Earth's biosphere is brimming with symbiotic relationships: from bacteria that became our cells' mitochondria, to mycorrhizal fungi that help plants grow, to the myriad mites, wasps, worms, and flies that make a living by parasitizing other animals.
Symbiosis often appears to be a one-way street: When it evolves, there is no going back as symbionts become irreversibly locked into relationships with their hosts or partner species. Such "obligate symbioses" are some of the most ancient ecological interactions, with examples dating back hundreds of millions of years, pillars around which the rest of the natural world has evolved.
New research at Caltech sheds light on why symbioses become irreversible but also why these lifestyles are not necessarily an evolutionary dead end.
The research was performed in the laboratory of Joe Parker , professor of biology and biological engineering and director of Caltech's Center for Evolutionary Science , and is described in two papers appearing recently in the journals Cell and Current Biology.
Parker's team has spent eight years studying ant colonies in the Angeles National Forest. This landscape is dominated by an ant species that is integral to the forest ecosystem: the velvety tree ant (Liometopum occidentale). These ants form colonies of millions of aggressive workers, but deep inside them lives a remarkable impostor beetle, named Sceptobius-a symbiotic insect found nowhere but these hostile nests.
Sceptobius is incapable of living outside of the ants' colonies, which are impenetrable fortresses to most other insects. The beetle has evolved a complex relationship with the ant where it is treated as a nestmate rather than attacked and is even fed mouth-to-mouth by the ants. It also feasts on the ants' eggs and larvae, living as a social parasite that exploits the colony, while providing little to nothing in return.
In a new study in the journal Cell, led by former graduate student Tom Naragon (PhD '25), Parker's team asked how Sceptobius can dupe the ants into believing it is one of them. They found that Sceptobius infiltrates colonies by developing an "invisibility cloak," switching off its own pheromones to become a stealth intruder. By doing so, its body becomes a blank canvas onto which it can "paint" the ant's own pheromones, stealing the colony's chemical identity to gain the ants' acceptance. Evolution of this ability has, however, come at a price, as the stealth strategy is likely evolutionarily irreversible.
The Beetle Invisibility Cloak: An Example of Symbiotic Irreversibility
Insects communicate through chemistry: On their body surface are cocktails of pheromones named cuticular hydrocarbons (CHCs). A CHC cocktail is unique to a given species, helping members of that species recognize each other. Ants have taken this further, with individual colonies having unique CHC cocktails. Workers in a nest sense CHCs on the bodies of other insects, checking if they are nestmate ants or intruders to be attacked. Naragon found that Sceptobius possesses the genetic machinery to produce its own CHCs. After the beetle emerges as a young adult, however, Naragon discovered that the genes encoding enzymes that produce CHCs become silenced, transforming the beetle into a stealth insect with no CHC scent. Under the ants' radar, the stealth beetle climbs onto an ant's body, using its brush-like feet (tarsi) to "groom" the ant, scooping up the signature CHCs, which it then smears over itself. Through this strategy, the beetle can assimilate, undetected, into the ants' social system.
While this may seem like an easy arrangement for Sceptobius, the strategy is irreversible. A consequence of the beetle's invisibility cloak is that, by outsourcing CHC production to the ant, Sceptobius has lost the ability to manufacture its own CHCs as a mature adult. CHCs, however, play a second essential role in insect biology: They create a waxy coating around the body, a barrier that stops insects losing water and drying out. Without ants to steal CHCs from, Sceptobius quickly desiccates and dies. The beetle can therefore never leave the nest.
Evolutionarily, the beetle is entrenched in a kind of catch-22. If mutations arose that enabled it to produce its own CHCs as a mature adult, it would be detected by the ants as foreign and killed. At the same time, if mutations arose that removed the beetle's attraction to ants, it would stop grooming ants and desiccate. Only if mutations arose simultaneously-removing both the CHC silencing mechanism and the ant attraction-could the beetle revert to being a free-living non-symbiotic species like its ancestors were. The probability of this happening is close to zero.
"Our study reveals how symbiosis can be irreversible through symbiotic traits that essentialize each other," says Parker. "For Sceptobius, interdependence between CHC silencing and ant grooming prevents either of these traits from reverting to their ancestral states. If this symbiont were to lose one of these traits, it would mean certain death by the other one. This creates an evolutionary barrier to living freely again."
According to Naragon, "Catch-22 scenarios are likely common in both microbial and multicellular obligate symbioses due to a simple fact: The surface of an organism is its boundary with the world; it maintains the integrity of everything inside the organism but also functions as an identifier for everyone outside the organism. Evolving intimate associations with your host can demand radical modifications to the chemical properties of your surface to avoid detection and death. These changes can compromise the integrity of that protective boundary, precluding survival away from the host context."
Symbionts Can be Forever
Sceptobius lives with a single Liometopum ant species. Locking yourself in for an evolutionary ride with just one partner might seem risky: If they go extinct, so do you. And yet the animal kingdom is packed with ancient symbiotic lineages. Never losing touch with your host is paramount for these organisms, and evidence from the field of neuroscience shows that symbionts are remarkably efficient at finding their hosts due to extreme attraction to host sensory information-chemical, visual, or tactile.
A paradox exists, however. Zoom out across evolutionary trees of symbionts, and one sees them switching to new hosts over evolutionary time. Host switching is important: It enables symbionts to cheat death if their original host goes extinct. Switching also enables the lineage to undergo speciation (one species splitting into two), generating biodiversity. How is a symbiont so seemingly attuned to one host able to shift to another?
