Spiroplasma in Glossina fuscipes fuscipes: a potential manipulator of male reproduction

: Glossina fuscipes fuscipes (Gff) is a riverine species of the Glossina genus (tsetse flies). It is one of the principal vector species responsible for transmitting human African trypanosomiasis across a broad swath of sub-Saharan Africa. Consequently, it remains a primary target for vector-control interventions in endemic regions. As with other Glossina species, G. fuscipes houses a highly streamlined microbiota, composed of maternally transmitted endosymbionts mandatory for survival, which also significantly impacts the vectorial capacity. Among them, the obligate Wigglesworthia provides essential nutrients and stimulates maturation of the fly's immune system, providing a refractory effect against Trypanosoma. It also harbors the commensal Sodalis and parasitic Wolbachia, the latter of which induces cytoplasmic incompatibility. In addition, Gff hosts a Spiroplasma strain that enhances host refractoriness to trypanosome infection but disrupts female metabolic and reproductive homeostasis. We used high-throughput RNA sequencing to examine the impact of Spiroplasma on male testes and accessory glands to reveal how this microbe exerts its influence on male reproductive physiology. We found that Spiroplasma substantially alters testes physiology, downregulating genes involved in energy metabolism and sperm function, suggesting reduced male reproductive fitness. In contrast, accessory gland gene expression was not significantly affected despite the critical role of these organs in semen production and reproductive success, considering also that the bacterium exploits the ejaculate to further increase its population prevalence. Spiroplasma infection also correlated with increased abundance of other symbionts, including Wigglesworthia.IMPORTANCEA complex host-symbiont interplay makes the Spiroplasma harbored in tsetse fly intriguing. The unexpected male-specific energetic costs of Spiroplasma infection keep open the biological question of why this bacterium is maintained in this species. On the other hand, Spiroplasma infection correlates with increased abundance of Wigglesworthia, promoting trypanosome refractoriness. The present work adds a valuable layer to further interpret the ecological and evolutionary narrative of Spiroplasma interactions, and lays the preliminary groundwork to explore the potential of using Spiroplasma as a biocontrol agent. While acknowledging that our findings were derived from a long-established laboratory colony, the data on the modulation of fly reproductive fitness and on the possible contribution to trypanosome refractoriness found make Spiroplasma intriguing for strategic integration into environmentally sustainable vector management programs, potentially in concert with the Sterile Insect Technique. The results provide significant advances in symbiont-host interactions, bringing new insight into understanding the role of symbionts in viviparous tsetse flies. We also provide a community forum for discussion on the possible use of bacteria as species-specific biocontrol.