Impact of Spiroplasma infection on reproductive physiology and behavior in Glossina fuscipes fuscipes

African Trypanosomiasis is a vector-born disease of major public health and veterinary importance in Sub-Saharan Africa. It is caused by the protozoa parasites of the genus Trypanosoma, transmitted by tsetse flies (Glossina spp.). The disease occurs in two forms: Human African Trypanosomiasis (HAT) better known as sleeping sickness, and Animal African Trypanosomiasis (AAT), or Nagana, which primarily affects livestock and cattle. According to World Health Organization (WHO), nearly one thousand new cases of HAT are reported annually. No effective vaccine is currently available, and existing drugs treatment are costly and difficult to implement. Consequently, vector control remains the most effective strategy to limit transmission. While conventional approaches such as insecticides and traps are still in use, eco-friendly strategies like the Sterile Insect Technique represent powerful alternatives (SIT). Tsetse flies are unusual among insects due to their reproductive strategy of adenotrophic viviparity: after egg fertilization, females nourish a single larva in utero, giving birth to a fully developed third instar larva. Their microbiota is also highly specialized, dominated by four main bacterial symbionts: the obligate endosymbiont Wigglesworthia, the commensal Sodalis, the facultative Wolbachia and Spiroplasma. These microbes play critical roles in host physiology, reproduction and vector competence. Particularly, Spiroplasma, a member of the Mollicutes class, has attracted growing interest because of its ability to confer refractoriness to trypanosome co-infection, potentially reducing disease transmission. However, its presence is restricted to certain Glossina species within the palpalis subgroup, including Glossina fuscipes fuscipes (Gff), the principal vector of HAT in Uganda. This thesis investigates the interaction between Spiroplasma and its tsetse fly host, with a focus on reproductive outcomes and mating behavior. Controlled mating experiments demonstrated that Spiroplasma infection increases mating propensity and revealed evidence of non-random mating, suggesting a form of pre-mating isolation linked to infection status. Furthermore, a horizontal transmission of Spiroplasma, from infected males to females was observed during copulation. Transcriptomic analysis of Gff revealed that Spiroplasma infection exerts an overall negative effect on male reproduction by reducing energy metabolism and compromising sperm quality and motility. In summary, this work provides new insight into the role of Spiroplasma in shaping tsetse reproductive biology. These findings highlight the dual potential of Spiroplasma as both a natural modifier of vector competence and a factor influencing fly fitness, with important implications for future applications, such as the development of innovative, symbiont-based biological control strategies against tsetse flies.