▪ Abstract  Recent theoretical studies suggest that the ability to tolerate consumer damage can be an important adaptive response by plants to selection imposed by consumers. Empirical studies have also found that tolerance is a common response to consumers among plants. Currently recognized mechanisms underlying tolerance include several general sets of traits: allocation patterns; plant architecture; and various other traits that may respond to consumer damage, e.g., photosynthetic rate. Theoretical studies suggest that tolerance to consumer damage may be favored under a range of conditions, even when the risk and intensity of damage varies. However, most of these models assume that the evolution of tolerance is constrained by internal resource allocation trade-offs. While there is some empirical evidence for such trade-offs, it is also clear that external constraints such as pollinator abundance or nutrient availability may also limit the evolution of tolerance. Current research also suggests that a full understanding of plant adaptation to consumers can only be achieved by investigating the joint evolution of tolerance and resistance. While tolerance to consumer damage has just recently received significant attention in the ecological literature, our understanding of it is rapidly increasing as its profound ecological and evolutionary implications become better appreciated.

Water limitation is one of the most important factors limiting crop productivity world-wide and has likely been an important selective regime influencing the evolution of plant physiology. Understanding the genetic and physiological basis of drought adaptation is therefore important for improving crops as well as for understanding the evolution of wild species. Here, results are presented from quantitative trait loci (QTL) mapping of flowering time (a drought escape mechanism) and carbon stable isotope ratio (d d d

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Many studies of herbaceous plant populations have illustrated the potential of adjacent subpopulations to adapt to local ecological conditions. However, the extent to which local adaptation on a small geographical scale can occur in outcrossing tree populations is not well understood. In this study, we reciprocally transplanted acorns from adjacent subpopulations of northern red oak (Quercus rubra L.) occupying north-, southwest-, and west-facing slopes within a 4-ha plot in a Missouri oak-hickory forest. The quantitative character we measured was leaf damage by herbivores on first-year seedlings, because it reflects resistance to insect herbivores-a quantitative trait that could be under different selective pressures in dissimilar microhabitats. We found that seedlings showed the least damage when planted at the site of the maternal plant. This finding provides initial but strong evidence of local adaptation and illustrates that selection associated with leaf harbivory may have a strong impact on the genetic structure of local tree populations. Such a result is unexpected for a widely outcrossing species on such a small geographical scale but indicates that genetic structuring is possible within other plant populations occupying heterogeneous environments.

To investigate the potential for and constraints on the evolution of compensatory ability, we performed a greenhouse experiment using Asclepias syriaca in which foliar damage and soil nutrient concentration were manipulated. Under low nutrient conditions, significant genetic variation was detected for allocation patterns and for compensatory ability. Furthermore, resource allocation to storage was positively, genetically correlated both with compensatory ability and biomass when damaged, the last two being positively, genetically correlated with each other. Thus, in the low nutrient environment, compensatory ability via resource allocation to storage provided greater biomass when damaged. A negative genetic correlation between compensatory ability and plant biomass when undamaged suggests that this mechanism entailed an allocation cost, which would constrain the evolution of greater compensatory ability when nutrients are limited. Under high nutrient conditions, neither compensatory ability nor allocation patterns predicted biomass when damaged, even though genetic variation in compensatory ability existed. Instead, plant biomass when undamaged predicted biomass when damaged. The differences in outcomes between the two nutrient treatments highlight the importance of considering the possible range of environmental conditions that a genotype may experience. Furthermore, traits that conferred compensatory ability did not necessarily contribute to biomass when damaged, demonstrating that it is critical to examine both compensatory ability and biomass when damaged to determine whether selection by herbivores can favor the evolution of increased compensation.

The evolutionary response of plant populations to selection for increased defense may be constrained by costs of defense. The purpose of this study was to investigate such constraints on the evolution of defense due to a cost of defense manifested as a trade-off between defense and tolerance. Variation in the response to artificial damage (tolerance) among lines of Brassica rapa that had been artificially selected for foliar glucosinolate content (defense) was examined. Leaf area was removed from replicates of three selection lines (high glucosinolates, control, and low glucosinolates) at three damage levels (0%, 20%, and 60% damage). An external cost of defense would result in a statistically significant selection line by damage treatment interaction, with those selected for high defense expressing less tolerance than those selected for low defense. Damage treatment had a significant overall effect on estimated total fitness, with fitness declining with increasing damage level. Further, selection line also had a significant overall effect on estimated total fitness, with low-defense selection lines having higher fitness compared to both control and high-defense selection lines. More importantly, a cost of defense in terms of tolerance was demonstrated by a significant selection line-by-damage treatment interaction. This interaction was in the direction to demonstrate a genetic trade-off between defense and tolerance, with low-defense selection lines decreasing estimated total fitness in response to damage less than both control and high-defense selection lines. Variation in tolerance among selection lines was due to the greater ability of low-defense lines to maintain fruit and seed production despite the presence of damage. In terms of tolerance, this cost of glucosinolate production in B. rapa could constrain the evolution of increased defense and, in so doing, maintain individuals within the population that are poorly defended yet tolerant.

A major goal of evolutionary biology is to understand the genetic architecture of the complex quantitative traits that may lead to adaptations in natural populations. Of particular relevance is the evaluation of the frequency and magnitude of epistasis (gene-gene and gene-environment interaction) as it plays a controversial role in models of adaptation within and among populations. Here, we explore the genetic basis of flowering time in Arabidopsis thaliana using a series of quantitative trait loci (QTL) mapping experiments with two recombinant inbred line (RIL) mapping populations [Columbia (Col) x Landsberg erecta (Ler), Ler x Cape Verde Islands (Cvi)]. We focus on the response of RILs to a series of environmental conditions including drought stress, leaf damage, and apical damage. These data were explicitly evaluated for the presence of epistasis using Bayesian based multiple-QTL genome scans. Overall, we mapped fourteen QTL affecting flowering time. We detected two significant QTL-QTL interactions and several QTL-environment interactions for flowering time in the Ler x Cvi population. QTL-environment interactions were due to environmentally induced changes in the magnitude of QTL effects and their interactions across environments--we did not detect antagonistic pleiotropy. We found no evidence for QTL interactions in the Ler x Col population. We evaluate these results in the context of several other studies of flowering time in Arabidopsis thaliana and adaptive evolution in natural populations.

Phenotypic plasticity is an important response mechanism of plants to environmental heterogeneity. Here, we explored the genetic basis of plastic responses of Arabidopsis thaliana to water deficit by experimentally mapping quantitative trait loci (QTL) in two recombinant inbred populations (Cvi x Ler and Ler x Col). We detected genetic variation and significant genotype-by-environment interactions for many traits related to water use. We also mapped 26 QTL, including six for carbon isotope composition (delta13C). Negative genetic correlations between fruit length and fruit production as well as between flowering time and branch production were corroborated by QTL colocalization, suggesting these correlations are due to pleiotropy or physical linkage. Water-limited plants were more apically dominant with greater root:shoot ratios and higher delta13C (higher water-use efficiency) when compared to well-watered plants. Many of the QTL effects for these traits interacted significantly with the irrigation treatment, suggesting that the observed phenotypic plasticity is genetically based. We specifically searched for epistatic (QTL-QTL) interactions using a two-dimensional genome scan, which allowed us to detect epistasis regardless of additive genetic effects. We found several significant QTL-QTL interactions including three that exhibited environmental dependence. These results provide preliminary evidence for proposed genetic mechanisms underlying phenotypic plasticity.