Cynthia Weinig

EVOLUTIONARY GENETICS OF RESISTANCE AND TOLERANCE TO NATURAL HERBIVORY IN ARABIDOPSIS THALIANA

Abstract.— Resistance and tolerance are widely viewed as two alternative adaptive responses to herbivory. However, the traits underlying resistance and tolerance remain largely unknown, as does the genetic architecture of herbivory responses and the prevalence of genetic trade‐offs. To address these issues, we measured resistance and tolerance to natural apical meristem damage (AMD) by rabbits in a large field experiment with recombinant inbred lines (RILs) ofArabidopsis thaliana (developed from a cross between the Columbia X Landsberg erecta ecotypes). We also measured phenological and morphological traits hypothesized to underlie resistance and tolerance to AMD. Recombinant inbred lines differed significantly in resistance (the proportion of replicates within an RIL that resisted herbivory), and early flowering plants with tall apical inflorescences were more likely to experience damage. Tolerance (the difference in fitness between the damaged and undamaged states), also differed significantly among RILs, with some lines over‐compensating for damage and producing more fruit in the damaged than undamaged state. Plastic increases in basal branch number, basal branch height, and senescence date in response to damage were all associated with greater tolerance. There was no evidence for a genetic trade‐off between resistance and tolerance, an observation consistent with the underlying differences in associated morphological and phenological characters. Selection gradient analysis detected no evidence for direct selection on either resistance or tolerance in this experiment. However, a statistical model indicates that the pattern of selection on resistance depends strongly on the mean level of tolerance, and selection on tolerance depends strongly on the mean level of resistance. These observations are consistent with the hypothesis that selection may act to maintain resistance and tolerance at intermediate levels in spatially or temporally varying environments or those with varying herbivore populations.

QTL architecture of resistance and tolerance traits in Arabidopsis thaliana in natural environments

Quantitative‐genetic approaches have offered significant insights into phenotypic evolution. However, quantitative‐genetic analyses fail to provide information about the evolutionary relevance of specific loci. One complex and ecologically relevant trait for plants is their resistance to herbivory because natural enemies can impose significant damage. To illustrate the insights of combined molecular and ecological research, we present the results of a field study mapping quantitative trait loci (QTL) for resistance and tolerance to natural rabbit herbivory in the genetic model, Arabidopsis thaliana. Replicates of the Ler × Col recombinant inbred lines were planted into field sites simulating natural autumn and spring seasonal germination cohorts. Shortly after flowering, herbivores removed the main flowering inflorescence (apical meristem). We found several main‐effect QTL for resistance within each seasonal cohort and significant QTL–season interactions, demonstrating that the loci underlying resistance to a single herbivore differ across seasonal environments. The presence of QTL × environment also shows that variation at specific loci is only available to selection in some environments. Despite significant among‐line variance components, no QTL for tolerance were detected. The combined results of the quantitative‐genetic and QTL analyses demonstrate that many loci of small effect underlie tolerance to damage by rabbits, and counter the hypothesis of locus‐specific tradeoffs between resistance and tolerance. The results also provide insights as to the locus‐specific nature of evolutionary constraints, i.e. some loci influence flowering time and resistance in both seasonal cohorts. Our results show how linking molecular‐genetic tools with field studies in ecologically relevant settings can clarify the role of specific loci in the evolution of quantitative traits.

PLASTICITY AND ENVIRONMENT-SPECIFIC COVARIANCES: AN INVESTIGATION OF FLORAL-VEGETATIVE AND WITHIN FLOWER CORRELATIONS

Abstract Floral traits are commonly thought to be more canalized than vegetative ones. In addition, floral and vegetative traits are hypothesized to be genetically decoupled, enabling vegetative structures to respond plastically to environmental heterogeneity, and to evolve in response to selection without disrupting the reproductive function of flowers. To test these hypotheses, we evaluate the genetic architecture of floral and vegetative traits in natural populations of Arabidopsis thaliana raised under variable light-quality environments. Plants were grown either under high or low ratios of red to far-red (R:FR) light, an aspect of light quality that varies with neighbor proximity and regulates competitive shade-avoidance responses. Across environments, we detected significant genetic variation for the average expression of all measured floral traits (petal length and width, stamen length, pistil length, stigma-anther separation, and exsertion of both the stamen and pistil beyond the corolla). Light quality significantly influenced the absolute size of several floral traits as well as the allometry (i.e., relative scaling) of all floral traits, and genotypes differed in the plasticity of floral traits to the light treatments. Exposure to low relative to high R:FR resulted in significantly greater elongation in the vegetative trait, petiole length, and genotypes again differed in the plasticity of this trait to R:FR. Consistent with prior studies, most floral traits were less plastic than the vegetative trait; herkogamy (i.e., stigma-anther separation) was the exception and expressed more variable trait values across environments than petiole length, apparently as a consequence of the independent responses of stamens and pistils. Flowers also showed strong phenotypic integration; genotypic correlations were significantly positive among floral traits within each light treatment. Although floral-vegetative correlations were not significant in the high R:FR light treatment, significant correlations were detected between petal traits, pistil length, and petiole length under low R:FR, in contrast to the widely held hypothesis that floral and vegetative traits are genetically independent. Finally, we detected selection for reduced herkogamy in the low R:FR light treatment. The observed correlation between functional trait groups suggest that vegetative plasticity may affect the expression of floral traits in some environments, and that environment-specific constraints may exist on the evolution of floral and vegetative traits.

