Lots of organisms rely on symbiotic relationships, in which two species rely on each other for survival and one lives inside the other. But citrus mealybugs enjoy a triply symbiotic relationship unlike any we've ever seen...with one absolutely crucial exception.
Microbiologist John McCutcheon of the University of Montana and biologist Carol von Dohlen of Utah State University teamed up to study how symbiosis helps the citrus mealybug survive. The bug must turn plant sap into nutrients, but it lacks the ability to do this on its own. It relies on the bacterium Tremblaya princeps to handle the conversion process...except this bacterium can't do it all either.
Instead, Tremblaya princeps handles one part of the conversion into amino acids, and then it hands things over to a second, smaller bacterium Moranella endobia. This bacterium lives inside Tremblaya princeps, and neither bacterium possesses the necessary complement of biomachinery to turn the plant sap into nutrients. So this means the mealybug relies on both bacteria equally for food, Moranella needs to be housed inside Tremblaya, and Tremblaya must survive inside the mealybug. It's the first triple symbiosis we've ever seen.
But that's not the end of the story - Tremblaya has the smallest genome of any organism that we've seen so far. It has only 121 genes, compared to the 20,000 or so that humans possess. No truly independent bacterium could ever survive with so few genes - its pair of symbiotic partners have allowed it to lose superfluous genes while settling into its easygoing routine. Crucially, the presence of a second bacterium in Moranella to handle some of its duties has allowed Tremblaya to lose even more genes that one might normally expect.
What we might be seeing here is a transitional stage in the development of organelles, which are semi-independent units found inside cells that possess a tiny portion of their own DNA and perform certain autonomous functions. In animals, those organelles are mitochondria, which are crucial for providing energy for the cell. The whole "tiny portion of their own DNA" thing is why mitochondrial DNA is so crucial to charting human ancestry.
In much the same way that Tremblaya and Moranella share metabolic functions and serve as equal partners in keeping their host alive, so too do organelles perform crucial functions for the survival of the cell and, by extension, the organism as a whole. With mealybugs, we might finally have a modern equivalent to the evolution of mitochondria and other organelles that took place millions of years ago.