1

696 for nicotine production, the nicotine has a relatively short half-life, 40% of it being converted to other metabolites (possibly sugars, amino acids, and organic acids) within 10 hours.

Thus, animals adapted to feeding on plants that produce toxins will be at a considerable advantage over animals that are not. Among herbivores, insects show the greatest ability to cope with the toxins. In part, this arises from the enormous period of time over which coevolution of insects and plants has occurred, but it is also related to insects' high reproductive rate and short generation time, which facilitate rapid adaptation to changes in the host plant. Through evolution, many insect species have not only developed increasing tolerance to a host plant's toxins but are now attracted by them. In other words, such insects locate food plants by the scent or taste of their toxic substance and frequently are restricted to feeding on such plants. For example, certain flea beetles, Phyllotreta spp., and cabbage worms, Pieris spp., feed exclusively on plants such as Cruciferae that produce glucosinolates (mustard oil). Colorado potato beetles, Leptinotarsa decemlineata, and various hornworms, Manduca spp., feed only on Solanaceae, the family that includes potato (Solanum tuberosum) (produces solanine), tobacco (Nicotiana spp.) (nicotine), and deadly nightshade (Atropa belladonna) (atropine) (Price, 1997).

The method most often used to overcome the potentially harmful effects of these chemicals is to convert them into non-toxic or less toxic products. Especially important in such conversions is a group of enzymes known as mixed-function oxidases (polysubstrate monooxygenases), which, as their name indicates, catalyze a variety of oxidation reactions (Schoonhoven etal., 1998). The enzymes are locatedinthemicrosome fraction* of cells and occur in particularly high concentrations in fat body and midgut. Perhaps unsurprisingly, it is these same enzymes that are often responsible for the resistance of insects to synthetic insecticides (Chapter 16, Section 5.5.).

Some insects are able to feed on potentially dangerous plants as a result of either temporal or spatial avoidance of the toxic materials. For example, the life history of the winter moth, Operophtera brumata, is such that the caterpillars hatch in the early spring and feed on young leaves of oak (Quercus spp.), which have only low concentrations of tannins, molecules that complex with proteins to reduce their digestibility. Though weather conditions are suitable and food is still apparently plentiful later in the season, a second generation of winter moths does not develop because by this time large quantities of tannins are present in the leaves. Spatial avoidance is possible for many Hemiptera whose delicate suctorial mouthparts can bypass localized concentrations of toxin in the host plant. Some aphids feed on senescent foliage where the concentration of toxin is less than that of younger, metabolically active tissue (Price, 1997).

Price (1997) proposed that at least four advantages may accrue to an insect able to feed on potentially toxic plants. First, competition with other herbivores for food will be much reduced. Second, the food plant can be located easily. Related to this, as members of a species will tend to aggregate on or near the food plant, the chances of finding a mate will be increased. Third, if an insect is able to store the ingested toxin within its tissues, it may gain protection from would-be predators. Many examples of this ability are known, especially among Lepidoptera (Blum, 1981; Nishida, 2002). Thus, most of the insect fauna associated with milkweeds are able to store the cardenolides produced by these plants. These substances, at sublethal levels, induce vomiting in vertebrates. Other well-known examples are pyrrolizidine alkaloids sequestered by arctiid moths, and cucurbitacins

* The microsome fraction is obtained by differential high-speed centrifugation ofhomogenized cells and consists of fragmented membranes of endoplasmic reticulum, ribonucleoproteins, and vesicles.

Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

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