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Honey bees have suffered dramatic losses in recent years, largely due to multiple stressors underpinned by poor nutrition [1]. Nutritional stress especially harms larvae, who mature into workers unable to meet the needs of their colony [2]. In this study, we characterize the metabolic capabilities of a honey bee larvae-associated bacterium, Bombella apis (formerly Parasaccharibacter apium), and its effects on the nutritional resilience of larvae. We found that B. apis is the only bacterium associated with larvae that can withstand the antimicrobial larval diet. Further, we found that B. apis can synthesize all essential amino acids and significantly alters the amino acid content of synthetic larval diet, largely by supplying the essential amino acid lysine. Analyses of gene gain/loss across the phylogeny suggest that four amino acid transporters were gained in recent B. apis ancestors. In addition, the transporter LysE is conserved across all sequenced strains of B. apis. Finally, we tested the impact of B. apis on developing honey bee larvae subjected to nutritional stress and found that larvae supplemented with B. apis are bolstered against mass reduction despite limited nutrition. Together, these data suggest a novel role of B. apis as a nutritional mutualist of honey bee larvae.

Mothers provide their offspring with symbionts. Maternally transmitted, intracellular symbionts must disperse from mother to offspring with other cytoplasmic elements, like mitochondria. Here, we investigated how the intracellular symbiontWolbachiainteracts with mitochondria during maternal transmission. Mitochondria andWolbachiamay interact antagonistically and compete as each population tries to ensure its own evolutionary success. Alternatively, mitochondria andWolbachiamay cooperate as both benefit from ensuring the fitness of the mother. We characterized the relationship between mitochondria andWolbachiatiters in ovaries ofDrosophila melanogaster. We found that mitochondria andWolbachiatiters are positively correlated in common laboratory genotypes ofD. melanogaster. We attempted to perturb this covariation through the introduction ofWolbachiavariants that colonize at different titers. We also attempted to perturb the covariation through manipulating the female reproductive tract to disrupt maternal transmission. Finally, we also attempted to disrupt the covariation by knocking down gene expression for two loci involved in mitochondrial metabolism:NADHdehydrogenase and a mitochondrial transporter. Overall, we find that mitochondria andWolbachiatiters are commonly positively correlated, but this positive covariation is disrupted at high titers ofWolbachia. Our results suggest that mitochondria andWolbachiahave likely evolved mechanisms to stably coexist, but the competitive dynamics change at highWolbachiatiters. We provide future directions to better understand how their interaction influences the maintenance of the symbiosis.