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194 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Series 4, Volume 60, No. 10 acid polymers are made without the participation of ribosomes. Rather there is a protein template. The peptides thus produced sometimes function as antibiotics (bacitracin is a familiar example), as do peptides made using ribosomes and DNA. Peptides having biological activity are of some importance in marine environments. Janolusimide (Atlas 600), from the nudibranch Janolus (Photo 119), is one example (Sodano & Spinella, 1986). Like all tripeptides it was obtained by coupling three amino acids, in this case N-methylalanine, 4-amino-3-hydroxy-2-methylpentanoic acid, and a cyclic hydroxyamide. Part 2. Biotransformation Natural products chemists often describe the “total synthesis” of a secondary metabolite. In principle that means that they have made it in the laboratory, starting with the elements of which the molecule is composed. In practice they do not bother to carry out all those steps themselves, but begin at a somewhat later stage, as has been explained in an amusing paper by Cornforth (1983). Likewise, “de novo” biosynthesis of a secondary metabolite does not mean that the organism begins with elemental carbon, hydrogen, oxygen and the like, but with primary metabolites. What is called “biosynthesis” may involve taking molecules apart as well as putting them together, and changing their structure as well as adding or replacing components. “Biotransformation” means altering the molecule. It is primarily of interest when the properties of that molecule are changed in a way that affects the biology of the organism. There are various reasons why a metabolite might be transformed in this manner. For example, a more stable metabolite might be stored more easily than a less stable one. This is probably a minor advantage, at least relative to two other ones: detoxification and enhancement of the toxic effect. Detoxification. An animal that feeds upon another animal or a plant that contains a defensive metabolite obviously needs to avoid any damage that the metabolite might inflict upon it. There are various ways of accomplishing that. One of the most straightforward is selective feeding, mainly by avoiding the part of the food item in which the defensive metabolite is most concentrated. However, there are no documented examples of that for opisthobranchs. Once the food has been ingested, it may be possible to avoid absorbing the defensive metabolite from the digestive tract. If it does make its way into the body, it can be excreted via the kidney. Or the physiology of the animal may be such that the metabolite does little harm. The nudibranch Hypselodoris orsini, a pair of which are shown in Photo 95 (also called by its junior synonym H. coelestis, and often misidentified as Glossodoris tricolor), provides an example of such detoxification (Cimino, Fontana, Giménez, Marin, Mollo, Trivellone & Zubia, 1993). This Mediterranean sea slug occurs in association with a sponge referred to in the literature as Cacospongia mollior (actually C. scalaris and more recently called Scalarospongia scalaris) (Demospongiae: Dictyoceratida: Thorectidae), upon which it feeds. The sponge contains the sesterterpenoid scalaradial (Atlas 486), which has a pair of aldehyde groups that are responsible for its activity. It is modified by the slug in two steps. First, it is converted by selective reduction of the aldehyde at C-17 to deoxoscalarin (Atlas 483). Then the molecule is converted by oxidation to 6-keto-deoxoscalarin (Atlas 489), which is then concentrated in specialized organs (mantle dermal formations) at the surface of the animal’s body. The location of 6-keto-deoxoscalarin suggests it is deployed against predators. However, the structure of the molecule suggests that it is biologically inactive, and therefore it has been proposed that the mantle dermal formations are a kind of excretory organ (Avila & Durfort, 1996). This interpretation is questionable. The glands are well positioned for a defensive function, and getting rid of metabolites via the gut or kidney would be much easier. One might reply, perhaps, that they represent an evolutionarily modified form of glands that previously did deploy some kind of defensive metabolite.
CIMINO & GHISELIN: CHEMICAL DEFENSE AND EVOLUTION OF GASTROPODS 195 Enhancement of toxic effects. Provided that an animal that derives defensive metabolites from food can tolerate them, the metabolites can be used in that animal’s own defense, and there are various ways in which the defense can be rendered more effective. One way, already just alluded to, is positioning the metabolites in a part of the body where they will be delivered to the attacking predator with maximal effect and minimal damage. This often means moving the metabolites from the digestive tract to the skin. A second way is increasing the concentration of the metabolite. Third, the molecule can be modified so that it is more toxic (or more distasteful) to predators. A good example of this is the Mediterranean Oxynoe olivacea, and some other animals of the order Sacoglossa (Photo 27). They feed upon a green alga, Caulerpa prolifera, in which there is a toxic sesquiterpene, caulerpenyne, with a terminal 1,4-diacetoxybutadiene moiety (Atlas 206). The mollusks modify caulerpenyne to the more toxic oxytoxin 1 by selective hydrolysis of one of the two enol acetate moieties (Atlas 207). Then they modify it again to form the still more toxic oxytoxin 2(Atlas 208). In general it is not easy to provide compelling evidence for biotransformation. In spite of some experimental evidence that records the absence of caulerpenyne in Oxynoe olivacea, in which oxytoxin 1 is compartmentalized in the detachable tail, this biotransformation could be refuted. Some authors (Guerriero, Marchetti, D’Ambrosio, Sinesi, Dini & Pietra, 1993) have suggested that minor dietary metabolites are accumulated selectively, whereas others (Jung & Ponhert, 2001) have hypothesized the biotransformation of the major metabolite mediated by algal lipases during grazing by the slug on the algae. Recently it has been rigorously proved that the biotransformation can be attributed to hydrolytic lipases produced by the slugs, whereas the activity is not present in the alga (Cutignano, Notti, d’Ippolito, Domenèch Coll, Cimino & Fontana, 2004). Part 3. Deployment of metabolites In order to explain how opisthobranchs obtain, modify, and utilize secondary metabolites we herein provide some additional details about their basic anatomy. The ancestral gastropod is generally thought to have had a coiled shell and an asymmetrical, twisted body. The gill was located near the front of the animal, over the head, as was the anus. Accompanying the gradual reduction of the shell, the gill and anus have moved backward toward the rear of the body, straightening the animal out somewhat. When a snail gets transformed into a slug, the soft part of the body gets expanded relative to the shell, and the animal loses the ability to withdraw into the shell. At later stages the shell becomes lost or virtually lost, except that in larvae it generally is relatively large and still protects the animal. The muscular foot continues to serve as the main organ of locomotion and extensions of it called parapodia may expand and cover much of the body. The foot allows the animal to move, albeit slowly, from place to place. It also permits the animal to cling to the substrate while moving about or when apt to be dislodged by waves or currents. Such an arrangement is effective for an animal that grazes upon food items that live attached to the surface (sessile organisms) such as plants and sponges, or pursues sedentary or slow-moving prey such as clams and snails. Combined with this mode of locomotion is a feeding mechanism that allows snails and slugs to subdue and utilize food sources that have effective mechanical defense. Gastropods are generally equipped with a rasp-like structure called a “radula,” which may be compared to a tongue covered with teeth. They can grab or scrape with it, usually with the two actions working in tandem. Food is taken into the gut and passed to a stomach, where it usually gets broken down somewhat. Then particles of food are passed to a structure called a digestive gland where digestion occurs intracellularly—i. e., inside the cells that line the gland. Nineteenth-century zoologists called the digestive gland a “liver” but, because it is only superficially similar to that of vertebrates (i.e., the
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