Inducible versus constitutive antioxidant defenses in algae along an environmental stress gradient

Optimal defense theory (ODT) predicts that antiherbivore defenses should be constitutive when plants are frequently attacked and inducible when the probability of attack is low. Like antiherbivore defenses, antioxidant defenses can be inducible or constitutive. We hypothesized the ODT predictions should apply to antioxidant defenses; thus, species inhabiting environments where oxidative stresses occur frequently should produce constitutive antioxidant defenses, whereas species in environments where stresses occur less frequently should produce inducible defenses. We tested this hypothesis by attempting to induce production of the antioxidant precursor dimethylsulfoniopropionate (DMSP) in 4 ulvoid algae species that experience different levels of environmental stress because they are zoned along a tidal gradient. The 2 lower intertidal species Ulvaria obscura and Ulva fenestrata, which experience oxidative stresses less frequently, induced DMSP production in response to applications of the chemical oxidant hydrogen peroxide within 7 d, whereas the higher intertidal species Ulva linza and Ulva intestinalis, which regularly experience oxidative stress, did not have increased DMSP concentrations. This study demonstrates a novel waterborne signaling mechanism for DMSP induction in marine macroalgae and provides evidence of selection for inducible antioxidant defenses in organisms experiencing less frequent environmental stresses.

Induced defenses alter the strength and direction of natural selection on reproductive traits in common milkweed

Evolutionary biologists have long sought to understand the ecological processes that generate plant reproductive diversity. Recent evidence indicates that constitutive antiherbivore defenses can alter natural selection on reproductive traits, but it is unclear whether induced defenses will have the same effect and whether reduced foliar damage in defended plants is the cause of this pattern. In a factorial field experiment using common milkweed, Asclepias syriaca, we induced plant defenses using jasmonic acid (JA) and imposed foliar damage using scissors. We found that JA-induced plants experienced selection for more inflorescences that were smaller in size (fewer flowers), while control plants only experienced a trend toward selection for larger inflorescences (more flowers); all effects were independent of foliar damage. Our results demonstrate that induced defenses can alter both the strength and direction of selection on reproductive traits, and suggest that antiherbivore defenses may promote the evolution of plant reproductive diversity.

Mucilage reduces leaf herbivory in Schreber’s watershield, Brasenia schreberi J.F. Gmel. (Cabombaceae)

Insect herbivores negatively affect plants by consuming biomass that could otherwise be used for growth or reproduction. To reduce their impact, plants have evolved a wide range of antiherbivore defenses. Schreber’s watershield (Brasenia schreberi J.F. Gmel.; Cabombaceae) is a freshwater, aquatic plant that produces a thick mucilage on the underside of leaves and all underwater organs. The mucilage has been proposed as a mechanism of defense against herbivory, but this hypothesis is untested. We conducted a comparative study to determine whether the quantity of mucilage on the underside of leaves is associated with herbivore damage on the leaves. We found that leaves with the thickest mucilage layer were the least damaged. To determine whether mucilage is directly responsible for defense against herbivores, we conducted a manipulative study where we removed the mucilage from the abaxial surface of leaves. We found that demucilated leaves experienced higher amounts of herbivore damage than leaves with their mucilage left intact. We conclude that the mucilage produced by B. schreberi functions to reduce herbivory on leaves, although its association with underwater herbivores and its specific antiherbivore properties remain unknown.

Phylogenetic Nonindependence and Meta-analysis

In ecological and evolutionary meta-analysis, pooling research from multiple species can be a problem because species form a nested hierarchy of phylogenetic relationships. This shared phylogenetic history can introduce a correlated structure to effect size data because studies on closely related species may yield similar outcomes, and therefore similar estimates of effect sizes. This similarity is the product of shared (i.e., phylogenetically conserved) morphological, physiological, or behavioral characteristics. This chapter describes statistical methods to account for phylogenetic nonindependence of species when pooling and testing for homogeneity of effect sizes. It also describes a method that compares the results of a traditional and a phylogenetically independent meta-analysis to evaluate which approach was more effective at explaining variation in research outcomes. Using these methods, it provides a worked example of a meta-analysis on trade-offs among plant antiherbivore defenses. The chapter concludes a discussion on approaches for collating phylogenetic information for meta-analysis.