Diversity, functionality, and evolutionary history of bacterial symbionts in false click beetles

Symbiotic relationships with microbes are widespread in insects, traversing a broad spectrum of reciprocal dependency, intimacy, duration, and cooperation. Many insect taxa are only able to establish in their ecological niche because of the help or bacterial partners. The false click beetles (Coleoptera, Throscidae) are a small, but globally distributed family of beetles that live in leaf litter and decaying wood and were previously reported to harbor intracellular symbionts in specialized bacteriomes. In this study, we investigated three of the four extant genera of Throscidae beetles for their symbiotic microbes and provide insights into host-symbiont interactions based on symbiont genomes and localization. The ancient Shikimatogenerans symbiont with a highly eroded genome of ~150 kbp and ~12% GC was present in all examined taxa, probably supplying tyrosine precursors for cuticle biosynthesis as its only contribution to the host. Beyond this, several secondary symbionts from the Enterobacterales (e.g. Sodalis and Symbiodolus ) and Flavobacteriales with variable tissue tropism, genome sizes, and encoded metabolic capabilities were present, suggesting multiple symbiont acquisition events. Their inferred roles based on genome content and localization range from parasitic by manipulating host reproduction, to mutualistic, supplying multiple amino acids and cofactors. Thus, beyond deepening our understanding of tyrosine-supplementing symbionts across a broad range of beetle families with different ecologies, the Throscidae provide insights into a dynamic evolutionary history with multiple co-occurring symbionts and expand our view on multipartite symbiotic interactions in insects.