Sexual behaviour

Some insects ingest plant matter containing alkaloids and use alkaloids in its own metabolism. For example, Ithomiine butterflies utilize pyrrolizidine alkaloids. Moreover, it was reported even in 1970s that some butterflies (Danaus spp.) use pyrrolizidine alkaloids from Senecio spp. directly in their sexual life729. They sequester and accumulate alkaloids during their entire adulthood and use them as pheromones. The males of the species intake alkaloids and use them for this purpose. It is known that the females are attracted to the males by strong pheromones: danaidal, hydroxydanaidal and danaidon. These pheromones are postcursors of pyrrolizidine alkaloids. In practice, the male of Danaus plexippus and Danaus chrysippus invites the female to enter into sexual contact by flying near her body and spraying danaidal, hydroxydanaidal and danaidon. The stronger the pheromones are, the better the male's possibility of copulation. The pheromones prepare the female to accept the male's proposition. As a result, the female is in the best possible position for sexual contact and for the opening of the copulation canal. Without pyrrolizidine alkaloids, the sexual life of these butterflies and their eventual reproduction would be limited. There are at least 40 different known insect species that possess the ability to collect and accumulate plant toxins including alkaloids700 729 (Figure 99).

Moreover, it is generally known that in some species of insects the location of eggs by females on plants is bio-chemically determined by plant-insect interaction. Some alkaloids play a very important role in the interaction and reproduction of insects. A good example is the egg-laying behaviour of Lep-tinotarsa decemlineata on potato plants. Potato alkaloids influence the female Leptinotarsa decemlineta choice of egg placement on the plant. This behaviour is determined by the chemical signals emitted by the female's taste and olfactory sense receptors during feeding or coming into contact with the parts of the plant623. The sexual and reproductive behaviour of some insects is connected

Figure 99. The copulation of butterflies. (Photo: T. Aniszewski)

with the acceptance of the plant as a beneficial environment for life. Alkaloids represent one group of compounds that are used as indicators of this acceptance.

3. Feeding attraction and deterrence

It has been mentioned earlier in this chapter that alkaloids have no selective toxicity to invertebrates. Therefore, for many insects, these compounds are more attractive than toxins (Figure 100). The cases of butterfly or beetle behaviours mentioned above are also a very evident example of the importance of plants for invertebrates. In both cases, alkaloids as secondary compounds take part in the

attraction for feeding. Moreover, it is also known that Manduca sexta flourish on alkaloid-rich Nicotiana plants and bruchid beetle (Bruchidius villosus) on quinolizidine alkaloid-rich plants703 732. Such examples are a consequence of the herbivore model for choosing a specific plant diet. This model is based on the plant-herbivore interaction. The plant can either attract or deter the insect or its larvae. Alkaloids represent only one group of the secondary compounds through which plants interact with insects and other animals.

The feeding attraction and deterrence by alkaloids are determined by the historical processes of the cell and cellular mutations, which lead to the evolution of mutualisms in ecosystems. Plant-provided food is an important base for the mutual benefit of the species595. Insects are not only consumers of alkaloids, but they are also vehicles for the transmission of alkaloids. Aphids ingest nutrients through plant juice, taking alkaloids from one plant and distributing them to the environment at large. Therefore, aphids function as alkaloid vectors. Through mediating alkaloid distribution from plants to the wider environment, aphis and other such organisms contribute to the evolution of life and the proliferation of alkaloidal plants. This puts increased pressure on the populations of animals that prefer non-alkaloidal plants. Evolution, in this sense, has not only a historical lens but also a future-oriented perspective. Evolution and micro-evolution perpetually occur in biocoenosis. Alkaloids are one group of chemicals that have central importance for and in the evolution of life.

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