Analysis of defensive secretion of a milkweed bug Lygaeus equestris by 1D GC-MS and GC×GC-MS: sex differences and host-plant effect

Abstract

The composition of defensive secretion produced by metathoracic scent glands was analysed in males and females of the milkweed bug Lygaeus equestris (Heteroptera) using gas chromatography with mass spectrometric detection (GC-MS). The bugs were raised either on cardenolide-containing Adonis vernalis or on control sunflower seeds in order to determine whether the possibility to sequester cardenolides from their host plants would affect the composition of defensive scent-gland secretion. Profiles of the composition of defensive secretions of males and females raised on sunflower were closely similar, with predominant presence of (E)-2-octenal, (E)-2-octen-1-ol, decanal and 3-octen-1-ol acetate. The secretion of bugs raised on A. vernalis was more sexually dimorphic, and some chemicals e.g. (E,E)-2,4-hexadienyl acetate and 2-phenylethyl acetate were dominant in males, but absent in females. Compared to bugs from sunflower, the scent-gland secretion of bugs raised on A. vernalis was characterized by lower overall intensity of the peaks obtained for detected chemicals and by absence of some chemicals that have supposedly antipredatory function ((E)-2-hexenal, (E)-4-oxo-hex-2-enal, 2,4-octadienal). The results suggest that there might be a trade-off between the sequestration of defensive chemicals from host plants and their synthesis in metathoracic scent-glands. Chemical defense belongs to the most widespread antipredatory strategies 1,2 and particularly insects employ a diverse arsenal of chemicals to defend themselves against their antagonists 3-5. Among insects, the true bugs (Hemiptera: Heteroptera) are characterized by the ancestral and almost universal presence of chemical defenses, that are highly diverse across various true bug taxa and that are frequently associated with aposematic coloration and mimicry 6,7. Chemical defense of adult true bugs is based on metathoracic scent gland secretions that are released upon attack of a predator 8. The scent-gland secretion of true bugs is a complex mixture of compounds, some of which could be toxic or irritating to predators while others may act as chemical warning signals perceived either by olfaction (highly volatile compounds) or by taste or chemesthesis 6,9,10. Very common compounds of defensive secretion of various true bug taxa include short-chain aldehydes, oxo-aldehydes, ketones, alcohols, organic acids, esters, etc 6,11,12. Nevertheless, the composition of the scent-gland secretion is taxon-specific and sexually dimorphic 13 , and also differs between adults and larvae, which possess a different scent-gland system than the adults (dorsoabdominal scent-glands) 14. Moreover, the relative content of individual compounds may depend on the season, physiological state and diet 15. Sampling of volatile secretions of true bugs is a crucial step of analyses. Majority of published methods require killing the insects (for dissection and extraction of sufficient quantities of secretion) 14,16-19. Only few studies have been focused on non-lethal sampling, but these procedu...

Dietary cardenolides enhance growth and change the direction of the fecundity‐longevity trade‐off in milkweed bugs (Heteroptera: Lygaeinae)

“…Since endogenous and sequestered defenses may trade‐off (Engler‐Chaouat & Gilbert, 2007 ), the lack of cardenolides for sequestration could lead to a higher investment towards endogenous defenses (i.e., defensive secretions) and therefore impair growth on the cardenolide‐free control diet. Although such a trade‐off has been suggested to occur in the milkweed bug Lygaeus equestris (Havlikova et al, 2020 ), metathoracic scent glands were shown to be reduced in O . fasciatus and other milkweed bugs (Aldrich, 1988 ; Schaefer, 1972 ) making this explanation rather unlikely.…”

HS–SPME–GC–MS Untargeted Analysis of Normal Rat Organs Ex Vivo: Differential VOC Discrimination and Fingerprint VOC Identification

“…In the Parkinson’s disease rat model experiment, it was also found that the content of 1-octen-3-ol in the blood increased significantly, at the same time, decreasing in striatum homogenate . ( Z )-2-Octen-1-ol is a pheromone associated with the adult male locust, Schistocerca americana, and isomer ( E )-2-octen-1-ol, in addition to being a sexually attractive pheromone substance, is also a major component of insect defensive secretions …”

“…22 (Z)-2-Octen-1-ol is a pheromone associated with the adult male locust, Schistocerca americana, 23 and isomer (E)-2-octen-1-ol, in addition to being a sexually attractive pheromone substance, is also a major component of insect defensive secretions. 24 1-Octen-3-ol and (Z)-2-octen-1-ol were detected in linoleic acid side chain and arachidonic acid side chain-oxidized phosphatidylcholine standards, 15 respectively, and 1-octen-3one was present in both standards. It indicated that these three VOCs belong to C8 chain substances produced by the oxidation of C18−C20 unsaturated fatty acids, but these three VOCs have not been reported in human studies so far.…”

Phenotypic plasticity in chemical defence of butterflies allows usage of diverse host plants

“…This has contributed to the hypothesis that diet specialization would often lead to an evolutionary and ecological ‘dead end’. With the advances of analytical chemistry and metabolomic approaches, we are now seeing that many insects can biosynthesize specialized metabolites [28,29], modify plant-acquired compounds [35] and even recycle them [27]. Our findings highlight that biochemical plasticity is not only possible, it may be more prevalent than currently assumed, and it may have far-reaching consequences for diet breadth, ecological niche partitioning and speciation.…”

“…For example: Chrysomela lapponica larvae (Coleoptera) increase 40 fold synthesis of defensive esters when effective sequestration of salicylic glycoside is not possible [30]. When raised on milkweed, Lygaeus equestris (Heteroptera) sequester cardenolides and reduce biosynthesis of volatile defences in their scent-gland in comparison to bugs fed sunflower seeds (no cardenolides) [31]. Even though in these examples autogenous and sequestered defence compounds belong to completely divergent chemical classes and are likely under different selection forces, there is still a trade-off between biosynthesis and sequestration.…”