Iwai, H., Mori, M., Tomita, M., Kono, N., & Arakawa, K. (2022). Molecular evidence of chemical disguise by the socially parasitic spiny ant Polyrhachis lamellidens (Hymenoptera: Formicidae) when invading a host colony. bioRxiv. https://doi.org/10.3389/fevo.2022.915517
Although ants can be regarded as unwanted pests that invade homes and businesses, they actually serve an important ecological purpose. They are instrumental in decaying wood into tiny pieces that will ultimately become part of the soil. Also, some feed on insects, including those that can be harmful forest pests. Carpenter ants belong to the genus Camponotus and chew through wood to create tunnels for a nest that will ultimately grow their colonies. The queen ant is the most important colony member, as she is the sole ant that lays all the eggs. Worker ants protect her and must continuously travel outside the colony to look for food. These hard-working ants must remain alert though, because a lazy parasitic queen ant might be waiting in the background, ready to take over, kill the reigning queen, and trick the workers of the original colony into working for her.
Off With Her Head!
How does this happen? It all begins with a parasitic queen ant of a different species that has just recently mated. She invades the host colony and kills the host queen. Afterwards, the impostor queen lays her eggs, and the deceived host workers raise her young. Eventually, the host workers will die, and the parasitic ants will completely take over the colony. The trick for the invading queen though, is to cleverly disguise herself. Workers are trained to attack any outsider, so she must not arouse the suspicion of the host workers. Ants recognize nestmates by their cuticle hydrocarbon profiles (CHCs). Hydrocarbons are long molecules composed only of hydrogen and carbon atoms. The outside of the ant (cuticle) is coated with these hydrocarbons, which serve as an important source of chemical signaling. If a parasitic queen ant marches in with a different CHC profile than that of the rest of the nest, workers will instantly recognize this and behave aggressively towards her. In order to successfully invade the colony and destroy the reigning queen, nonresident queens must find a way to exactly mimic the CHC profile of the host group.
This occurs via a process known as rubbing. In the rubbing process, the parasitic queen ant straddles a worker ant and rubs the body with her legs. This is all it takes to somehow acquire the host CHC profile. Up until now, little was known about the exact mechanism of CHC transfer: was it chemical camouflage or chemical mimicry? In the process of chemical camouflage, the parasite directly transfers the CHC profile from the cuticle of the host. Chemical mimicry, on the other hand, involves the parasite synthesizing the host CHC profile through its own biochemical pathways. Iwai et al. set out to clarify what exactly occurs during and after parasitic ant rubbing behavior.
The scientists used a carpenter ant species, Camponotus japonicus, as the host. The selected parasitic ant species was Polyrhachis lamellidens. First, individuals from both species of ants were collected from colonies in the wild and reared in a laboratory. Before they could go any further, there had to be a way to follow the CHC profile of the workers to determine if it was directly transferred to the P. lamellidens queens during rubbing. To accomplish this, the researchers constructed a special mix of hydrocarbons and applied this to the workers’ cuticle prior to any rubbing behavior. This mix of hydrocarbons contained carbon atoms that have just slightly different chemical properties; this allows the scientists to track or ‘label’ the carbon atoms and see if they are transferred. Then, newly mated P. lamellidens queens were introduced to C. japonicus workers. The queens were allowed to practice their rubbing behavior for 3 days. Afterwards, the queens were removed and introduced to a different set of C. japonicus workers. The scientists closely observed and recorded the behavior of the workers towards the disguised queen. They constructed four categories of behaviors: ignore, threaten, rush, and bite; the latter three were considered ‘aggressive.’ Workers accepted queen ants that had practiced rubbing behavior; no aggressive behavior was noted. The parasite queens clearly had the host ant CHCs, but had they synthesized them by themselves or had they stolen them?
Then, to determine if the CHC profile of the workers were directly transferred to the queens, they first extracted the hydrocarbons from the ants (both the hosts and the parasites). They vaporized the complex mix of hydrocarbons and used special machinery to analyze the constituents. The chemical components were separated based on properties such as boiling point and mass. Using the labeled carbon atoms, the scientists found that between 10-20% of the CHC profile of the queens contained the labeled carbon atoms that had been applied to the workers. Interestingly, the labeled CHC was also discovered in the postpharyngeal gland of the P. lamellidens queens. Perhaps most remarkable is that the queens retained the CHC disguise acquired form the host workers for at least 9 days, with no notable decrease measured.
The Easy Way
Finally, to confirm that chemical mimicry was not part of the ant’s disguise, expression levels of CHC-synthesis genes were studied. This was accomplished by first extracting RNA from both queen ants that had exhibited rubbing behavior and control queen ants that did not. RNA is ultimately used to make proteins, and in this case, the proteins of interest are the ones involved in synthesizing CHCs. In general, the amount of RNA at any given moment corresponds to the amount of proteins being synthesized. RNA comes from DNA, where genes are found. The researchers used a special process to construct DNA from the RNA they had extracted. The DNA was sequenced, and the expression levels of genes related to CHC-synthesis were compared. There was no significant difference in the expression levels of the ants that had engaged in rubbing behavior vs. those that did not. This confirms that the queen ants acquire their new CHC profile solely from the host; there is no biosynthesis of new hydrocarbons.
Carpenter ants play an important role in a forest ecosystem but are sometimes in danger of parasitic species. These parasites destroy an entire colony by engaging in chemical disguise. This study not only reveals a spectacular mechanism of deceit but demonstrates a variety of approaches that can be used to address a single question. The scientists who directed this study used a combination of behavioral studies, chemistry and genetic sequencing to arrive at a fascinating conclusion.