Alkaloid Parsimony

Ecological theory would predict that the natural products produced by plants and animals would be multifunctional so as to be maximally adaptive for the producers. This phenomenon, described as alkaloid parsimony, has been detected with some frequency in spite of the fact that more appropriate in vivo ecological studies utilizing realistic bioassays are still required.

At least 50 alkaloids have been demonstrated to inhibit the germination or the growth of seedlings. This phenomenon, referred to as allelopathy, is important in competition between plants and is frequently directed against seedlings by mature plants of the same species.37 A diversity of alkaloids, including quinine,

cinchonine, ergotamine, harmaline, strychnine, berberine, colchicine, morphine, cocaine, caffeine, coniine, and nicotine, have been demonstrated to possess allelopathic activity, but many of the compounds possess additional ecological roles. In essence, these usually bitter and toxic alkaloids are also herbivore deterrents, and in many cases they possess antibacterial and antifungal activities.3 These examples of alkaloid parsimony are illustrative of the mutifunctionality of these compounds and serve to focus on the great adaptiveness of these nitrogen heterocycles.

Alkaloid parsimony has also been detected in animals that produce alkaloids. Fire ants in the genus Solenopsis produce venoms dominated by novel 2,6-dialkylpiperidines that are delivered into vertebrates subcutaneously. The venoms constitute outstanding examples of alkaloid parsimony as they provide their ant producers with a diversity of deterrents. The alkaloids cause dermal necrosis in humans and are very algogenic in addition to being very lytic. The piperidines also perturb enzymatic pathways and block neuromuscular junctions. Fire ants have obviously adapted their piperidines to function as extreme exanples of alkaloid parsimony.

Some low molecular-weight alkaloids subserve the role of pheromones but in addition they possess at least one other function of great ecological significance. Ant workers in a variety of genera synthesize trisubstituted alkylpyrazines in their mandibular glands and these compounds generally function as alarm or alerting pheromones.36 Workers of Odontomachus species frequently generate a very effective alarm signal with 2,5-dimethyl-3-isopentylpyrazine, but they also use this compound parsimoniously as an effective repellent against other ants in physical confrontations. It has also been suggested that these alkaloids may be utilized as antimicrobials in the moist environment of the nest, as is the case for the fire ant alkaloids.

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