Reciprocal fitness consequences in the defense mediated interaction between a toxic plant and a sequestering herbivore

• Applicants: Dr. Jörn Pagel; Professor Dr. Georg Petschenka; Professor Dr. Andreas Schweiger; Professorin Dr. Anke Steppuhn
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems ; Evolution and Systematics of Plants and Fungi ; Organismic Interactions, Chemical Ecology and Microbiomes of Plant Systems
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 535991727

The proposed project is part of the package proposal "Fitness Landscapes of biotic INTeractions and their role for eco-evolutionary biodiversity dynamics: towards a theory-based synthesis across interaction types (FLINT)”, which aims to bridge the gap between theoretical and empirical research on the impact of biotic interactions on biodiversity dynamics. In this empirical project, we focus on the reciprocal fitness consequences of the interaction between a toxic plant and its specialized seed predator that sequesters plant toxins for defense. Secondary metabolites are key mediators of biotic interactions at multiple scales: Chemical properties of secondary metabolites influence global distribution patterns of plant interactions and local differences in herbivore composition are known to select for geographic variation in plant chemical defense. However, our understanding of the reciprocal fitness consequences in antagonistic plant-herbivore interactions mediated by defense chemistry is very limited. In particular, how the interplay of biotic and abiotic factors at large and local scales affects fitness on both sides has never been investigated. Here, we propose a set of hierarchical objectives to investigate the abiotic and biotic drivers of antagonistic plant-herbivore interactions and their fitness consequences at scales ranging from the molecular level of defense chemistry to large-scale distribution patterns. We hypothesize that the reproductive fitness and chemical defenses of plants will vary across their distributions due to interactions with their abiotic and biotic environments. This variation will affect toxin-sequestering seed predators, which have a direct fitness impact by feeding on plant reproductive organs and benefit from plant toxins for their own defense. As a model, we will use the Eurasian milkweed bug Spilostethus saxatilis (Heteroptera: Lygaeinae), which is obligately associated with autumn crocus (Colchicum autumnale) and sequesters high levels of colchicine alkaloids for defense. We will study C. autumnale populations across Germany and Europe, colonized or not by S. saxatilis, along gradients of temperature, precipitation and soil nutrient availability. In our project, we will integrate (1) chemical analyses of alkaloids and other plant metabolites in insects and plants across geographic and biotic gradients, (2) characterization of the abiotic and biotic environment along with fitness assessments of plants and specialist herbivores, (3) reciprocal feeding, sequestration, and seed predation assays to test for local adaptation, and (4) ecological/biogeographic modeling to unravel how fitness feedbacks from biotic interactions drive plant-insect co-occurrence across their distributional ranges. The proposed project will advance the general, theoretical framework for conceptualizing species interactions and their fitness consequences, as envisioned for the FLINT consortium.

• DFG Programme: Research Grants