Sequestration and biosynthesis of cyanogenic glucosides in passion vine butterflies and consequences for the diversification of their host plants

Abstract

The colorful heliconiine butterflies are distasteful to predators due to their content of defense compounds called cyanogenic glucosides (CNglcs), which they biosynthesize from aliphatic amino acids. Heliconiine larvae feed exclusively on Passiflora plants where ~30 kinds of CNglcs have been reported. Among them, some CNglcs derived from cyclopentenyl glycine can be sequestered by some Heliconius species. In order to understand the evolution of biosynthesis and sequestration of CNglcs in these butterflies and its consequences for their arms race with Passiflora plants, we analyzed the CNglc distribution in selected heliconiine and Passiflora species. Sequestration of cyclopentenyl CNglcs is not an exclusive trait of Heliconius, since these compounds were present in other heliconiines such as Philaethria, Dryas and Agraulis, and in more distantly related genera Cethosia and Euptoieta . Thus, it is likely that the ability to sequester cyclopentenyl CNglcs arose in an ancestor of the Heliconiinae subfamily. Biosynthesis of aliphatic CNglcs is widespread in these butterflies, although some species from the sara‐sapho group seem to have lost this ability. The CNglc distribution within Passiflora suggests that they might have diversified their cyanogenic profile to escape heliconiine herbivory. This systematic analysis improves our understanding on the evolution of cyanogenesis in the heliconiine– Passiflora system.

Sequestration and functional diversification of cyanogenic glucosides in the life cycle of Heliconius melpomene

“…In this study, it was observed that larvae of H. melpomene can sequester not only cyclopentenyl CNglcs, but also aromatic and aliphatic CNglcs (Figure 2b and Table 1). Sequestration of sulphated cyclopentenyl CNglcs (tetraphyllin B-sulphate) from P. caerulea was not detected (Figure 2b), which suggested that addition of a sulphate group to CNglcs in Passiflora could be a recent modification to avoid sequestration by heliconiines, as previously hypothesized (de Castro et al, 2019). Larvae did not sequester prunasin from P. edulis and amygdalin from P. platyloba (Figure 2b), even though we demonstrated that they have the machinery to sequester these compounds when they are painted on leaves in high concentration.…”

“…Plants from the Passiflora genus also produce CNglcs, and it has previously been demonstrated that CNglcs biosynthesized from cyclopentenyl glycine by Passiflora can be sequestered by Heliconius (Engler et al, 2000;Engler-Chaouat and Gilbert, 2007). Some of these cyclopentenyl CNglcs, most likely remnants from larval feeding, were even found in adults of several Heliconius and heliconiine species, suggesting that these compounds are kept after pupation (de Castro et al 2019) (Figure 1). However, an enormous diversity of CNglc structures is present in the Passiflora genus, which raise the question whether Heliconius larvae could sequester other CNglcs apart from the ones with a cyclopentenoid structure.…”

“…Nevertheless, recently emerged Heliconius butterflies have overall similar total concentration of CNglcs to other heliconiines (de Castro et al, 2019), which suggests that Heliconius might become more toxic than other heliconiines as they become mature adults (3-4 weeks after eclosion). Corroborating this hypothesis, we found that all three copies of the putative genes involved in the biosynthesis of CNglc are expressed during adulthood ( Figure 4) and mature adults contain twice as much CNglc as virgin adults (Figure 3a).…”

“…Small amounts of radiolabelled linamarin and lotaustralin were even retrieved from some eggs produced by these females. However, sequestration of linamarin and lotaustralin is rare in heliconiine, and in general, they rely on obtaining aliphatic CNglcs via biosynthesis and not sequestration (Castro et al, 2019).…”

“…In some Passiflora species these cyclopentenyl CNglcs are modified with additional unusual sugars and sulphate groups, such as passibiflorin and tetraphyllin B, respectively ( Figure 1). The aliphatic CNglcs linamarin and lotaustralin are present in some Passiflora species (Spencer et al, 1986), whereas few others contain the aromatic CNglcs prunasin, amygdalin, and passiedulin, of which has only been reported in P. edulis (Christensen and Jaroszewski, 2001;de Castro et al, 2019). Spencer (1988) hypothesized that Passiflora plants diversified their CNglc profile in response to the herbivory of Heliconius and closely related genera.…”