Small genome symbiont underlies cuticle hardness in beetles

Beetles, representing the majority of the insect species diversity, are characterized by thick and hard cuticle, which plays important roles for their environmental adaptation and underpins their inordinate diversity and prosperity. Here, we report a bacterial endosymbiont extremely specialized for sustaining beetle’s cuticleformation. Many weevilsare associated with a γ-proteobacterial endosymbiont lineage Nardonella , whose evolutionary origin is estimated as older than 100 million years, but its functional aspect has been elusive. Sequencing of Nardonella genomes from diverse weevilsunveiled drastic size reduction to 0.2 Mb, in which minimal complete gene sets for bacterial replication, transcription, and translation were present but almost all of the other metabolic pathwaygenes were missing. Notably, the only metabolic pathwayretained in the Nardonella genomes was the tyrosinesynthesis pathway, identifying tyrosineprovisioning as Nardonella ’s sole biological role. Weevilsare armored with hard cuticle, tyrosineis the principal precursor for cuticleformation, and experimental suppression of Nardonella resulted in emergence of reddish and soft weevilswith low tyrosinetiter, confirming the importance of Nardonella -mediated tyrosineproduction for host’s cuticleformation and hardening. Notably, Nardonella ’s tyrosinesynthesis pathway was incomplete, lacking the final step transaminase gene. RNA sequencing identified host’s aminotransferase genes up-regulated in the bacteriome. RNA interference targeting the aminotransferase genes induced reddish and soft weevilswith low tyrosinetiter, verifying host’s final step regulation of the tyrosinesynthesis pathway. Our finding highlights an impressively intimate and focused aspect of the host–symbiont metabolic integrity via streamlined evolution for a single biological function of ecological relevance.