Mealybugs rely on a nested arrangement of symbiotic bacteria for essential nutrients. This interconnected system might illuminate the evolution of some of the essential components of plant and animal cells. Image: Charles Olsen, USDA
With just 121 protein-coding genes, the diminutive Tremblaya princeps, a symbiotic bacterium that lives inside specialized cells of the sap-eating mealybug, has the smallest known genome of any cellular organism on the planet. Tremblaya helps to supply the mealybug with essential amino acids and likely receives nutrients and other life-sustaining molecules in return. And even as it tests the lower limits of genome size, the Tremblaya genome may still be shedding genes.
Even more surprisingly, scientists discovered in 2011 that Tremblaya plays host to its own bacterial guest. Called Moranella endobia, the bacterium is smaller in physical size than its host but has more than three times as many genes. Together the three organisms form a complex, co-dependent web; the nested bacteria complement each other and their insect host, creating a genetic patchwork of enzymes needed to produce amino acids lacking in the mealybugs’ sap diet.
Tremblaya presents something of a paradox that some biologists think could help illuminate the evolution of cell parts. The combination of host and symbiont has allowed Tremblaya to cast off many of its genes, surviving with a genome size once thought to be impossible.
“It’s quite remarkable how these bacteria have pushed the lower limit of what we consider to be a viable organism,” said John Archibald, a microbiologist at Dalhousie University in Halifax, Nova Scotia. “Ten years ago, people would have laughed at the idea of bacteria with such a small gene set.”
Given its extreme minuteness and the fact that it must get many essentials from both its host and resident microbes, some suggest that Tremblaya blurs the boundaries between cellular organisms and organelles, specialized structures within cells such as the energy-producing mitochondria. It has been officially designated an endosymbiont, an organism that lives within the cells of another organism. But its genome size resembles that of some organelles. “When do these things stop being bacteria?” asked John McCutcheon, a biologist at the University of Montana in Missoula who studies these organisms.
Indeed, scientists now know that some organelles evolved from endosymbiont bacteria, raising hopes that studying tiny endosymbionts like Tremblayacould shed light on the evolution of those organelles. “There is no bright line between endosymbionts and organelles,” McCutcheon said. “We might be looking at something pretty darn similar to the endosymbiont-to-organelle transition.”
In a paper published June 20 in the journal Cell, McCutcheon and collaborators reveal a striking new level of interdependency among the Tremblaya troika. The mealybug genome appears to include genes from other varieties of bacteria distinct from Tremblaya and Moranella, and the two endosymbiont bacteria may use the protein products of these genes to manufacture nutrients and to make their membranes.
Archibald, who was not involved in the study, described it as “a lot of mixing and matching taking place in evolutionary time.”