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 Hassan Salem, lead author of the study and a post-doctoral fellow in Emory University’s Department of Biology. “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,” Salem 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.
The tortoise beetle, which eats thistle leaves, has evolved a symbiotic relationship with bacteria that allows it to have such a specialized diet. Photo by Hassan Salem.