Bacteria may have bad reputations but in fact, all animals -- us included -- rely on them in critical ways. In the case of sap-feeding insects, intimate associations with microbes offer a source for essential nutrients that their sugary diets just don't include. Now, researchers reporting in the August 11th Current Biology have new insight into organisms that have taken this symbiotic lifestyle to the extreme; they have sequenced the genomes of two species of bacteria that live together, one inside of the other, inside mealybugs.
The effort has revealed a level of molecular-level integration between bacterial species that scientists have never before seen.
"This symbiosis teaches us a lot about extremes," said John McCutcheon of the University of Montana. "The mealybug, Tremblaya and Moranella represent an extreme level of integration between organisms, each requiring fragments of essential amino acid pathways from the other to survive."
Scientists first discovered the bacteria living within mealybugs in the mid-1900s, but at that time they were thought to be a single species. More recently, scientists confirmed that there were in fact two endosymbionts in the mealybugs. That those bacteria, Tremblaya and Moranella, were in fact nested one inside the other came to light only much more recently, based on the discovery of study co-author Carol von Dohlen of Utah State University.
McCutcheon and his colleagues had studied other examples of dual symbionts living within insects before, and their genomes had shown remarkable levels of metabolic complementarity. For example, he says, one bacterial partner might make eight of the ten essential amino acids, leaving the other to make the remaining two.
The nested organization of the mealybug symbionts made them an irresistible choice for further exploration. "We hoped to learn if and how the duties of nutrient production were distributed between the two symbionts," McCutcheon said. "Frankly, we thought that the system was so cool and unique that we just had to do it. It just had to be interesting!"
And indeed, it is. As in previous examples, the bacterial genomes have each lost genes such that they now show little overlap in retained genes involved in nutrient production. But, in this instance, several of the amino acid pathways apparently require a patchwork of interspersed gene products from Tremblaya, Moranella, and possibly the mealybug too.
McCutcheon and von Dohlen don't yet know quite how it works, but it suggests that metabolites or enzymes or both must get traded back and forth among species in order to come up with the complete set of essential amino acids needed by all three organisms.
Also notable, the researchers found that Tremblaya's genome has shrunk to an impressive degree, making its genome the smallest ever described in any cell.
"Tremblaya has undergone a massive amount of genome decay," McCutcheon said. "It shows how much reduction is possible in bacterial genomes, given the right permissive environment."
The mealybugs and their symbionts may be unusual, but the new findings hold important life lessons nonetheless. "These results really represent one extreme example, but the symbiosis theme replays itself over and over again in biology at many different levels," McCutcheon says.
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