Investigating Symbioses between Spiroplasma and Drosophilid Flies

Abstract

Insects commonly harbor microbial symbionts that can strongly influence their biology, ecology, and evolutionary trajectories (Douglas, 2014). Among these, facultative bacterial symbionts are of particular interest because their persistence within host populations depends on a balance between costs, benefits, and environmental conditions. In this thesis, I investigate the interaction between Spiroplasma and the drosophilid fly Zaprionus kolodkinae, a system that has received little attention compared to well- studied Drosophila–Spiroplasma associations. I first characterise the basic biology of Spiroplasma infection in Z. kolodkinae, examining patterns of vertical transmission, infection stability, and the feasibility of generating cured (uninfected) laboratory lines. I then assess whether Spiroplasma manipulates host reproduction or influences host fitness traits, including vitality, climbing ability, and resistance to starvation. These experiments reveal that Spiroplasma infection in Z. kolodkinae is maternally inherited, does not induce strong reproductive manipulation but does enhance survival of the fly following parasitisation. Next, I explore how environmental stressors influence Spiroplasma–host interactions. By comparing infected and uninfected lines across multiple drosophilid species (Z. kolodkinae, Drosophila hydei, and Drosophila melanogaster), I test whether Spiroplasma alters tolerance to desiccation, thermal stress, and metabolic rate. I show that the impact of Spiroplasma on host stress tolerance is context dependent and varies across host species and environmental conditions, highlighting that symbiont- mediated effects cannot be generalised across systems. Finally, I examine the role of ecological factors in shaping Spiroplasma transmission and persistence, focusing on the effects of food composition and temperature on vertical transmission and segregational loss in laboratory populations of Z. kolodkinae. These experiments demonstrate that temperature did not impact vertical transmission in this symbiosis (contrasting to other systems), but subtle effects of diet were observed. Collectively, this thesis demonstrates that the Zaprionus–Spiroplasma association represents a distinct form of facultative symbiosis, shaped by host biology, symbiont traits, and environmental context. Future work should consider this symbiosis in its ecological context and determine the reasons why Spiroplasma-Drosophila interactions vary according to Spiroplasma strain and fly host.