Mealybugs are the Russian nesting dolls of the animal world. Their bodies harbor a bacterium that helps them turn plant sap into usable nutrients, and this bacterium itself harbors an even smaller bacterium, creating a three-tiered symbiotic relationship that hasn't been observed in any other animal. Now scientists have figured out how both microbes pitch in to keep the mealybugs well fed—a finding that could offer a glimpse at how our own mitochondria evolved.
Microbiologist John McCutcheon of the University of Montana, Missoula, was intrigued by the citrus mealybug's (Planococcus citri's) unique setup. So he teamed up with biologist Carol von Dohlen of Utah State University in Logan, who had discovered the Matryoshka-style arrangement  in 2001, to look at the genes the two bacterial partners contained. They soon noticed something strange: Individually, neither the larger bacterium, Tremblaya princeps, nor its internal partner, Moranella endobia, had the biochemical machinery needed to convert sap into the amino acids mealybugs need. But together they did.
The bacteria tag-team the process , McCutcheon says. Tremblaya handles some steps then turns to Moranella to do the rest.
The tag-team relationship and the tiny size of the Tremblaya genome suggest each bacterial partner has lost some of its genes and their corresponding functions over evolutionary history. With just 121 genes (compared with humans' 20,000 to 25,000), the Tremblaya genome is the smallest ever found in an organism that isn't an organelle, a small subunit within a cell that has a little bit of its own DNA. Gene loss tends to happen when bacteria settle into a comfortable environment and no longer need certain functions, von Dohlen says, and it is what researchers think happened to organelles.
Scientists believe that mitochondria, the organelles that act as power plants for animal cells, and chloroplasts, which enable plants to photosynthesize, got their start as bacterial friends performing Tremblaya- and Moranella-like kinds of metabolic tasks; as they lost parts of their genomes that were going unused, they became a permanent part of their hosts.
The team's results, published online today in Current Biology, may have opened a window on that loss, says Patrick Keeling, a protistologist at the University of British Columbia in Vancouver, Canada. "How bacteria become organelles is something we don't understand so well, because our only examples are millions of years old," he says. "This study ... gives us an opportunity to look at how it's unfolding."