ABSTRACT
The thick and hard cuticle of beetles plays an essential role in their adaptation to a variety of ecological niches, thereby underpinning their enormous diversity and prosperity. In recent years, symbiotic bacteria involved in cuticle formation have been reported in several insect species. Weevils (Curculionoidea) are armored with hard cuticle and ubiquitously associated with an ancient γ-proteobacterial endosymbiont lineage Nardonella. Nardonella genomes are extremely reduced to 0.2 Mb with strikingly limited gene repertoire. Besides genes for core cellular processes such as DNA replication, transcription and translation, the genomes retain only a few metabolic pathways, of which the most complete one is the synthetic pathway for tyrosine, the principal precursor amino acid needed for cuticle formation. Experimental suppression of Nardonella results in low tyrosine titer and the emergence of adult beetles with reddish and soft cuticles, confirming the critical role of Nardonella for the host's cuticle formation. Notably, the Nardonella's tyrosine synthesis pathway is incomplete, where the final step reaction is regulated by host's aminotransferase genes preferentially expressed in the symbiotic organ. We suggest that the symbiont-mediated cuticle hardening may be found not only in weevils but also in other insects more widely than previously envisaged.
