Bacterial Symbiont Responsible for Weevils’ Hardness

– Discovery of symbiotic bacteria specialized for tyrosine synthesis and essential for hardening and coloring of host’s exoskeleton –

Researchers:Takema Fukatsu, Chief Researcher, the Bioproduction Research Institute; Hisashi Anbutsu, Senior Researcher and Minoru Moriyama, Senior Researcher, the Symbiotic Evolution and Biological Functions Research Group of the institute; The Open University of Japan; Kyushu University; Kagoshima University; Kyoto University; the University of Tokyo; Okinawa Institute of Science and Technology; and the National Institute for Basic Biology


The researchers have demonstrated that symbiotic bacteria of weevils are specialized for synthesis of tyrosine, an amino acid. In addition, they identified the host-side genes that regulate the final step of the tyrosine synthesis pathway.

Genomic structure of Nardonella endosymbionts from four weevil species
From PNAS doi: 10.1073/pnas.1712857114 Fig. 2 with modification

Background of Research

Weevils constitute the most species-rich group among the beetles, and many of them possess a hard exoskeleton. Many weevils are associated with endosymbiotic bacteria called Nardonella. On the ground that the molecular phylogeny of host weevils agrees well with that of their symbiotic bacteria, it has been estimated that the origin of the symbiotic relationship dates back to the common ancestor of the extant weevil species over 100 million years ago. However, the function of Nardonella endosymbiont has been elusive thus far.

Details of Research

The researchers determined the complete Nardonella genome sequences derived from four weevil species. The genomes were extremely small in size, at about 200,000 base pairs, with only the genes minimally needed for bacterial survival and, additionally, the genes constituting the tyrosine synthesis pathway. These results indicated that Nardonella is an endosymbiont specialized for tyrosine synthesis. However, in the tyrosine synthesis pathway, only the final step gene was missing. In the host insect cells, the genes that are capable of catalyzing the final step reaction were highly expressed. Hence, it is conceivable that the symbiont-mediated tyrosine synthesis is subjected to the final step regulation by the host genes.

Future Plans

The researchers will investigate the possibility that other insects, which are dependent on symbiotic bacteria for their growth, survival and reproduction, may have a symbiotic relationship similar to the weevil-Nardonella relationship.

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