The Plasmodium transmission-blocking symbiont, Microsporidia MB, is vertically transmitted through Anopheles arabiensis germline stem cells.

Microsporidia MB is a promising candidate for developing a symbiont-based strategy for malaria control because it disrupts the capacity of An. arabiensis to transmit the Plasmodium parasite. 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 MB has the potential to spread through target vector populations and become established at high prevalence. Stable and efficient vertical transmission of Microsporidia MB is important for its sustainable use for malaria control, however, the vertical transmission efficiency of Microsporidia MB can vary. In this study, we investigate the mechanistic basis of Microsporidia MB vertical transmission in An. arabiensis. We show that vertical transmission occurs through the acquisition of Microsporidia MB by Anopheles cystocyte progenitors following the division of germline stem cells. We also show that Microsporidia MB replicates to increase infection intensity in the oocyte of developing eggs when mosquitoes take a blood meal suggesting that symbiont proliferation in the ovary is coordinated with egg development. The rate of Microsporidia MB transmission to developing eggs is on average higher than the recorded (mother to adult offspring) vertical transmission rate. This likely indicates that a significant proportion of An. arabiensis offspring lose their Microsporidia MB symbionts 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 that Microsporidia MB has a highly specialized vertical transmission strategy in An. arabiensis, which may explain host specificity.