Defense by Microbial Symbionts

Besides the own production of toxins or antifeedants for defense, it turns out that many organisms also rely on the help of microbial (endo)symbionts (Figure 4). This explains, at least in part, why very different organisms use similar toxins for their defense. For example, tetro-dotoxin 16 has been isolated not only from pufferfish (Tetraodontidae) but also from other fish (Pomacanthus semicirculatus and Scarus gibbus), marine snails (Naticidae), crabs (Atergatis sp.), and the octopus Hapalochlaena macu-losa. Interestingly, tetrodotoxin 16 is also the toxin of amphibians such as newts (Taricha spp. and Triturus spp.), the frog Colosthetus inguinalis, and toads (Atelopus spp.). This broad occurrence in diverse animals from different habitats led to the hypothesis that tetrodotoxin

16 is produced by symbiotic microorganisms. The hypothesis was further supported by the observation that frogs (Colosthetus inguinalis) kept in zoos do not contain tetrodotoxin 16 any more, suggesting that under natural conditions the toxin-producing microorganism is taken up from their environment. Meanwhile several microorganisms have been isolated from marine sediments as well as a Pseudomonas sp. from the skin of pufferfish that are able to synthesize tetrodotoxin (16). Also, cyanobacteria that live in association with other marine organisms are suspected to be the producers of many complex natural products, for example, saxitoxin

17 isolated from macroorganisms. Likewise, many marine polyketides such as palytoxin (18; see Defense Strategies of Marine and Aquatic Organisms) are expected to be of microbial origin but so far it was not possible to identify

Figure 3 Interference with microbial signaling compounds (quorum quenching).

Bryostatin 20 20

Figure 4 Examples of defense compounds produced or likely to be produced by symbiotic microorganisms.

Bryostatin 20 20

Figure 4 Examples of defense compounds produced or likely to be produced by symbiotic microorganisms.

the microorganism responsible for its production. The tendency of marine organisms to be protected by highly complex molecules - often of polyketide origin - fuels the suspicion that many other defensive compounds may be of microbial origin. The associations between microbes and marine macroorganisms are assumed to be ancient because the host organisms need to have evolved resistance against the toxins from their symbionts before using them for protection.

In the symbiotic association of the squid Euprymna scolopes with Vibrio fischeri bacteria, the microorganisms play a pivotal role in the defense of their hosts. Squids make use of their light organs to blur recognition by optical illusion pretending to be in another position in the water than where they really are, which supports their escape. In the light organ of the squids, it is the Vibrio fischeri bacteria that are responsible for the light emission.

It is usually very difficult to prove the symbiotic contribution because the producing microorganisms cannot be cultivated separately from their host. In the case of the polyketide toxin pederin (19; see Animal Defense Strategies) occurring in Pederus beetles, sequencing of the gene cluster responsible for the pederin biosynthesis revealed that pederin is generated by a so-far noncultivable microorganism. Interestingly, pederin-related compounds occur in marine sponges also being produced by endosymbionts.

From the marine environment bryostatins such as 20 have been characterized as defense compounds of the bryozoan Bugula neritina. In this case, it was possible to isolate the symbiotic bacterium Endobugula sertula and to prove that indeed it produces bryostatins, for two bryozoan.

Also, tyrosol 21 and isatin 22, which provide the shrimp Palaemon macrodactylus and the lobster Homarus americana protection against fungal infection by the pathogen Lagenidium callinectes, are generated by bacteria that grow in biofilms on developing embryos of the shrimp and lobster.

Interestingly, sometimes defensive compounds are even produced by both the host organism and its endophyte. For example, the complexly functionalized diterpenoid taxol 23 is produced by both the yew tree (Taxus baccata) and endophytic fungi, for example, Taxomyces andreanae, posing the fascinating question whether and how the whole set of genes necessary for the biosynthesis of taxol has been exchanged between the plant and the fungus.

Another example for the intricate interactions between micro- and macroorganisms is that of leaf-cutting ants (Acromyrmex) that rely on a fungus as food source. The ants obtain their food from the fungus which they feed in turn with leaves. The attine ants also indirectly defend their fungus from attack by pathogenic fungi (e.g., Escovopsis) because the ants carry filamentous microorganisms belonging to the genus Pseudonocardia on their body that produce an antifungal compound that is highly active against Escovopsis.

Symbiotic relationships are not limited to the production of toxins by associated microorganisms. As already mentioned during the discussion of induced defense mechanisms of plants, many plants emit a bouquet of volatiles that serves both to inform other plants about upcoming dangers and to attract predators such as parasitic wasps or predatory spider mites. In order to attract help from organisms of other trophic levels, some plants provide nectar to attract defenders (see Plant Defense Strategies).

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