The gut bacteria of the humble stinkbug could reveal intriguing insights into the evolutionary origins of disease-causing bacteria, researchers say. Scientists thought that gut symbionts in insects and many animals are vulnerable to competition or predation by other microbes and are less bound to their hosts.

Japanese researchers from the National Institute of Advanced Industrial Science and Technology in Tsukuba, found this to be wrong, at least in the case of the stinkbugs belonging to Plataspidae family. The plataspid stinkbugs and their gut symbionts show evidence of cospeciation, with the gut microbes showing unique genetic traits typical of endocellular symbionts.

When plataspid stinkbugs lay their eggs, they deposit bellow the eggs mass small, brown capsules filled with symbionts. Hatchings nymph will consume the symbionts from the capsules. As the nymphs develop, their anterior gut becomes a sac for digesting plant juice, while the posterior gut transform into a fermentation chamber where the symbiont bacteria provide the insect with nutrients. Adult stinkcbugs deprived of their symbionts showed developmental delays, grew smaller, failed to copulate or reproduce, and died prematurely.

The team analyzed 12 Japanese populations of stinkbugs, from three genera and seven species. Takema Fukatsu and colleagues made a family tree of the bacteria's DNA to show how the bacterial species in different insect species were related.

DNA research showed that to each bug species there corresponded a distinct bacterial species belonging to their own class of Proteobacteria, and were closely related to the obligate endosymbiont Buchnera aphidocola of aphids. Plataspid stinkbugs depend on their gut symbionts to survive in the same way aphids depend on their endosymbiont bacteria.

Moreover, DNA analysis showed that, like Buchnera, the gut endosymbionts co-evolved with their hosts. The plataspid DNA family tree pattern proved identical to the bacterial DNA family tree pattern.

The finding was a completely unexpected for bacteria living freely in an insect's gut. The researchers found the same pattern of a small genome, a high percentage of A and T nucleotides in their DNA, and accelerated molecular evolution, possibly resulted from population genetic occurrences like small population size and bottlenecks.

But with this unique plataspid stinkbug system, they will be well equipped to study insect symbiosis and its influence on genome evolution. The changes found in these bacteria have been seen in many pathogenic bacteria, like Chlamydia. They may reflect the way bacteria enter a host, as symbionts or parasites.

The Japanese team hopes to discover more about the evolution of pathogen bacteria by manipulating host and bacteria separately.