Local and Global Costs of Adaptive Plasticity to Density in Arabidopsis thaliana

Although phenotypic plasticity is demonstrably adaptive in a range of settings, organisms are not perfectly plastic. Costs of plasticity comprise one factor predicted to counter the evolution of this adaptive strategy, yet evidence of costs is rare. Here, we test the fitness effects of plastic life‐history and morphological responses to density and costs of this plasticity in recombinant inbred lines of Arabidopsis thaliana. Several costs of plasticity and homeostasis were detected. Of particular relevance, there was a significant cost of plasticity to active stem‐elongation responses, an adaptive trait in many species. There was also a cost of plasticity to apical branch production at both high and low density, which resulted from the greater suppression of basal branching in genotypes with plastic apical branch production relative to genotypes with fixed apical branch production. The presence of a cost in multiple environments (i.e., a global cost) is predicted to counter the evolution of plasticity. Experimental segregating progenies such as the one used here are expected to have higher genetic costs of plasticity than arrays of genotypes sampled from natural populations because selection should remove genotypes with costs resulting from linkage disequilibrium or epistasis. The use of experimental progeny arrays therefore increases the ability to evaluate genetic costs.

Environmental Effects on the Expression of Quantitative Trait Loci and Implications for Phenotypic Evolution

Abstract Organisms in natural populations experience environmental heterogeneity over a range of temporal and spatial scales, and this heterogeneity has significant evolutionary implications. By affecting patterns of selection and the expression of genetic variation, environmental heterogeneity can play an important role in determining the evolutionary dynamics of phenotypic traits and the maintenance of genetic variation. Although mapping quantitative trait loci (the loci that underlie continuously varying quantitative traits) has a long history in agricultural and applied studies, the technique has only recently been applied to evolutionary studies. This application has made it possible to identify the specific loci underlying trait variation in different environments, to measure environmental variation in natural selection on those loci, and to test assumptions of models regarding the maintenance of genetic variation under environmentally heterogeneous selection. Here we review recent studies that have examined interactions between quantitative trait loci and ecologically relevant environments to address evolutionary questions.

TESTING ADAPTIVE PLASTICITY TO UV: COSTS AND BENEFITS OF STEM ELONGATION AND LIGHT-INDUCED PHENOLICS

Abstract On exposure to ultraviolet radiation (UV), many plant species both reduce stem elongation and increase production of phenolic compounds that absorb in the UV region of the spectrum. To demonstrate that such developmental plasticity to UV is adaptive, it is necessary to show that the induced phenotype is both beneficial in inductive environments and maladaptive in non‐inductive environments. We measured selection on stem elongation and phenolic content of seedlings of Impatiens capensis transplanted into ambient‐UV and UV‐removal treatments. We extended the range of phenotypes expressed, and thus the opportunity for selection in each UV treatment, by pretreating seedlings with either a low ratio of red:far‐red wavelengths (R:FR), which induced stem elongation and reduced phenolic concentrations, or high R:FR, which had the opposite effect on these two phenotypic traits. Reduced stem length relative to biomass was advantageous for elongated plants under ambient UV, whereas increased elongation was favored in the UV‐removal treatment. Selection favored an increase in the level of phenolics induced by UV in the ambient‐UV treatment, but a decrease in phenolics in the absence of UV. These results are consistent with the hypotheses that reduced elongation and increased phenolic concentrations serve a UV‐protective function and provide the first explicit demonstration in a wild species that plasticity of these traits to UV is adaptive. The observed cost to phenolics in the absence of UV may explain why many species plastically upregulate phenolic production when exposed to UV, rather than evolve constitutively high levels of these compounds. Finally, pretreatment with low R:FR simulating foliar shade did not exacerbate the fitness impact of UV exposure when plants had several weeks to acclimate to UV. This observation suggests that the evolution of adaptive shade avoidance responses to low R:FR in crowded stands will not be constrained by increased sensitivity to UV in elongated plants when they overtop their neighbors.

Antagonistic multilevel selection on size and architecture in variable density settings.

