In collaboration with the Max Planck Institute (Germany), Emory University (the United States), and Johannes Gutenberg University (Germany), the researchers determined the whole-genome sequence of Stammera, a bacterial symbiont of the tortoise leaf beetle Cassida rubiginosa, and found that the genome of Stammera is specialized for production of enzymes to degrade pectin, one of the main components of the plant cell wall.

Effects of symbiont sterilization on host’s survival and pectin-degrading enzymatic activities (Caplet is a symbiont-encasing structure located at the anterior tip of the egg. Symbiont infection occurs when the larva chews and breaks the eggshell during hatching.)

Leaf beetles are the insects that are specialized for feeding on plant leaves. They include many agricultural pests, such as Cassida rubiginosa that consumes thistle leaves. As for symbiotic bacteria of leaf beetles, since the pioneering microscopic observations reported by Stammer in the 1930s, almost no studies have been conducted thus far.

The researchers investigated Cassida rubiginosa and identified a bactrerial symbiont. They named it Stammera and determined its whole-genome sequence. The sequence is small, at about 270,000 base pairs, containing only a minimal number of genes required for bacterial life plus pectin-degrading enzyme genes. When the larvae were deprived of Stammera by sterilization, the pectin-degrading activities in the alimentary tract significantly decreased, and their growth and survival were inhibited. Pectin is one of the main components of plant cell walls. It is likely that the host insect breaks the plant cell wall with the help of Stammera that is specialized for production of pectin-degrading enzymes, thereby efficiently utilizing the plant cytoplasm as a source of nutrients.

Some leaf beetles other than Cassida rubiginosa were also reported to have symbiotic bacteria. The researchers will investigate the microbiological nature, diversity and biological function of these symbiotic bacteria.