Beetles get by with a little help from their friends

In a remarkable display of teamwork, bacteria living inside the gut of the tortoise leaf beetle supply vital enzymes so that their host can digest leaves (Cell 2017, DOI: 10.1016/j.cell.2017.10.029).

The beetle (Cassida rubiginosa) cannot make the enzymes that break down the two principal forms of pectin, a tough polysaccharide found in the cell walls of plants. Fortunately, it has an assistant: a bacterium named Candidatus Stammera capleta that lives within a sac-like reservoir in the beetles’ guts and churns out the necessary enzymes, polygalacturonase and rhamnogalacturonan lyase.

Once these enzymes have chopped up the pectin, the beetle can use its own enzymes to crack open other cell wall components, such as cellulose and hemicellulose, to access the nutrients within. Meanwhile, the microbes get a steady supply of essential amino acids and vitamins from their host.

This symbiotic relationship is so important to the beetle’s survival that female beetles funnel a tiny fraction of their bacteria into a little blob on the end of each of their eggs, so that their offspring can dine on the beneficial bacteria and start to cultivate their own colonies.

Although symbiotic relationships between insects and bacteria are very common, the bacteria usually live inside the animals’ cells—a nurturing environment that enables the bacteria to shed many of the genes required for normal metabolic function.

“What’s unusual here is that these bacteria are outside of the host’s cells, yet they still have this tiny genome,” says Martin Kaltenpoth of Johannes Gutenberg University and the Max Planck Institute for Chemical Ecology, who was part of the team, along with lead author Hassan Salem of Emory University, to make the discovery.

The team found that Candidatus Stammera capleta has a stripped down genome that contains only 270,000 base pairs—just 6% the size of Escherichia coli—making it the smallest genome of any symbiotic organism living outside its host’s cells.

“It tells you that a tiny genome is quite possible for a bacterium that is not intracellular,” says Nancy A. Moran of the University of Texas, Austin.

Moran notes that the work could interest synthetic biology researchers, who have been pruning genomes down to their bare necessities in an attempt to create a minimal genome that only contains genes essential for life. Candidatus Stammera capleta demonstrates that the definition of ”minimal” strongly depends on the organism’s environment. “If you make the environment hospitable enough, you don’t need many genes to live there,” Kaltenpoth says.