Chemical Defenses Against Floral Enemies: Costs and Benefits of Toxic Nectar to Nectar Robbing, Pollination, and Plant Fitness

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

ADLER - 0211480 In natural systems, organisms face simultaneous direct and indirect interactions with mutualists and antagonists, yet our understanding of the relative importance of these competing pressures in shaping the evolution of phenotypic traits is still rudimentary. For example, flowering plants may face the dilemma of attracting pollinators while discouraging floral enemies, such as nectar robbers. Because robbers are competing with pollinators for the same resource (e.g., nectar), it may be particularly difficult for plants to evolve defensive mechanisms that are effective against antagonists without also deterring mutualists. The goal of the proposed research is to determine how a ubiquitous but poorly understood floral trait, toxic nectar, influences interactions with both nectar robbers and pollinators, and how these simultaneous interactions shape selection on this phenotypic trait. Toxic nectar, or nectar that contains secondary compounds, is a widespread trait that may confer resistance to floral robbers, but at a potential cost to pollinator attraction. Currently, the costs and benefits of toxic nectar are unknown. In addition to resistance traits, plants may cope with antagonists via tolerance. The concepts of resistance and tolerance have traditionally been used in reference to herbivores; however, nectar robbers, like herbivores, have profound effects on plant fitness and plants are likely to use resistance and tolerance as defensive mechanisms against robbers. The proposed research will extend resistance and tolerance concepts to a wider range of plant-animal interactions through an examination of the role of nectar traits, such as nectar toxicity and production, as mechanisms of resistance and tolerance to nectar robbing. The aim of this work is to understand the ecological role of toxic nectar within the context of interactions between mutualists (pollinators) and antagonists (nectar robbers), and to determine the extent to which resistance and tolerance mediate the impact of nectar robbers on plant genotypes. The research proposed will experimentally manipulate nectar toxicity in Gelsemium sempervirens, a perennial vine with documented alkaloids in its nectar, to determine the impact of toxic nectar on the community of insect pollinators and robbers as well as on male and female plant reproduction and offspring success. In addition, this study will manipulate nectar robbing and pollination in genotypes of G. sempervirens with known variability in nectar toxicity to determine the direct and indirect effects of nectar robbing on plant fitness, genetic variation in resistance and tolerance to nectar robbing, and costs of toxic nectar in the absence of nectar robbing. The research combines manipulative field experiments with observations to address four main questions: 1) Is there genetic variation in nectar toxicity? 2) Does toxic nectar benefit plants by increasing resistance to nectar robbing without reducing pollinator attraction? 3) Does nectar robbing reduce plant fitness directly or indirectly, and do plant genotypes vary in their resistance or tolerance of nectar robbing? and 4) Is there a cost of toxic nectar in the absence of robbing? The work proposed here is novel in that it integrates the costs and benefits of resistance associated with a ubiquitous but poorly understood floral trait, toxic nectar, in the context of joint forces of selection exerted by both mutualistic and antagonistic floral visitors. This work extends an existing conceptual framework and provides empirical insight into the diversity of roles a single floral trait can serve, from attraction to defense. Consideration of the broader context in which organisms evolve is fundamental to our understanding of how multiple-species interactions shape genetic and biological diversity at ecological, microevolutionary, and macroevolutionary scales.