Lysinibacillus sphaericus exposure impedes Anopheles dirus's oviposition via downregulating vitellogenin

Vector control using Lysinibacillus sphaericus is an effective strategy for preventing the transmission of mosquito-borne diseases. Our previous study demonstrated that exposure to L. sphaericus during the larval stage of Anopheles dirus significantly reduced the fecundity of surviving adult mosquitoes. However, the underlying mechanisms driving this reduction remain unclear. Sublethal doses of L. sphaericus, often resulting from insufficient or delayed application, can still impact mosquito populations. Therefore, this study aimed to investigate how sublethal doses of L. sphaericus inhibit the reproductive capacity of An. dirus mosquitoes.

First, the staining method was used to detect L. sphaericus in surviving adult mosquitoes that had been exposed to sublethal doses during the larval stage. Second, adult mosquitoes were fed a sucrose solution containing L. sphaericus, and the effects on the reproductive capacity were observed. Third, transcriptome sequencing and qPCR were employed to identify and validate genes associated with oviposition suppression in An. dirus following treatment with sublethal doses of L. sphaericus. Finally, we assessed the effects of sublethal doses and direct feeding of L. sphaericus on vitellogenin (Vg) expression and activation of the target of rapamycin (TOR) signaling pathway using qPCR and Western blotting.

Our findings demonstrated that L. sphaericus persists in adult An. dirus mosquitoes that survived larval exposure to sublethal doses. Additionally, feeding adult female mosquitoes with L. sphaericus significantly suppressed their oviposition ability. Transcriptome analysis revealed substantial alterations in gene expression profiles among surviving mosquitoes exposed to sublethal doses of L. sphaericus. Notably, L. sphaericus inhibit lysosomal function and lipid metabolism, which are critical for mosquito physiology. Furthermore, L. sphaericus significantly downregulated the Akt-TOR signaling pathway and Vg expression in adult mosquitoes.

Exposure An. dirus larvae to L. sphaericus resulted in the persistence of L. sphaericus in surviving adult mosquitoes and significantly suppressed female oviposition by downregulating Vg expression via inhibition of lysosomal function and the TOR signaling pathway. This study offers novel insights into the interaction between L. sphaericus and its mosquito host and identifies potential molecular targets for controlling mosquito population density by modulating oviposition behavior.