effects associated with the presence of knock-down resistance allele in major malaria vectors An. gambiae s.s. carrying kdrR allele.
Although all mosquito females subjected to oviposition assay were blood-fed, fecundity was significantly lower in homozygous KisKdr individuals compared to that of susceptible specimens. This result suggests that a physiological cost could be associated with the presence of kdrR allele in pyrethroid resistant mosquitoes when they are in an insecticide-free environment. Similar pattern was observed in the main dengue vectors Aedes aegypti. Indeed, Brito and collaborators reported that a few number of pyrethroid resistant Aedes aegypti mosquitoes was able to lay eggs and a smaller amount of eggs was produced . The KisKdr females displayed significantly lower fertility compared to that of their counterpart, Kisumu. This finding suggests that kdrR allele could reduce hatchability of eggs in homozygote [kdrRR] An. gambiae mosquitoes. Under laboratory conditions, three hypotheses could support these observations in mosquitoes: i) a decreasing egg maturation, ii) a reduced egg laying ability and iii) an unsuccessful mating associated with lower mating success which has been reported in resistant mosquitoes [30, 34, 52]. The significant difference in fecundity and fertility between resistant and susceptible mosquito strains suggests yet other likely interesting pathways that could be targeted for disruption in malaria transmitting vectors. However, in field settings, there are other resistance mechanisms which were probably co-selected with kdrR allele in Anopheles mosquitoes. Also, the wild mosquitoes are continuously under insecticide pressure  and consequently, differences in physiological responses in resistant individuals are important factors to be taken into consideration. Further molecular analyses could be considered in order to understand the differentially expressed genes between KisKdr female mosquitoes that have laid eggs and those that have not. This will pave the way to the identification of new genetic targets as alternative tools for vector control in the face of insecticide-resistance.
Larval survivorship and pupation rate were also assessed. The results showed that while 50% of Kisumu larvae were died at day 10, more than 50% KisKdr larvae were still alive or had reached pupation. The KisKdr larvae had a lower risk of death compared to their counterpart Kisumu suggesting that this fitness advantage might be associated with the kdr resistant [kdrRR] genotype. Further, we reported a significant pupation rate in KisKdr compared to Kisumu mosquitoes. Unfortunately, this is evidence that the strong survivorship during the aquatic phase and the high pupation rate in KisKdr resistant mosquitoes will threaten current vector control interventions. This information could help for designing a most effective tools for insecticide resistance management. However, present findings were obtained with standardized larval density and controlled diet which is not the case in natural settings where mosquito larval development could also be affected by density-dependent competition and mortality .
After emergence, adult mosquito females need to feed on blood in order to have the crucial protein source required for egg development . The present study provides the first evidence of blood feeding advantage associated with the knock-down resistance using An. gambiae s.s. mosquitoes carrying only kdrR (L1014F) allele. Noteworthy, our results indicate a significant association between kdrR allele and the high blood feeding success in An. gambiae s.s. The blood feeding success in mosquitoes depends on their craving for blood. Further, the presence of kdrR allele could enhance the mosquito avidity for blood feeding. The significant blood feeding success in both [kdrRR] and [kdrRS] KisKdr mosquitoes with respect to the susceptible genotype [kdrSS] could suggest a linkage disequilibrium (LD) at the loci of the para voltage-gated sodium channel (Vgsc) gene and the gene encoding the craving for blood meal taking. The benefit of this knock-down resistance to strengthen blood feeding ability might increase the human biting rate index and consequently the expected number of malaria infected individuals according to the Ross-MacDonald model . An important parameter susceptible to favour the blood feeding success is the host seeking-behaviour which is likely reinforced by the interactions with kdrR allele. Indeed, it was previously reported that the host-seeking performance in presence or absence of insecticide was increased in kdr heterozygous [kdrRS] An. gambiae .
Furthermore, no significant difference was noticed in blood meal volume ingested by homozygote resistant [kdrRR] mosquitoes compared to homozygote susceptible [kdrSS] individuals. However, heterozygous KisKdr F1-1 and F1-2 mosquitoes ingested a higher blood volume compared to Kisumu specimens. A high blood meal size could lead to the ingestion of a high gametocyte density if the heterozygote kdr resistant females feed on an infected host. Moreover, it has been demonstrated that the heat shock protein 70 (Hsp70) protects An. gambiae mosquitoes against the high temperature stress through thermoregulation process during the blood meal on a warm-blooded host . Accordingly, we hypothesize that this thermoregulation could strengthen the differentiation of ingested gametocytes into ookinetes in the midgut lumen of heterozygous [kdrRS] KisKdr female since the parasite exflagellation could be initiated by fast fall of temperature within mosquito midgut . Therefore, the risk of malaria transmission might remain very huge in regions where there is a high heterozygote kdrR genotype frequency. Indeed, it was demonstrated that kdr homozygote resistant An. gambiae mosquitoes are more susceptible to the P. falciparum infection than the susceptible Kisumu  and consequently, this fact could be more striking in heterozygote mosquitoes.
Gametocyte-infected mosquitoes must survive long enough to become infectious and transmit the sporozoites to a new host . So, one of the key factors modulating malaria transmission is the vector longevity post blood feeding. This study demonstrates that, the presence of kdrR allele seems to increase the longevity of heterozygote KisKdr specimens while no survival advantage was observed in homozygous individuals compared to the susceptible strain Kisumu. This benefit in heterozygote [kdrRS] over homozygote [kdrRR], makes kdr an overdominance gene for this specific trait. We observed that heterozygote mosquitoes survive until 24 days. Thus, these specimens have sufficient lifespan to allow extrinsic incubation period of the Plasmodium parasites if they ingest gametocyte-infected blood. However, further investigations are needed to evaluate the cost of Plasmodium infection on heterozygote resistant KisKdr mosquito’s survivorship. In fact, the intensity of infection (i.e. oocyst burden) affecting the vector immune responses  could relatively lessen the fitness of mosquitoes regarding their survival and fecundity .