Novel 3-Acetoxy Fatty Acid Isoprenyl Esters from Androconia of the Ithomiine Butterfly Ithomia salapia

Abstract

Male ithomiine butterflies (Nymphalidae: Danainae) have hairpencils on the forewings (i.e. androconia) that disseminate semiochemicals during courtship. While most ithomiines are known to contain derivatives of pyrrolizidine alkaloids, dihydropyrrolizines or g-lactones in these androconia, here we report on a new class of fatty acid esters identified in two subspecies, Ithomia salapia aquinia and I. s. derasa. The major components were identified as isoprenyl (3-methyl-3-butenyl) (Z)-3-acetyloxy-11-octadecenoate, isoprenyl (Z)-3-acetyloxy-13-octadecenoate (12) and isoprenyl 3-acetyloxyoctadecanoate (11) by GC/MS- and GC/IR-analyses, microderivatizations, and synthesis of representative compounds. The absolute configuration of 12 was determined to be R. The two subspecies differed not only in the composition of the ester bouquet, but also in the composition of more volatile androconial constituents. While some individuals of I. s. aquinia contained ithomiolide A, a PA derived g-lactone, I. s. derasa carried the sesquiterpene a-elemol in the androconia. These differences might be important for the reproductive isolation of the two subspecies, in line with previously reported low gene exchange between the two species in regions where they co-occur. Furthermore, the occurrence of positional isomers of unsaturated fatty acid derivatives indicates activity of two different desaturases within these butterflies, D9 and D11, which has not been reported before in male Lepidoptera.

First chromosome scale genomes of ithomiine butterflies (Nymphalidae: Ithomiini): Comparative models for mimicry genetic studies

“…Chemical signals such as sex pheromones have been suggested to a play a role in reproductive isolation in mimetic butterflies (González-Rojas et al, 2020;Darragh et al, 2020), particularly among co-mimetic species (Mérot, Frérot, Leppik, & Joron, 2015). In ithomiine butterflies sexual pheromones have long been studied (Schulz et al, 2004), and have been shown to diverge between closely related taxa (Mann et al, 2020;McClure et al, 2019a;Stamm, Mann, McClure, Elias, & Schulz, 2019), suggesting a possible role in reproductive isolation (McClure et al, 2019b). More broadly, butterflies are phytophagous during the larval stage, and hostplant adaptation, mediated by chemical communication, has been hypothesized to be a major driver of speciation (Ehrlich & Raven, 1964;Jousselin & Elias, 2019).…”