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Passport for an interloper

How parasitic silverfish get the right “rubber” stamp

Munich, 12/01/2011

Ant colonies are very attractive targets for parasites and predators, as they offer the successful invader secure living quarters and an abundant source of food. To protect their nests from interlopers, ants use chemical odorants to distinguish nestmates from potential intruders. However, parasites can subvert this defense by using mimicry, i.e., by adopting and emitting the same signals. “How parasites actually acquire the relevant chemical compounds has been something of a mystery,” says Professor Volker Witte of the Department of Biology at LMU Munich. Together with his doctoral student, Christoph von Beeren, Witte has now shown that silverfish that form close associations with ants do not make the odor compounds themselves. Instead, they steal them directly from their hosts, in this case an Asian species of army ant. Simply by rubbing itself against the surface cuticle of colony members, the silverfish picks up the characteristic mixture of chemicals that identifies the bearer as an inhabitant. This enables the intruder to successfully infiltrate and become socially integrated into the nest. (BMC Ecology 1 Dec 2011)

To protect their colonies from invasion, social insects, such as ants, bees and wasps, must be able to recognize their nestmates and exclude predators and parasites. They do so by means of chemical communication, based on the perception of substances associated with the surface cuticle. Every member of an ant colony, for instance, possess a complex mixture of molecules – mainly hydrocarbons – on its body surface, which identifies each individual as a bona fide inhabitant of the nest. Nevertheless, parasitic species are able to gain entry to colonies and plunder their food supplies or kill and consume their young. Many of them use a strategy known as chemical mimicry by adopting the chemical identity of their hosts. How exactly they come by the correct odor signals has in most cases remained unclear. “In principle, they can either make the substances themselves or steal them from their hosts,” says Christoph von Beeren.

To distinguish between these two possibilities and identify the source of the camouflaging odors, the researchers marked worker ants in a colony of army ants with molecules that were labeled with deuterium. Deuterium is a heavy isotope of hydrogen which behaves like the more prevalent lighter form, but can be differentiated from it by mass spectrometry.  “We were able to detect the deuterium label on the silverfish. Since it can only have come from the labeled ants, the silverfish must have stolen surface compounds from their hosts,” von Beeren explains. The odor compounds are transferred by intensive physical contact. The silverfish simply rub against their hosts to acquire the chemical signature that identifies them as colony members. They do not seem to synthesize any cuticular signals of their own, as the overall concentration of surface substances dropped when they were kept in isolation.

Another issue that has been much debated is whether or not the acceptability of an intruder is a function of the degree of similarity between its body odors and those of the host.  The researchers have now confirmed that individuals whose chemical mimicry is deficient are more likely to be attacked than intruders that are perfectly masked. “Silverfish that had been removed from the nest and isolated until the concentration of surface substances dropped resulting in reduced chemical similarity to the host were attacked and often captured by the army ants,” von Beeren reports. This shows that, unless they come equipped with the correct combination of odors, silverfish have little chance of obtaining asylum among the ants. (göd/PH)

Acquisition of chemical recognition cues facilitates integration into ant societies.
C. von Beeren, S. Schulz, R. Hashim, V. Witte
BMC Ecology 1 Dec 2011

Prof. Dr. Volker Witte
Department of Biology II
Phone: +49 89 / 2180-74265

Christoph von Beeren
Department of Biology II
Phone: +49 89 / 2180-74209

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