The insect steroid hormone 20-hydroxyecdysone as a key regulator of the interplay between Anopheles gambiae and Plasmodium falciparum

Abstract

Plasmodium falciparum parasites are responsible for infecting millions of people with malaria each year, with the major disease burden occurring in Africa. These parasites are transmitted by the blood feeding behavior of female Anopheles mosquitoes, with Anopheles gambiae serving as the main African vector. Specific interactions between the mosquito and the parasite are required in order for malaria transmission to occur. As female An. gambiae need to blood feed to produce eggs, after taking a Plasmodium-infected blood meal, egg and parasite development proceed concurrently. Despite this temporal link, however, it is not clearly understood how these two important processes interact. In this dissertation, we explore the interactions between An. gambiae oogenesis and P. falciparum infection. Specifically, we investigate whether female Anopheles suffer reproductive costs due to infection, and reciprocally examine how oogenetic processes affect Plasmodium. In particular, this thesis focuses on the insect steroid hormone, 20-hydroxyecdysone, which is produced by the female after blood feeding and serves as the master regulator of oogenesis— we study its role in regulating the interplay between egg and parasite development. In Chapter 1, a literature review of vector-pathogen interactions is given with a focus on Anopheles mosquitoes and Plasmodium parasites. In Chapter 2, we explore the relationship between An. gambiae egg development and P. falciparum infection, revealing a positive correlation between the number of eggs a female develops and the number of oocysts in her midgut, and demonstrate that parasite infection does not compete with oogenesis. We determine that this interaction is mediated by 20E produced by the female after a blood meal, as impairing 20E signaling breaks the positive egg-oocyst correlation and results in fewer oocysts. We find that 20E-regulated processes affect Plasmodium during the ookinete to oocyst transition, limiting the ability of parasites to establish midgut infection. Strikingly, we also show that females with impaired 20E function support faster developing parasites, critically shortening the time it takes for P. falciparum to reach transmission stages. We demonstrate that faster parasite growth is linked to the accumulation of midgut lipids in females that invest less toward reproduction, via the function of a mosquito lipid transporter important for oogenesis. Together, these findings suggest that P. falciparum parasites are well-adapted to their An. gambiae vectors, as they modulate their development relative to the reproductive investment of the female, a strategy that enables them to promote their transmission while limiting fitness costs to their mosquito host. In Chapter 3, we expand our investigation of Anopheles-Plasmodium interactions by conducting field studies in Burkina Faso using multiple naturally circulating isolates of P. falciparum parasites and two Anopheles species. We first demonstrate that An. gambiae females do not suffer reproductive costs due to P. falciparum either in terms of fecundity or fertility, and we furthermore confirm the positive egg-oocyst correlation. We also determine that P. falciparum field isolates can modulate their growth in response to physiological changes in both An. gambiae and An. coluzzii mosquitoes, developing faster in females with impaired 20E signaling. By using different P. falciparum isolates in two mosquito vectors, we reveal possible genotype-by-genotype interactions affecting oocyst growth, suggesting that both parasite and mosquito genetic factors contribute to the parasite’s likelihood of being transmitted. Overall, these studies describe previously unknown vector-pathogen interactions between oogenesis and parasite development. While these findings may partially explain the formidable spread of malaria in Africa, they also have important implications for currently proposed vector controls, particularly those that aim to suppress mosquito populations by manipulating mosquito egg development. In Chapter 4, a discussion of these studies is given along with final considerations.