ThePlasmodiumtransmission-blocking symbiont,Microsporidia MB, is vertically transmitted throughAnopheles arabiensisgermline stem cells

Abstract

Microsporidia MBis a promising candidate for developing a symbiont-based strategy for malaria control because it disrupts the capacity ofAn. arabiensisto transmit thePlasmodiumparasite. The symbiont is predominantly localized in the reproductive organs and is transmitted vertically from mother to offspring and horizontally (sexually) during mating. Due to the contribution of both transmission routes,Microsporidia MBhas the potential to spread through target vector populations and become established at high prevalence. Stable and efficient vertical transmission ofMicrosporidia MBis important for its sustainable use for malaria control, however, the vertical transmission efficiency ofMicrosporidia MBcan vary. In this study, we investigate the mechanistic basis ofMicrosporidia MBvertical transmission inAn. arabiensis. We show that vertical transmission occurs through the acquisition ofMicrosporidia MBbyAnophelescystocyte progenitors following the division of germline stem cells. We also show thatMicrosporidia MBreplicates to increase infection intensity in the oocyte of developing eggs when mosquitoes are given a blood meal suggesting that symbiont proliferation in the ovary is coordinated with egg development. The rate ofMicrosporidia MBtransmission to developing eggs is on average higher than the recorded (mother to adult offspring) vertical transmission rate. This likely indicates that a significant proportion ofAn. arabiensisoffspring lose theirMicrosporidia MBsymbionts during development. The stability of germline stem cell infections, coordination of symbiont proliferation, and very high rate of transmission from germline stem cells to developing eggs indicate thatMicrosporidia MBhas a highly specialized vertical transmission strategy inAn. arabiensis,which may explain host specificity.Author SummaryMosquito vectors of diseases are associated with a broad range of microbes. Some of the microbes significantly affect vector biology including pathogen transmission efficiency. Anopheles mosquitoes, which transmit the malaria parasite,Plasmodium falciparum,harbor a native microbe known asMicrosporidia MB.This microbe interferes with the formation of transmissible stages of the parasite that are transferred to humans by female mosquitoes when taking a blood meal. This phenotype can be exploited to develop a novel strategy for controlling malaria similar to the control of dengue fever using Aedes mosquitoes carryingWolbachiabacteria. Mother-to-offspring transmission of protective microbes is important in sustainable application of microbe-based technologies to control vector-borne diseases because it ensures maintenance of the microbe in target vector populations across many generations. Here, we investigated stability ofMicrosporidia MBinfections and efficiency of mother-to-offspring transmission during early stages of egg formation and development. We found that this microbe has a specialized transmission mechanism that involves infecting the germline cells that are important in egg production. We also demonstrated a very high transmission rate (97%) of theMicrosporidia MBfrom infected germline cells into daughter cells during cell division. As the germline daughter cells developed into eggs,Microsporidia MBestablished itself in the egg yolk through active replication which only occured after the female mosquitoes had a blood meal. Our study gives insights into an efficient mother-to-offspring transmission route ofMicrosporidia MBthat can be utilized sustainably in microbe-based intervention to control malaria.