A leaf-eating beetle has evolved a symbiotic relationship with bacteria that allows the insect to break down pectin -- part of a plant's cell wall that is indigestible to most animals.
The journal Cell published the findings on the novel function of the bacterium, which has a surprisingly tiny genome -- much smaller than previous reports on the minimum size required for an organism not subsisting within a host cell.
"This insect is a leaf eater largely because of these bacteria," says lead author of the study. "And the bacteria have actually become developmentally integrated into the insect's body."
Two organs alongside the foregut of the beetle Cassida rubiginosa house the bacteria and appear to have no other function than to maintain these microbes. "The organs are equivalent to the liver in humans, in the sense that they contain the tools to break down and process food," lead author says.
The newly characterized bacterium has only 270,000 DNA base pairs in its genome, compared to the millions that are more typical for bacterial strains. That makes its genome closer to that of intracellular bacteria and organelles than to free-living microbes. Mitochondria, for example, the organelles that regulate metabolism within cells, have 100,000 base pairs.
A human gut holds about 10,000 species of bacteria. These microbial communities, which can be genetically characterized as microbiomes, are transferred generationally but are also dynamic and respond to environmental changes. The microbiome of an urbanite, for example, has different characteristics from that of a hunter-gatherer.
Unlike humans, insects tend to have specialized feeding ecologies. They offer simple models to study symbiotic relationships between microbes and their hosts.
Authors used genome sequencing technology to characterize the microorganisms as a new species of bacterium. Despite its tiny genome, the bacterium has the power to degrade pectin.
"Just as an apex predator has claws and strong mandibles to obtain the nutritional value that it needs from its prey, the bacterium has pectin-digesting genes that enable the beetle host to deconstruct a plant cell," lead author says.
After the bacterium breaks down the pectin, the beetle's digestive system can then access all of the amino acids and vitamins within the plant's cells for its nutrients.
Comparative transcriptomics revealed pectinase expression to be enriched in the symbiotic organs, consistent with enzymatic buildup in these structures following immunostaining with pectinase-targeting antibodies. Symbiont elimination results in a drastically reduced host survivorship and a diminished capacity to degrade pectin.
The lead author christened the new bacterium Candidatus Stammera capleta, after Hans-Jurgen Stammer, the ecologist who first glimpsed it and wondered about it more than 80 years ago.
"The most amazing thing to me is that we made this discovery because I read a really old book," lead author says. "It speaks to the importance of natural history collections and libraries for old journals. We truly stand on the shoulders of giants, extending the work of those who came before us."