Scientist John McCutcheon, shown here with a cicada, studies mutually beneficial relationships between insects and bacteria.
Research reveals fascinating relationships between
insects and one-celled stowaways
By Deborah Richie
“If you study diversity, you find interesting things,” says John McCutcheon, who joined the faculty of UM’s Division of Biological Sciences in 2010. “And this diversity does not need to be found deep in a remote jungle or on the bottom of the ocean, but can be right outside your door.”
You might start with spittlebugs, cicadas, aphids and mealybugs, which are all sap-feeding insects. Sap is a nutrient-poor food that cannot supply the insects with the 10 essential amino acids needed for life. Instead, they rely on bacteria to make it for them, and in turn the bacteria have a cozy place to inhabit inside the insect — a symbiotic relationship.
Bacteria, plants and fungi have the ability to manufacture all 20 amino acids, while animals must obtain the 10 essential amino acids from their diet.
McCutcheon’s recent co-discovery of the metabolic partitioning between two bacterial species dwelling within mealybugs borders on the fantastic. He and Utah State University’s Carol von Dohlen found a level of integration never before observed among three organisms. Their research appeared in the Aug. 23 issue of Current Biology and is titled “An Interdependent Metabolic Patchwork in the Nested Symbiosis of Mealybugs.”
“In this symbiosis, both bacteria live in the cytoplasm of special mealybug cells, and one bacteria lives inside of the other,” he says. “To make some essential amino acids, different enzymes are contributed from all three organisms in the symbiosis. The survival of the entire system is dependent on these enzymes, which are produced in a remarkable patchwork pattern.”
Specifically, a bacterium called Moranella (named after McCutcheon’s mentor, Nancy Moran) nests within a bacterium called Tremblaya, providing it with parts of the essential amino acid pathways it lacks. The Tremblaya-Moranella composite nests within mealybug cells and supplies essential amino acids (or some of their precursors) to the mealybug.
The amazing relationships McCutcheon explores in his work may lead to practical applications for agriculture. Many sap-feeding insects are serious crop pests, such as the citrus mealybug and the glassy-winged sharpshooter. He has a new U.S. Department of Agriculture grant to study the stinkbugs that are invading soy crops in the southeast in high numbers.
“If you kill the symbionts, you kill the insect,” he says. “So there’s a real interest in exploiting knowledge of the symbionts for pest control.”
McCutcheon began his research delving into sap-feeding insects and their bacterial symbionts as a postdoctoral fellow at the University of Arizona in 2006 under the tutelage of Moran, who has long studied the evolution and ecology of aphids and their beneficial bacteria.
Aphids live by sucking sap from the phloem (nutrient-transporting tissue) of plants and secreting sweet honeydew that attracts ants. For anyone strolling the UM campus this past summer, they personally experienced the sticky leaves stuck to the bottom of their shoes — all honeydew residue from a bountiful aphid season.
The challenge that intrigued McCutcheon as a postdoctoral fellow was to expand Moran’s study from aphids, which have one symbiont, to a different group of sap-sucking bugs with more than one bacterial symbiont.
While Moran came to her subject of insect and bacteria co-evolution because of a lifelong interest in bugs, McCutcheon admits he hadn’t thought much about them until his postdoctoral work. His doctorate is in computational biology.
“I’m really into them now,” he says with a grin. Some of his main subjects live just outside his office, such as the spittlebug larvae that cover themselves in bubbly foam for protection and the cicadas that sing in the trees on campus.
“You can actually see the organs that contain the bacterial symbionts in some spittlebugs,” he says. “Just pick one out of the spittle and turn it over and look for a couple of bright orange, red or yellow dots near the back end of the insect.”
Spittlebugs, cicadas and sharpshooters are the group of sap-feeding insects that McCutcheon selected for the array of potential symbiotic bacteria to study. The group differs from aphids and mealybugs, which passively sip the sap from a plant’s phloem. Instead, they are xylem feeders that have to suck hard to extract liquid from the primarily water-carrying part of a plant. The insects tend to have big heads to house the large sucking muscles that help with the arduous task. In addition to being under negative pressure, xylem sap is more nutrient-poor than phloem.
“From old microscopy, there was known to be a huge diversity of bacteria within the insect group containing spittlebugs, cicadas and sharpshooters,” he says. “But these bacteria cannot be grown in the lab, so it wasn’t until the advent of modern genome sequencing methods that we had the opportunity to study how these bacteria are associated with insects.”
He focused on the Sulcia bacteria that has co-evolved with the three insects for 270 million years. Over time Sulcia lost the ability to make all 10 amino acids and had to pick up a partner. The two bacteria share the same cells, rather than nesting in each other, like with the mealybug.
Sulcia produces seven of the 10 essential amino acids in the spittlebug. Bacteria called Zinderia makes the other three. In the cicada, Sulcia makes eight, and the bacteria Hodgkinia adds the missing two amino acids. The sharpshooter has a similar ratio, but Sulcia has a different partner symbiont called Baumannia.
“In each case, the insect depends on the metabolic contributions from the two bacteria. Their duties are distributed in complementary and nonoverlapping ways,” McCutcheon explains.
The complex symbiosis of bacteria within the sap-feeding insects is even more remarkable when you consider the genetics of these one-celled microorganisms.
“The bacteria we’re studying have such reduced genomes that they live at the extreme end of the spectrum,” he says. “With so few genes, it’s a complete mystery how the symbionts survive.”
A genome carries the instructions for an organism to make the proteins and enzymes it needs to live. Tremblaya has the smallest bacterial genome in the world, with only 121 protein-coding genes. In comparison, the E. coli bacterium has 4,100 genes. People have somewhere around 24,000 protein-coding genes.
McCutcheon hopes that examining bacteria with super small genomes may help illuminate the evolutionary point where bacteria become organelles — organ-like structures within cells — and lead to more knowledge of mitochondria, the power producers of cells. Like the symbionts McCutcheon studies, mitochondria used to be free-living bacteria. In the process of becoming an organelle, mitochondria also lost many genes, similar to what is observed in the insect symbionts such as Tremblaya.
His lab applies the latest technology to rapidly, efficiently and cheaply sequence bacterial genomes. The researchers remove the insect organs that have the bacteria and purify the DNA. Since about 80 percent of the DNA comes from the insect, it takes many sequences to find and identify the bacteria DNA.
“For mealybugs, we sequenced over 2 billion bases,” he says, “and only then could we pull out the bacterial symbiont genomes.”
McCutcheon makes a compelling case for the wonders and necessities of diversity revealed when you tease apart life at miniscule levels.
His next line of inquiry is to examine how such small genomes can function. “How can these bacteria have such tiny genomes, encoding so few genes, and still be alive?”
These mealybugs, Planococcus citri, contain two distinct species of bacteria – one living inside the other – which provide the insects with essential amino acids. (Photo by Lyle Buss, University of Florida)
Bacteria often have a bad rap. While a minority of these single-celled organisms cause nasty diseases, humans rely on thousands of bacterial species living in and on us to help us digest food, protect our skin, build up proper immune responses and several other key functions. From 500 to 1,000 species of bacteria inhabit the human gut alone.
“We’re more bacterial than human in terms of cell count,” McCutcheon says. “There are about 10 times more bacterial cells in your gut alone than there are human cells in your entire body.”
In addition to their relevance to us, bacteria are amazing in their own right. According to geologic studies, bacteria have been on Earth for some 3.5 billion years.
“This antiquity has given them time to evolve biochemical pathways to live just about anywhere and eat just about anything,” McCutcheon says. Bacteria thrive everywhere, from the hot springs in Yellowstone National Park to the underside of Antarctic glaciers.
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