Abstract In some ecological settings, an individuals fitness depends on both its own phenotype (individual-level selection) as well as the phenotype of the individuals with which it interacts (group-level selection). Using contextual analysis to measure multilevel selection in experimental stands of Arabidopsis thaliana, we detected significant linear selection that reversed across individual versus group levels for two composite phenotypic traits, “size” and “elongation.” In both cases, selection at the individual level acted to increase values of these traits, presumably due to their positive effect on resource acquisition. Group selection favored decreased values of the same traits. Nonlinear selection was weak but significant in several cases, including stabilizing selection on developmental rate; individuals with very rapid development likely had lower than average fitness due to their reduced resource level at reproduction, while very delayed reproduction may have resulted in lower fitness if prolonged competition for resources reduced overall environmental quality and fitness of all individuals in a group. Under this scenario, stabilizing selection on individual traits is evidence of selection at the group level. Significant density-dependent selection suggests that a threshold density must be reached before group selection acts. Below this threshold, selection at the individual level affects phenotypic evolution more strongly than group selection. A second experiment measured multilevel selection in progeny stands of the original experimental plants. Multilevel selection again acted antagonistically on a composite trait that included size and elongation as well as on an architectural trait, branch production. The magnitude of individual versus group selection was relatively similar in the progeny generation, and the observed balance of individual versus group selection across densities is generally consistent with the hypotheses that multilevel selection can contribute to phenotypic evolution and to important demographic phenomena, including soft selection and the “law of constant yield.”

Susceptibility to UV damage in Impatiens capensis (Balsaminaceae): testing for opportunity costs to shade-avoidance and population differentiation

Plastic increases in leaf secondary compounds may be an adaptive strategy that reduces the damaging effects of high-energy, ultraviolet radiation (UV). Here, we examine (1) the relationship between fitness and anthocyanin and flavonoid concentrations in experimental, UV environments, (2) the effects of UV on Impatiens capensis plants derived from woodland and clearing sites, and (3) whether susceptibility to UV damage is reduced by exposure to high ratios of red : far-red wavelengths (R : FR), which also stimulate the production of leaf compounds. Seedlings from each site were exposed to either high R : FR typical of sunlight or low R : FR characteristic of foliar shade, after which plants were moved into ambient UV or UV-removal treatments. Ultraviolet radiation stimulated the production of anthocyanins and flavonoids. However, higher anthocyanin concentrations were associated with lower biomass in the UV environment. Relative to the clearing population, reproductive output of the woodland population was more detrimentally affected by exposure to UV, despite its higher concentration of anthocyanins. Increased anthocyanin production may therefore be a stress response rather than an adaptive one. The greater tolerance of the clearing population to UV suggests that populations with an evolutionary history of UV exposure evolve mechanisms to limit damage. The R : FR pretreatments did not influence susceptibility to UV damage.

Sequence diversity and haplotype associations with phenotypic responses to crowding: GIGANTEA affects fruit set in Arabidopsis thaliana

Identifying the molecular genetic basis of intraspecific variation in quantitative traits promises to provide novel insight into their evolutionary history as well as genetic mechanisms of adaptation. In an attempt to identify genes responsible for natural variation in competitive responses in Arabidopsis thaliana, we examined DNA sequence diversity at seven loci previously identified as members of the phytochrome B signalling network. For one gene, GIGANTEA (GI), we detected significant haplotype structure. To test for GI haplogroup–phenotype associations, we genotyped 161 A. thaliana accessions at GI and censused the same accessions for total fruit set and the expression of three phenotypic traits (days to flowering, petiole length, and inflorescence height) in a greenhouse experiment where plants were grown in crowded and uncrowded environments. We detected a significant association between GI and total fruit set that resulted in a 14% difference in average fruit set among GI haplogroups. Given that fruit set is an important component of fitness in this species and given the magnitude of the effect, the question arises as to how variation at this locus is maintained. Our observation of frequent and significant epistasis between GI and background single nucleotide polymorphisms (SNP), where the fitness ranking of the GI allele either reverses or does not differ depending on the allele at the interacting SNP, suggests that epistatic selection may actively maintain or at least slow the loss of variation at GI. This result is particularly noteworthy in the light of the ongoing debate regarding the genetic underpinnings of phenotypic evolution and recent observations that epistasis for phenotypic traits and components of fitness is common in A. thaliana.

Resolving the genetic basis of invasiveness and predicting invasions

Considerable effort has been invested in determining traits underlying invasiveness. Yet, identifying a set of traits that commonly confers invasiveness in a range of species has proven elusive, and almost nothing is known about genetic loci affecting invasive success. Incorporating genetic model organisms into ecologically relevant studies is one promising avenue to begin dissecting the genetic underpinnings of invasiveness. Molecular biologists are rapidly characterizing genes mediating developmental responses to diverse environmental cues, i.e., genes for plasticity, as well as to environmental factors likely to impose strong selection on invading species, e.g., resistance to herbivores and competitors, coordination of life-history events with seasonal changes, and physiological tolerance of heat, drought, or cold. Here, we give an overview of molecular genetic tools increasingly used to characterize the genetic basis of adaptation and that may be used to begin identifying genetic mechanisms of invasiveness. Given the divergent traits that affect invasiveness, “invasiveness genes” common to many clades are unlikely, but the combination of developmental genetic advances with further evolutionary studies and modeling may provide a framework for identifying genes that account for invasiveness in related species.