Marked Effects of Larval Salt Exposure on the Life History and Gut Microbiota of the Malaria Vector Anopheles merus (Diptera: Culicidae)

SIMPLE SUMMARY: Anopheles merus is a malaria-transmitting mosquito with the unusual ability to breed in a range of saltwater concentrations. Although previous studies have looked at the consequences of this capacity on the immature mosquitoes present in the saltwater, what this means for the adults that emerge from this water is unknown. In this study, we look at the consequences of breeding in a range of saltwater concentrations on laboratory-bred An. merus. We found that high salt concentrations delayed the time to pupation but does not affect adult longevity. Therefore, although the larvae are at risk for a longer period, it does not affect the lifespan of the adult. Increased salt exposure also results in increased insecticide tolerance, which makes them harder to control. Finally, the salt concentration affects the composition of the mosquito’s gut microbiota. The significance of this is that the gut bacteria composition affects the capacity to transmit the malaria parasite. This study suggests that due to changes in the gut bacteria, mosquitoes that breed in very salty water are less likely to carry the malaria parasite in their gut. Therefore, the saltiness of the water that these mosquitoes breed in may affect how well they transmit malaria. ABSTRACT: Anopheles merus can breed in a range of saltwater concentrations. The consequences of this ability on the life history of adult An. merus are poorly understood. This study examined the effects of exposure to 0, 2.1875, 4.375, 8.75, and 17.5 g/L of sodium chloride on An. merus. The effects on larval development, adult longevity, fertility, and fecundity, as well as deltamethrin tolerance were examined. The effect of larval salt exposure on the expression of defensin-1 in adults was examined by quantitative Real-Time PCR. Finally, the effect of the larval salt concentration on microbial dynamics was assessed by 16S Next Generation Sequencing. High concentrations of saltwater increased larval development time and number of eggs laid, as well as deltamethrin tolerance. Larval exposure to salt also reduced the expression of defensin-1. The exposure also had a significant effect on microbial diversity in larvae and adults. The diversity of larvae decreased once adults emerged. Salt-tolerant bacterial genera predominated in larvae but were absent in adults. High salt concentrations resulted in greater abundance of Plasmodium-protective genera in adults. Although this study was conducted on a laboratory strain of An. merus, these data suggest that osmoregulation has a significant effect on the life history of the species with potential epidemiological consequences.