Institut für Phytomedizin

Permanent URI for this collectionhttps://hohpublica.uni-hohenheim.de/handle/123456789/14

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Antioxidant availability trades off with warning signals and toxin sequestration in the large milkweed bug (Oncopeltus fasciatus)
(2023) Heyworth, H. Cecilia; Pokharel, Prayan; Blount, Jonathan D.; Mitchell, Christopher; Petschenka, Georg; Rowland, Hannah M.
In some aposematic species the conspicuousness of an individual's warning signal and the concentration of its chemical defense are positively correlated. Several mechanisms have been proposed to explain this phenomenon, including resource allocation trade‐offs where the same limiting resource is needed to produce both the warning signal and chemical defense. Here, the large milkweed bug (Oncopeltus fasciatus: Heteroptera, Lygaeinae) was used to test whether allocation of antioxidants, that can impart color, trade against their availability to prevent self‐damage caused by toxin sequestration. We investigated if (i) the sequestration of cardenolides is associated with costs in the form of changes in oxidative state; and (ii) oxidative state can affect the capacity of individuals to produce warning signals. We reared milkweed bugs on artificial diets with increasing quantities of cardenolides and examined how this affected signal quality (brightness and chroma) across different instars. We then related the expression of warning colors to the quantity of sequestered cardenolides and indicators of oxidative state—oxidative lipid damage (malondialdehyde), and two antioxidants: total superoxide dismutase and total glutathione. Bugs that sequestered more cardenolides had significantly lower levels of the antioxidant glutathione, and bugs with less total glutathione had less luminant orange warning signals and reduced chroma of their black patches compared to bugs with more glutathione. Bugs that sequestered more cardenolides also had reduced red–green chroma of their black patches that was unrelated to oxidative state. Our results give tentative support for a physiological cost of sequestration in milkweed bugs and a mechanistic link between antioxidant availability, sequestration, and warning signals.
New species based on the biological species concept within the complex of Lariophagus distinguendus (Hymenoptera, Chalcidoidea, Pteromalidae), a parasitoid of household pests
(2023) Pollmann, Marie; Kuhn, Denise; König, Christian; Homolka, Irmela; Paschke, Sina; Reinisch, Ronja; Schmidt, Anna; Schwabe, Noa; Weber, Justus; Gottlieb, Yuval; Steidle, Johannes Luitpold Maria
The pteromalid parasitoid Lariophagus distinguendus (Foerster) belongs to the Hymenoptera, a megadiverse insect order with high cryptic diversity. It attacks stored product pest beetles in human storage facilities. Recently, it has been shown to consist of two separate species. To further study its cryptic diversity, strains were collected to compare their relatedness using barcoding and nuclear genes. Nuclear genes identified two clusters which agree with the known two species, whereas the barcode fragment determined an additional third Clade. Total reproductive isolation (RI) according to the biological species concept (BSC) was investigated in crossing experiments within and between clusters using representative strains. Sexual isolation exists between all studied pairs, increasing from slight to strong with genetic distance. Postzygotic barriers mostly affected hybrid males, pointing to Haldane's rule. Hybrid females were only affected by unidirectional Spiroplasma‐induced cytoplasmic incompatibility and behavioural sterility, each in one specific strain combination. RI was virtually absent between strains separated by up to 2.8% COI difference, but strong or complete in three pairs from one Clade each, separated by at least 7.2%. Apparently, each of these clusters represents one separate species according to the BSC, highlighting cryptic diversity in direct vicinity to humans. In addition, these results challenge the recent ‘turbo‐taxonomy’ practice of using 2% COI differences to delimitate species, especially within parasitic Hymenoptera. The gradual increase in number and strength of reproductive barriers between strains with increasing genetic distance also sheds light on the emergence of barriers during the speciation process in L. distinguendus.
A simple artificial diet for feeding and sequestration assays for the milkweed bugs Oncopeltus fasciatus and Spilostethus saxatilis
(2023) Espinosa del Alba, Laura; Petschenka, Georg
Insect artificial diets are not only an important tool for mass rearing, nutritional research, and maintaining laboratory colonies but also for studying insect‐plant interactions. For herbivorous insects able to sequester plant toxins, feeding and sequestration assays based on artificial diet allow for the investigation of physiological, ecological, and evolutionary questions which may be difficult to study using real plants representing complex chemical environments. We developed a simple artificial diet, consisting of sunflower meal pressed into pills, for the milkweed bugs Oncopeltus fasciatus (Dallas) and Spilostethus saxatilis (Scopoli) (Heteroptera: Lygaeidae), which are capable of sequestering cardenolides and colchicum alkaloids, respectively. We assessed insect performance, suitability of the diet for sequestration assays, and its shelf life. Compared to sunflower seeds which are widely used as a laboratory maintenance diet for milkweed bugs, no differences were found in terms of weight development, presence of deformities, speed of development, or mortality. Importantly, after feeding O. fasciatus and S. saxatilis sunflower pills enriched with crystalline ouabain (cardenolide) or colchicine (colchicum alkaloid), respectively, sequestration was observed in both species. Moreover, as a prerequisite to test ecological hypotheses, our method allows for adequate concentration control and homogenous distribution of toxins across the diet. Under relatively warm conditions (27 °C and 60% r.h.), the new diet was stable for up to 10 days when used for feeding assays with adult bugs. Therefore, studies focusing on the role of plant toxins in predator–prey interactions and plant defense, but also insecticide research could benefit from using this approach.
Spatial metabolomics reveal divergent cardenolide processing in the monarch (Danaus plexippus) and the common crow butterfly (Euploea core)
(2023) Dreisbach, Domenic; Bhandari, Dhaka R.; Betz, Anja; Tenbusch, Linda; Vilcinskas, Andreas; Spengler, Bernhard; Petschenka, Georg
Although being famous for sequestering milkweed cardenolides, the mechanism of sequestration and where cardenolides are localized in caterpillars of the monarch butterfly (Danaus plexippus, Lepidoptera: Danaini) is still unknown. While monarchs tolerate cardenolides by a resistant Na+/K+‐ATPase, it is unclear how closely related species such as the nonsequestering common crow butterfly (Euploea core, Lepidoptera: Danaini) cope with these toxins. Using novel atmospheric‐pressure scanning microprobe matrix‐assisted laser/desorption ionization mass spectrometry imaging, we compared the distribution of cardenolides in caterpillars of D. plexippus and E. core. Specifically, we tested at which physiological scale quantitative differences between both species are mediated and how cardenolides distribute across body tissues. Whereas D. plexippus sequestered most cardenolides from milkweed (Asclepias curassavica), no cardenolides were found in the tissues of E. core. Remarkably, quantitative differences already manifest in the gut lumen: while monarchs retain and accumulate cardenolides above plant concentrations, the toxins are degraded in the gut lumen of crows. We visualized cardenolide transport over the monarch midgut epithelium and identified integument cells as the final site of storage where defences might be perceived by predators. Our study provides molecular insight into cardenolide sequestration and highlights the great potential of mass spectrometry imaging for understanding the kinetics of multiple compounds including endogenous metabolites, plant toxins, or insecticides in insects.

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