In recent years, the basic, applied, and medical importance of symbiotic microorganisms has attracted even greater attention, reflecting the recognition that internal microorganisms play important roles in the spread and infestation of agricultural and medical pest insects and that gut microorganisms substantially affect human health and diseases.

Metamorphosis, in which insects develop from eggs through larvae and pupae to adults, is among the bases of the insect diversity. For example, in butterflies and moths, larvae (caterpillars) eat plant leaves while adults suck flower nectar or tree sap. In this way, despite belonging to the same species, they can utilize different food sources and ecological niches depending on their developmental stages. It was known that symbiotic microorganisms play important roles in various insects, but the relationship between symbiotic microorganisms and insect metamorphosis was not well understood.

In collaboration with Johannes Gutenberg University, the Max Planck Institute for Chemical Ecology, and the University of Hamburg in Germany, the researchers determined the genome and analyzed the biological functions of Macropleicola sp., the symbiotic bacterium found in the digestive tract and Malpighian tubules of leaf beetles of subfamily Donaciinae, which inhabit wetlands and have a unique lifestyle of sucking sap from plant roots in the water as larvae and eating plant leaves on land as adults. They demonstrated that the biological functions of the symbiont genome, which was extremely reduced in size due to massive gene losses, are specialized for provisioning of nutrients like essential amino acids needed for synthesis of proteins that are lacking in the plant sap, and also for production of pectinases that are necessary for digestion of plant cell walls but lacking in the leaf beetles themselves.

This study revealed for the first time that one species of symbiotic bacteria can perform completely different functions in larvae and adults of the same host insect. This is an interesting new finding for understanding the process of symbiotic evolution, and since some Donaciinae leaf beetles, such as Donacia provostii infesting rice and lotus roots, are known as agricultural pests, it is also expected that the finding may lead to the development of new pest control methods targeting symbiotic bacteria.