Three-Part Harmony: USU Biologist Probes Nested Symbiosis of Mealybugs

Recent findings about a tiny insect that harbors two microbes it couldn’t survive without reveals a striking glimpse of how all organisms evolve.

Utah State University biologist Carol von Dohlen discovered the three-tiered symbiotic relationship of citrus mealybugs and two bacterial partners, Tremblaya princeps and Moranella endobia, back in 2001.

“To our knowledge, this was the first documented report of intracellular symbiosis involving two different species of bacteria, where one bacterium lives inside the other,” says von Dohlen, associate professor in USU’s Department of Biology.

Close cousins of aphids, mealybugs feed on sweet plant sap but are unable to convert the sugary substance into life-sustaining protein. In recent years, research on aphids and related sap-feeding insects has shown that bacterial residents provide missing essential amino acids, the building blocks of protein, to nourish their host.

“In mealybugs, we had discovered the physical relationship of the insects and their bacteria, none of which presumably could survive without the others,” von Dohlen says. “But we didn’t know exactly how this weird association worked.”

Enter microbiologist John McCutcheon of the University of Montana, who teamed with von Dohlen to sequence the bacteria’s genomes and piece together each microbe’s role in the trio’s communal survival. The team’s findings appear in “An Interdependent Metabolic Patchwork in the Nested Symbiosis of Mealybugs,” the Aug. 11 online edition of Current Biology.

To conduct the study, von Dohlen painstakingly dissected the microbe-containing organs from the tiny bugs, to obtain DNA for genome sequencing.

“It was very delicate work,” she says. “I used a dissecting scope. Each mealybug is less than two millimeters long.”

McCutcheon, assistant professor in UM’s Division of Biological Sciences, conducted the meticulous process of mapping each bacterium’s genes and their functions. His findings reveal that each bacterium has a very small, simple genome.

“The results are intriguing,” von Dohlen says. “We discovered that each bacterium handles different steps of amino acid synthesis pathways and they don’t duplicate efforts. Furthermore, we discovered that the mealybug also has a gene that contributes to this process that the bacteria don’t possess. This may be the first documented evidence of this kind of triple ‘tag-team’ relationship.”

The simple genomes and complementary nature of these microbes, she says, suggests that each partner has lost genes as they’ve settled into a comfortable, symbiotic environment.

“Metabolic pathways that parallel and complement each other have been found in other organisms,” von Dohlen says. “But to find one interdependent pathway where each partner has specific, unduplicated roles is extraordinary.”

The findings, she says, shed light on the development of organelles: semi-independent units found inside the cells of all living things that possess a tiny portion of their own DNA and perform varied functions.

“Our studies could help us understand the evolution of mitochondria and other organelles essential for each living cell’s survival,” von Dohlen says.

Related links: