Pyrrolizidine alkaloid sequestration

Pyrrolizidine alkaloid sequestration by insects is done for defense and mating purposes. Various species of insects have been known to utilize molecular compounds from plants for their own defense and even as their pheromones or precursors to their pheromones. A few Lepidoptera have been found to sequester chemicals from plants which they retain throughout their life and Arctiidae is no exception to this strategy. Starting in the mid-twentieth century researchers investigated various members of Arctiidae, and how these insects sequester pyrrolizidine alkaloids (PAs) during their life stages, and utilize these chemicals as adults for pheromones or pheromone precursors. PAs are also used by members of the Arctiidae for defense against predators throughout the life of the insect.

Pyrrolizidine alkaloids are a group of chemicals produced by plants as secondary metabolites, all of which contain a pyrrolizidine nucleus. This nucleus is made up of two pyrrole rings bonded by one carbon and one nitrogen. There are two forms in which PAs can exist and will readily interchange between: a pro-toxic free base form, also called a tertiary amine, or in a non-toxic form of N-oxide.

Researchers have collected data that strongly suggests that PAs can be registered by taste receptors of predators, acting as a deterrent from being ingested. Taste receptors are also used by the various moth species that sequester PAs, which often stimulates them to feed. As of 2005, all of the PA sequestering insects that have been studied have all evolved a system to keep concentrations of the PA pro-toxic form low within the insect’s tissues.

Researchers have found a number of Arctiidae that utilize PAs for protection and for male pheromones or precursors of the male pheromones, and some studies have found evidence suggesting PAs have behavioral and developmental effects. Estigmene acrea,Cosmosoma myrodora,Utetheisa ornatrix,Creatonotos gangis and Creatonotos transiens are all members of the family Arctiidae and found to use PAs for their defense and/or male pheromones. Parsimony suggests that the sequestering of PAs in the larval stage evolved in the subfamily Arctiinae common ancestor. The loss of ability to sequester and utilize PAs has occurred in a number of species, along with the switch from larval uptake to adult uptake of PAs occurring multiple times within the Arctiinae taxon.

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