13-year cicada. Credit: Patchattack. CC BY 2.0
13-year cicada. Credit: Patchattack. CC BY 2.0

The Slow-Motion Symbiotic Train Wreck of the 13-Year Cicada

Round about now, in various US states, a vast swarm of cicadas will start crawling out of the ground. These black-bodied, red-eyed insects have stayed underground for 13 or 17 years, drinking from plant roots. When they greet daylight for the first time, they devote themselves to weeks of frenzied sex and cacophonous song, before dying en masse. They’ll be picked off by birds, snagged by squirrels, and crunched under shoes and tyres, but none of that will dent their astronomical numbers—which is perhaps the point of their lengthy underground stints, and their synchronous emergence.

But the cicada’s weird lifestyles have also left them with a different legacy. It involves the bacteria that live in their bodies, and it’s so weird that when John McCutcheon first discovered it, he thought he had made a technical error.

Many insects carry bacteria inside their cells. These ‘endosymbionts’ are especially common among sap-sucking bugs, like cicadas, and help their hosts to make nutrients that they can’t get through their food. They almost always have exceptionally tiny genomes. Once they get inside insect cells, they become isolated from other bacteria and restricted to small populations. This deprives them of opportunities for shuffling or acquiring genes, and allows harmful mutations to build up in their DNA. One by one, their genes break and disappear, leaving them with shrivelled minimalist genomes.

McCutcheon is one of a small cadre of scientists who, over the past 15 years, have deciphered the weird genomes of many insect symbionts. When he started his own lab at the University of Montana, he decided to look at Magicicada tredicim—one of the periodical 13-year species. He did the usual thing: he dissected out the organs where the bacteria live, pulled out their DNA, cut it into fragments, sequenced the pieces, and used a computer to assemble those portions into a coherent whole.

Except, it didn’t work. The sequences just wouldn’t assemble neatly. It was as if someone had taken several similar but incomplete jigsaw puzzles, and jumbled all the pieces together. “It was just such a mess,” he says. “I thought it was something technically wrong but I couldn’t figure out what.”

Perplexed, he moved on to a different cicada—a South American species called Tettigades undata. There, he found yet more weirdness. It contained a bacterium called Hodgkinia, which had somehow split into two distinct species inside its insect host. As I wrote last year, these daughter species are like two halves of their ancestor. They’ve each lost different genes so that individually, each is a pale shadow of the original Hodgkinia, but collectively, they complement each other perfectly.

When McCutcheon worked out what was going on in T.undata, he suddenly realised what was happening in the 13-year cicada. It also contained Hodgkinia symbionts that had split into separate lineages—and not just two.

Graduate students Matthew Campbell and James Van Leuven eventually showed that the DNA from this cicada’s symbionts form at least 17 distinct circles. It’s not clear if each of these represents a Hodgkinia chromosome, or an entire Hodgkinia genome on its own, but at least four of them are found in distinct cells You can see this in the images below, where the blue, green, purple, and orange dots all represent cells that have just one of the 17 circles.

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Hodgkinia cells in cicada tissue. Credit: Campbell et al, 2015. PNAS.

As in the earlier discovery, these circles complement each other; they share sets of genes for making nutrients that matter to the host, but none of them has the full complement. They’re also found in other species of periodical cicadas. And they might just be the tip of the iceberg: the team could confidently identify 17 circles, but the insects likely harbour many more. “If I had to guess, I’d say there’s between 20 and 50,” says McCutcheon. “It’s incredible. It’s a mess.”

From Hodgkinia’s point of view, one lineage has clearly split into several, and irreversibly so. “That’s the baseline definition of speciation,” says McCutcheon. “It’s happening in an asexual population, but the lineage has fractured and it’s not going back.” But if you take the cicada’s perspective, the collective symbionts are still doing the same thing as the original. And while they parcelled their genes into separate cells, the total amount of bacterial DNA has increased. Each part became smaller, but collectively, their genome got bigger.

So are the Hodgkinia circles different species or lineages? Is the Hodgkinia genome the total of the circles in a single cicada, or does each distinct lineage have its own genome? It’s really hard to say. “The problem is that when we write a paper, we have to use words, and words mean something,” says McCutcheon. “It is very hard to put labels on this stuff, and I will not just give this a new name willy-nilly, because I don’t think we understand it well enough.”

There are other mysteries too. The cicada also has another bacterial symbiont called Sulcia, which shows no sign of this ridiculous fragmentation. There’s just one Sulcia and it’s the same in all cicada cells. Why has this microbe stayed whole, while its neighbour rent itself asunder? No one knows. A reasonable guess is that Hodgkinia evolves much faster than Sulcia, and more quickly builds up mutations that disable its genes.

Also, why has Hodgkinia fractured into many lineages within cicadas, when other insect symbionts have not in their respective hosts? McCutcheon thinks the answer lies in the insects’ long lives. Most sap-sucking bugs are lucky to make it past their first birthday. They lead short, fast lives, and if their symbionts developed detrimental mutations, they and their hosts would be weeded out by natural selection. Cicadas, by contrast, can live for 2 to 19 years, and for most of that time, they’re barely moving or growing. During those slow years, their symbionts aren’t that important, and are free to build up detrimental mutations without affecting their hosts or falling foul of natural selection—at least, not in the short term.

The long-term outlook may not be that rosy. Partnerships with microbes often furnish animals with incredible and valuable skills—in this case, the ability to drink plant sap without becoming deficient in important nutrients. But with great opportunity comes great risk. Once host and bacterium become dependent on each other, they can enter into a kind of symbiotic trap—or, as Nancy Moran puts it, they could jointly “spiral down the symbiosis rabbit hole”.

Take Hodgkinia. If it continues to fragment and degenerate, it—they?—may eventually be unable to sustain the cicada. “It just looks like it’s going off the rails,” says McCutcheon. “It’s like watching a train wreck or a slow-motion extinction event. It makes me think differently about symbiosis.”

Reference: Campbell, Van Leuven, Meister, Carey, Simon, and McCutcheon. 2015. Genome expansion via lineage splitting and genome reduction in the cicada endosymbiont Hodgkinia. PNAS http://dx.doi.org/10.1073/pnas.1421386112