IAB Research Project Description

The Functional Significance of Variation in the Production of Extrafloral Nectaries by Aspen

Extrafloral nectaries on an acacia plant in the Institute of Arctic Biology Research Greenhouse. Date unknown. Credit: Courtesy IAB Research Greenhouse

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Extrafloral nectaries (EFNs) are nectar-secreting glands located on stems, leaves, and bracts (modified or specialized leaves) that occur in a wide taxonomic range of plants. The relationship between EFNs and ants is a classic example of a defense mutualism. Ants and other natural enemies, attracted by food rewards (and sometimes nesting space), kill or dislodge herbivores which can damage plants.

In this project principal investigator Diane Wagner and her team are studying the function and maintenance of variation in EFN expression in the quaking aspen (Populus tremuloides), an ecologically important and widespread serving as wildlife habitat, fire break, recreational backdrop, and a source of wood pulp. In aspen, EFN expression varies at multiple spatial scales: leaves within ramets (an independent member of a clone), ramets within genets (groups of genetically identical individuals), and among genets. Preliminary evidence collected over the past three years in Interior Alaska suggests that the presence of EFNs on leaves decreases herbivory (feeding on plants), both indirectly, by attracting ants and parasitoid wasps, and directly, by affecting the oviposition or egg-laying behavior of at least one specialist herbivore, the aspen leaf miner (Phyllocnistis populiella). In addition, the defensive benefit of EFNs appears to decline with increasing tree height, probably because visitation by natural enemies declines with canopy height.

Dozens of studies conducted in the past 50 years document that ants reduce herbivory on plants to which they have access, and some have demonstrated resulting benefits to correlates of plant fitness. Other natural enemies including coccinelid beetles, mites, spiders, and wasps, also collect nectar from EFNs. Several studies have shown that parasitism rates of herbivores tend to be higher on plant species with EFNs. Yet experimental evidence of parasitoids as a selective agent for EFNs is lacking in natural systems. Field-based exclusion of parasitoids while allowing access to other natural enemies has not been feasible and it is often difficult to determine if parasitism actually reduces herbivore damage to plants.

Little attention has been paid to natural variation in the presence and abundance of EFNs within and between plants either because this variation is absent or overlooked. Yet the study of this natural variation has enormous potential to improve our understanding of the evolution of EFNs. As metabolically active secretory organs, EFNs are often assumed to be costly and maintained in populations by conferring a fitness benefit. In species where it has been examined, the expression of EFNs has a genetic component and the presence of EFNs in many species appears to be maintained by natural selection.

By relating EFN expression to herbivory in natural systems, we can better understand current selection on this trait. In addition, the existence of leaves with and without EFNs on the same plant provides the opportunity to study behavioral responses of herbivores and natural enemies to EFNs, while controlling for plant-level sources of variation. Previous studies have convincingly shown that EFNs defend plants through indirect means, by attracting predators that remove herbivores. We suggest that the direct behavioral responses of herbivores to EFNs may play an important and previously unrecognized role in EFN evolution. If herbivores can perceive the presence of EFNs, and leaves with EFNs impose a higher rate of herbivore mortality, then selection is expected to favor individuals who avoid visiting leaves with EFNs. Herbivore avoidance of leaves with EFNs would combine with higher mortality on EFN-bearing leaves and strengthen selection for expression of EFNs. Furthermore, avoidance of EFNs might persist for some time even in the absence of differential mortality on leaves with and without EFNs.

Project Funding

National Science Foundation
$360,000.00
1 Apr 2006 – 1 Mar 2009
IAB Proposal #06-002
UAF Grant #G3021
IAB Project #144


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