Now things get really strange. Other species of mealybugs are also Russian dolls, with the same bug-in-a-bug-in-a-bug set-up. One of them—the long-tailed mealybug—even seems to have two kinds of inner bacteria living inside its outer one.
No matter the mealybug species, the outer bacterium is always Tremblaya. But the inner bacterium varies considerably—it’s Moranella in the citrus mealybug, but different microbes in the other insects. The most obvious explanation for this pattern is that some ancestral mealybug became infected with its two nested microbes. As the insects diverged into different species, so did the microbes within them. For whatever reason, the outer one stayed the same, while the inner one changed into new forms—Moranella being just one of them. “It’s so weird and so uncommon to get these bacteria inside of each other that I thought it had to happen once,” says McCutcheon.
If that were the case, you’d expect the family tree of the inner microbes to match that of the mealybugs themselves. But it doesn’t. Not even close. Once again, with mealybugs, the obvious explanation is completely wrong.
By sequencing the genomes of five mealybugs and all their associated bacteria, Filip Husnik, one of McCutcheon’s graduate students, has shown that these insects seem to regularly replace their inner bacterium. Tremblaya is constant, but the microbe within gets periodically swapped out for a fresh partner. That’s why some of these inner microbes have genomes that are 10 times bigger than others—they are recent acquisitions that haven’t had time to lose disposable genes.
So, even though the mealybug and its bacteria cannot survive without each other, their three-way alliance turns out to be surprisingly flexible. New partners come and go. Various genes are lost and retained.
This constant symbiotic turmoil leaves its mark on the genomes of the mealybugs themselves. Husnik showed that each of these insects carries bacterial genes in its DNA, and these genes almost never come from their current partners. Instead, they belonged to bacteria that once colonized these insects but have since been ousted. Before they vanished, they transferred genes to their hosts, and the mealybugs use these genes to make nutrients. So, each of these insects is a mash-up of many species of microbes, several of which aren’t even there anymore.
In their new paper, Husnik and McCutcheon show all of these genetic patchworks in one enormously complicated diagram. When I ask McCutcheon to explain it to me, he says: “F*********ck.” Then, after a pause: “If you look at just the citrus mealybug, you think: Oh, it’s that way. But this dynamism gives me a sense that we should be careful drawing conclusions from one example.”
“The study is an exceptional accomplishment,” says von Dohlen. “I think the next major phase of research in symbiosis will be studies like this,” which look at many related species to see how the relationships between animals and microbes evolve over time. That’s the best shot of answering the many lingering questions about these symbioses.