In a second study appearing in the journal Current Biology, led by former graduate student Julian Wagner (PhD '24), the team asked what the future may hold for entrenched symbionts like Sceptobius. By performing behavioral experiments, they identified ant chemical cues that enable Sceptobius to find and recognize its host. The beetle is highly sensitive to ant CHCs, which trigger it to mount and groom the ant, stealing these compounds. The beetle also walks along ant chemical trails so that it never loses contact with ants for very long.
Wagner experimented by placing Sceptobius with other ant species that are not the beetle's natural Liometopum host. Usually, these killed Sceptobius-the CHCs on the beetle's body coming from Liometopum, hence identifying it as foreign. Curiously, if Wagner prevented these non-host ants from attacking Sceptobius, he saw the beetle mount and groom them, much like it does with Liometopum.
Even ants that split away from Liometopum 100 million years ago were accepted by Sceptobius, and through its grooming behavior it adopted their chemical profiles. "We were really surprised that a host-specific beetle is actually very promiscuous if you give it a chance," says Wagner. "Really any ant will do." If the beetle so easily switches hosts in lab, why hasn't this happened in nature?
The team collaborated with James Boedicker's group at USC to simulate how Sceptobius might move across a landscape populated by ants of the same or different species. Their computer model found that Sceptobiusbegins losing CHCs and dies rapidly from desiccation outside of nests unless it encounters an ant to groom. Any residual CHCs on its body will be those from its host ant, so if it encounters its typical host again, it can replenish its CHCs and survive. If, however, it encounters a different ant species, its CHCs won't match, leading to its likely demise in the jaws of the other ant species-most of the time.
Wagner then built a real-life version of the model, with living beetles and ants, and found that the predictions held up. The specificity of the otherwise promiscuous beetle to its natural host comes not from its own preference for that ant. Instead, specificity is enforced on the beetle because of its inability to travel long distances before it desiccates and aggression from non-host ants with different CHCs.
The team believe that evolving within the confines of Liometopum colonies means Sceptobius has needed only a crude ability to sense its host. Its nervous system is only coarsely tuned to detect certain LiometopumCHCs, and these CHCs are, in fact, shared by many other ant species. "Given the chance, Sceptobius can and will accept many possible ant species as hosts and, moreover, can socially integrate into their nests despite having never encountered them in its evolutionary history," Parker says.
Such promiscuity may be far more common. "Many symbionts are so closely associated with their partners, living in or on them for their whole lifecycle, that they will rarely, if ever, encounter alternatives," says Wagner. "They don't need a fine-tuned ability to tell their hosts apart from other species. Paradoxically, it's this closeness that predisposes them to switch hosts if the rare chance arises." Both the model, and Wagner's real-life version of it, found there is wiggle room for Sceptobius-conditions where the beetle can successfully switch to a different ant. For example, if the CHC levels on its body are very low, it is less detectable to ants. Even if those CHCs don't match, it has the chance to groom a new ant species and steal its identity just before desiccation kicks in.
"Host switches are rare events but ultimately inevitable given sufficient time," says Parker. "The reason we have big ancient radiations of obligate symbionts at all is probably because of these chance encounters."
A Natural Laboratory on Caltech's Doorstep
The two papers underscore the richness of Caltech's local ecology and the importance of curiosity-driven research-Caltech's true strength, according to Parker. As Wagner recounts: "We spent so much time out in the forest getting bitten by ants, watching and collecting beetles from nests, and chatting about science and life. The fact that we had no predefined goal for where our projects would lead was so critical; it allowed us to let the natural system do the talking. It was really such an honor to spend so much time with the insects and uncover their secrets."
"Sceptobius was just living up the street. It turned out to be an evolutionary gold mine," says Parker. "This work started with basic natural history of a tiny insect and ended up teaching us something universal about the biosphere."
The Cell paper is titled "Symbiotic entrenchment through ecological Catch-22." Naragon is the study's first author. In addition to Naragon, Parker, and Wagner, Caltech co-authors are graduate students Joani W. Viliunas, K. Esther Okamoto, and Hannah M. Ryon; former research associate Mina Yousefelahiyeh; visitors in biology and biological engineering Adrian Brückner and Sheila A. Kitchen; former Caltech undergraduate Danny Collinson (BS '24); former postdoctoral scholar Reto S. Wijker; and Alex L. Sessions, the Nico and Marilyn van Wingen Professor of Geobiology. Funding was provided by the National Science Foundation, the Army Research Office, the Alfred P. Sloan Foundation, a Pew Biomedical Scholarship, a Klingenstein-Simons Fellowship, the Shurl and Kay Curci Foundation, and the Okawa Foundation.
The Current Biology paper is titled "Enforced specificity of an entrenched symbiosis." Along with Wagner, Parker and Naragon, Caltech co-authors are Visitor in Biology and Biological Engineering Adrian Brückner. Additional co-authors are Jason H. Wong, Enes Haxhimali, and James Q. Boedicker of USC; and Jocelyn G. Millar of UC Riverside. Funding was provided by the Army Research Office, the National Science Foundation, the National Institutes of Health, the Alfred P. Sloan Foundation, a Pew Biomedical Scholarship, a Klingenstein-Simons Fellowship, the Shurl and Kay Curci Foundation, and the Okawa Foundation.
