A new perspective on co-evolution Plants are not only a source of food for insects, but also toxins that can provide protection against predators, which holds clear importance to their evolutionary prospects. By investigating the interactions between plants and insects, Dr Georg Petschenka and his colleagues at the University of Giessen aim to build a deeper understanding of their co-evolution. A wide variety of chemical compounds for defence have evolved within plants over time to ward off herbivorous parasites, yet these defences can almost invariably be overcome by adapted species of insects. Many insects also sequester these compounds, suggesting that they play a wider role. “This means that not only can these insects cope with these compounds, but they also store them in their bodies. A famous example is the monarch butterfly (Danaus plexippus) whose caterpillars acquire heart poisons from milkweed (Asclepias spp.), then the caterpillar and the resulting butterfly are protected against predators,” explains Dr Georg Petschenka. As an Emmy Noether Group Leader at the University of Giessen, Dr Petschenka aims to probe deeper into insect resistance to plant toxins, as well as other questions around the relationship between insects and plants, including the evolutionary implications. “We want to find out how insects overcome plant defence mechanisms and what this means for insect-plant co-evolution,” he outlines.
Co-evolution The concept of co-evolution between plants and insects, which is classically thought of as bi-trophic, is central to this work. In this concept, an insect occupying a novel dietary niche might evolve a resistance trait to overcome its new host’s defences, after which it can then thrive on the available dietary resources. “This is the classic assumption. However, some of the insects we’re studying - especially the milkweed bugs, the Lygaeinae - seem to interact with specific plants only because toxins are there,” says Dr Petschenka. While these insects may feed on many plants, they also interact with individual plant species because they have particular compounds which they can then use for their own defence. “This uptake of compounds for use as a defence mechanism is called sequestration. We’ve found that this is an evolutionary driver, that makes an insect stick to a specific plant species in some cases,” continues Dr Petschenka. Researchers in the group are also studying a number of other topics in this area as part of their work in looking at sequestration on a multi-layered scale. Different insects are being
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Many herbivorous insects use plants not only as a dietary resource, but also to sequester plant toxins as a defense against predators. Conceptually, this means that insects exploit plants in at least two ways, i.e. for nutrition and defense. While insects may gain food and toxins from the same plant in many cases, other species seem to use multiple plants for feeding but acquire toxins exclusively from specific plant species. In conclusion, sequestration of toxins for defense is an evolutionary driver selecting for specialized insect-plant interactions also in insects that are generalists from a nutritional perspective.
studied in the laboratory, with Dr Petschenka and his colleagues looking at how they perform on different diets. “We investigate whether the inclusion of what we think are evolutionarily novel sources providing toxins leads to dietary benefits. We study natural histories and do lots of analytical chemistry, to figure out what insects actually store from plants, and also work in the predator-prey sphere,” he explains. The aim here is to assess whether the sequestration of toxins provides protection against predators. “We work with lacewings as insect predators, and also have a collaborator experimenting with birds. We are trying to get the full picture of co-evolution on different hierarchical levels,” says Dr Petschenka. This research holds important implications with respect to the management of insect populations. Just as insects can develop resistance to plant toxins, they can also evolve resistance to insecticides. “This is a big problem for insect and pest control,” stresses Dr Petschenka. The
group’s research has led to deeper insights into insect biology, which could prove important from a conservational perspective. “It may seem surprising that some species are ecologically fragile, as they are adaptable and have a high level of dietary flexibility. However, we have shown that they depend on specific plants to acquire certain toxins – if this plant is not present, then they may not thrive in the population,” continues Dr Petschenka. “So it’s clear that we need to understand the ecology of different species in much more detail if we are to protect them effectively.” The side-effects of insecticides are currently tested mainly against bees, yet as a dietary generalist it is already pre-adapted to lots of noxious compounds, so is unlikely to be affected as severely as a more ecologically fragile species that is dependent on a single source of food. A more evolutionary perspective is required to accurately assess risk, believes Dr Petschenka. “We need to integrate other concepts into risk assessment strategies,” he says.
Differential processing of host plant toxins by insect herbivores as a driver of multi-trophic interactions Dr. Georg Petschenka Emmy Noether research group leader Justus-Liebig-Universität Giessen Institut für Insektenbiotechnologie Heinrich-Buff-Ring 26-32 35392 Giessen, Germany T: +49 641 99 37603 E: Georg.P.Petschenka@ agrar.uni-giessen.de W: http://www.uni-giessen. de/fbz/fb09/institute/iib/ ento/ag/petschenka Dr. Georg Petschenka studied Biology in Tübingen and Bayreuth. Since 2017 he is leading an Emmy Noether Research group (DFG) and a research group funded by the Hessen State Ministry of Higher Education, Research and the Arts (HMWK) via the ‘LOEWE Center for Insect Biotechnology and Bioresources at Justus Liebig University Giessen. His research focuses on insect resistance to plant toxins and sequestration of plant toxins by insects.
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