Antioxidant availability trades off with warning signals and toxin sequestration in the large milkweed bug (Oncopeltus fasciatus)

Abstract

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.

Differential accumulation of cardenolides from Asclepias curassavica by large milkweed bugs does not correspond to availability in seeds or biological activity on the bug Na+/K+-ATPase

“…Such research could also shed more light on whether there are costs associated with the biosynthesis of different cardenolides (e.g., those with extra glycosylation, or further modifications to the sugar moiety), and the cost-benefit of producing specific cardenolides in relation to differences in insect diversity, herbivore specialization, and the constraints on those herbivores. Although Agrawal et al (2022) found no significant effects of the most potent inhibitors on O. fasciatus growth and performance, we have recently shown that sequestration can be costly in terms of oxidative state (Blount et al, 2023;Heyworth et al, 2023) which is visible in the wing colors that milkweed herbivores use as anti-predator defenses. More tests on the costs of sequestration will also be important for understanding the evolutionary and ecological interactions between the plant and the herbivore.…”

Cardenolides in the defensive fluid of adult large milkweed bugs have differential potency on vertebrate and invertebrate predator Na + /K + -ATPases

“…The variability in the bug's sequestration behaviour in our study cannot alone be accounted for by the cardenolide content of the host plant because the bugs in our study were provided with an ad libitum supply of seeds of A. curassavica. This within population variation may reflect genetic differences in the individuals' capacity to sequester [54], differences in individual physiological state [55,56] or the absence of predation pressure and the relaxed selection in the laboratory [53]. The higher variance in the concentration of cardenolides in the defensive fluid could also be owing to the fact that we were only able to collect a fraction of the DLS content during manual stress.…”

“…The variance could also reflect sex differences in sequestration, but we found no differences between male and female cardenolide concentrations in the DLS fluid (see the electronic supplementary material, S6 and also Moore & Scudder [17]). We did not control for insects' reproductive stage, or age, which might contribute to variation [56]. Testing whether predators are able to detect the variation present in the chemical defence will be important for understanding if such variation is subject to differential selection and therefore how intraspecific variation in chemical defence concentration can be maintained [6].…”

“…[27]). For example, certain cardenolides can be detrimental to insect growth [72], as well as redox state [56]. Sequestration might also be constrained by transport and modification of specific compounds [48,73].…”

No-cost meals might not exist for insects feeding on toxic plants

“…Recently, a study showed a physiological association in D. plexippus between oxidative stress (measured by oxidative lipid damage), cardenolide sequestration, and warning colouration ( Blount et al, 2023 ). Interestingly, in Oncopeltus fasciatus , the amount of sequestered cardenolides depleted the redox state (measured by the total glutathione level) and antioxidant availability was traded off with brightness and chroma signals ( Heyworth et al, 2023 ). Although very few studies have supported the physiological cost of toxin sequestration in aposematic insects, there is still a lack of evidence that toxin sequestration, warning signals (pigments), and oxidative stress are interlinked.…”