The Ecological Role of Alkaloids

Abstract Alkaloids are a special group of secondary compounds and are part of an organism s adaptation mechanism to its living environment. They are not toxic when stored, but become toxic as a result of cell pH change. The defensive function of alkaloids is only secondary, and connected to internal immune and regulation processes. Animal responses to alkaloids are very diverse. Some animals can tolerate alkaloids relatively well, while others are harmed or even poisoned by them. Animal behaviour in relation to alkaloids depends on evolutionary and co-evolutionary factors. Sequestration of alkaloids is connected with these processes. Alkaloids are a part of plant-derived nutrition. A selective toxicity of these compounds in vertebrates is clearly observed. Vertebrates have the capacity to recognize alkaloids. 

Alkaloids take part in the life processes of some invertebrates as pheromones, inducers of sexual behaviour, and in reproduction. A case study of quinolizidine alkaloids and population changes proved that these alkaloids occur in all legume species studied but not, however, in all individuals. The distribution and frequency changes of alkaloidal and non-alkaloidal plants in populations is a direct expression of natural selection natural hybridization and micro-evolution can be considered as an evidence of current evolutionary responses by ecological and genetic systems. 

Key words alkaloids, attraction, case smdy, deterrence, ecology, food, MEC, micro-evolution, pheromones, quinolizidine alkaloids, selective toxicity, sexual life 

From the beginnings of alkaloid research (from the discovering of morphine) to today, one of the most interesting questions has been and remains the function 

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Davis, A. R., Alkaloids and ascidian chemical defense evidence for the ecological role of natural products from Eudistoma olivaceum, Mar. Biol., Ill, 375, 1991. 

Thompson et al. have investigated the ecological role of the brominated isoxazoline alkaloids, aerothionin (Structure 2.84) and homoaerothionin (Structure 2.89), in Californian specimens of A. fistularis.134 139 The alkaloids caused behavioral modification in marine invertebrates, were toxic to dorid nudibranchs other than one specialized feeder, inhibited the settlement and/or metamorphosis of invertebrate larvae, and were strongly antimicrobial and cytotoxic. Sponges exuded significantly more of these two alkaloids when wounded,155 and, consistent with their defensive role, the metabolites were found to be localized in spherulous cells close to the aquiferous exhalant canals.156... 

In contrast with the seed caffeine of Coffea species, relatively little attention has been paid to that of tea. This is in part because the fruit of tea, including the seeds, is of minor economic importance compared with that of coffee moreover earlier studies revealed little caffeine in the tea seed (13.141. Recently we (H) found that the pericarp contains the greatest concentrations of alkaloids in the dry fruit of tea, and that appreciable amounts occur in the seeds, especially in the coats. Thus, from physiological and ecological viewpoints, our concerns are the roles of purine alkaloids and seed coats of tea during fruit development (seed formation) and seed germination. Caffeine in Coffea arabica seed is synthesized in the pericarp, transported to the seed, and accumulated there during fruit... 

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