Spiroplasma Are Protective Heritable Symbionts With Low Physiological Impact in the Drosophilid Fly Zaprionus kolodkinae

Spiroplasma bacteria are widespread associates of insects, with Drosophila serving as a key model for understanding maternally inherited symbioses. Most research has focused on the poulsonii–citri clade of Spiroplasma, leaving other lineages comparatively understudied. Here, we characterise the symbiosis between the drosophilid Zaprionus kolodkinae and its ixodetis group Spiroplasma (sZko). We assembled a complete genome for sZko, which encodes multiple candidate symbiosis factors, including ankyrin repeat domain proteins and diverse ribosome-inactivating protein (RIP) toxins typically linked to protective interactions. Notably, the genome also harbours a gene with a predicted ricin B lectin-binding domain, a candidate for establishing microbe-insect interactions at the eukaryotic cell surface. Phenotypic assays confirmed maternal inheritance of sZko with no evidence of reproductive parasitic phenotypes. Infected flies were protected against attack by the generalist parasitoid wasp Leptopilina heterotoma. There was no detectable impact of sZko on its host's starvation tolerance, suggesting minimal physiological cost to the host, and this low impact was mirrored for the protective symbiont sHy1 in D. hydei but contrasted with previous results for D. melanogaster. We conclude the Z. kolodkinae–Spiroplasma association is primarily defensive, and genomic analysis raises the possibility that protection involves a novel coupling between lectin-binding domains and RIPs.