The Genetics Underlying Natural Variation in the Biotic Interactions of Arabidopsis thaliana: The Challenges of Linking Evolutionary Genetics and Community Ecology

In the context of global change, predicting the responses of plant communities in an ever-changing biotic environment calls for a multipronged approach at the interface of evolutionary genetics and community ecology. However, our understanding of the genetic basis of natural variation involved in mediating biotic interactions, and associated adaptive dynamics of focal plants in their natural communities, is still in its infancy. Here, we review the genetic and molecular bases of natural variation in the response to biotic interactions (viruses, bacteria, fungi, oomycetes, herbivores, and plants) in the model plant Arabidopsis thaliana as well as the adaptive value of these bases. Among the 60 identified genes are a number that encode nucleotide-binding site leucine-rich repeat (NBS-LRR)-type proteins, consistent with early examples of plant defense genes. However, recent studies have revealed an extensive diversity in the molecular mechanisms of defense. Many types of genetic variants associate with phenotypic variation in biotic interactions, even among the genes of large effect that tend to be identified. In general, we found that (i) balancing selection rather than directional selection explains the observed patterns of genetic diversity within A. thaliana and (ii) the cost/benefit tradeoffs of adaptive alleles can be strongly dependent on both genomic and environmental contexts. Finally, because A. thaliana rarely interacts with only one biotic partner in nature, we highlight the benefit of exploring diffuse biotic interactions rather than tightly associated host-enemy pairs. This challenge would help to improve our understanding of coevolutionary quantitative genetics within the context of realistic community complexity.

Diet creates metabolic niches in the "immature gut" that shape microbial communities

Although diet composition has been implicated as a major factor in the etiology of various gastrointestinal diseases, conclusive evidence remains elusive. This is particularly true in diseases such as necrotizing enterocolitis where breast milk as opposed to commercial formula appears to confer a "protective effect" to the "immature gut." Yet the mechanism by which this occurs continues to remain speculative. In the present study we hypothesize that the basic chemical composition of diet fundamentally selects for specific intestinal microbiota which may help explain disparate disease outcome and therapeutic direction. Complimentary animal and human studies were conducted on young piglets (21 d.)(n = 8) (IACUC protocols 08070 and 08015) and premature infants (adjusted gestational age 34-36 weeks) (n = 11) (IRB Protocol 15895A). In each study, cecal or stool contents from two groups (Breast milk-fed (BF) vs. Formula-fed (FF)) were analyzed by gas chromatography/mass spectrometry (GC/MS) and comprehensive metabolic profiles generated and compared. Concurrently, bacterial community structure was assayed and respective representative microbiota of the groups determined by 16S rRNA gene amplicon pyrosequencing. Statistical modeling and analysis was done using SIMCA-P+ and R software. GC/MS metabolomics identified clear differences between BF and FF groups in the intestinal environment of piglets and humans. Sugars, amino-sugars, fatty acids, especially unsaturated fatty acids, and sterols were identified as being among the most important metabolites for distinguishing between BF and FF groups. Joint analysis of microbiota and metabolomics pinpointed specific sets of metabolites (p < 0.05) associated with the dominant bacterial taxa. The chemical composition of diet appears to have a significant role in defining the microbiota of the immature gut. Tandem analysis of intestinal microbial and metabolic profiles is potentially a powerful tool leading to better understanding of the role of diet in disease perhaps even leading to specific strategies to alter microbial behavior to improve clinical outcome.

Distribution of genetic variation within and among local populations of Arabidopsis thaliana over its species range

A detailed description of local population structure in Arabidopsis thaliana is presented, including an assessment of the genetic relatedness of individuals collected from the same field. A hierarchical sample of four individuals from 37 local populations, including North America, England, Eastern and Western Europe, and Asia, and a selection of ecotypes, were analysed for variation in Adh, ChiA, FAH1, F3H, Rpm1, Rps5, and five microsatellite loci. Twenty-eight of the 37 population samples contained individuals with identical multilocus haplotypes, 12 of which were fixed for a single haplotype. These monomorphic populations were evenly distributed over the species range. Only in North America did we find a single multilocus haplotype shared among different populations, perhaps indicating a continental founder event. Despite the occurrence of local inbreeding, a considerable amount of genetic variation was found segregating within and among local populations. A novel analysis of haplotype differences reveals that genetic differentiation occurs at every geographic scale in A. thaliana, where we find a surprising under-representation of recent migrants between local populations. This leads us to hypothesize that most dispersal between A. thaliana populations is by pollen rather than seed. Based on the structure of A. thaliana populations, it appears that regional groups of local populations may provide the most appropriate genetic material for linkage disequilibrium mapping of adaptive traits.

Salicylic acid inhibits jasmonic acid-induced resistance of Arabidopsis thaliana to Spodoptera exigua

The role of salicylic acid (SA) in plant responses to pathogens has been well documented, but its direct and indirect effects on plant responses to insects are not so well understood. We examined the effects of SA, alone and in combination with jasmonic acid (JA), on the performance of the generalist herbivore, Spodoptera exigua, in wild-type and mutant Arabidopsis thaliana genotypes that varied genetically in their ability to mount SA- and JA-mediated defence responses. In one experiment, growth of S. exigua larvae was highest on the Wassilewskija wild-type, intermediate on the Columbia wild-type and the JA-deficient fad mutant, and lowest on the nim1-1 and jar1-mutants, which are defective in the SA and JA pathways, respectively. Activity of guaiacol peroxidase, polyphenoloxidase, n-acetylglucosaminidase, and trypsin inhibitor varied by genotype but did not correlate with insect performance. SA treatment increased growth of S. exigua larvae by approximately 35% over all genotypes, but had no discernable effect on activities of the four defence proteins. In a second experiment, growth of S. exigua was highest across treatments on the cep1 mutant, a constitutive expressor of high SA levels and systemic acquired resistance, and lowest on the fad mutant, which is JA-deficient. JA treatment generally increased activity of all four defence proteins, increased total glucosinolate levels and reduced insect growth by approximately 25% over all genotypes. SA generally inhibited expression of JA-induced resistance to S. exigua when both hormones were applied simultaneously. Across genotypes and treatments, larval mass was negatively correlated with the activity of trypsin inhibitor and polyphenoloxidase and with total glucosinolate levels, and insect damage was negatively correlated with the activity of polyphenoloxidase. SA had little effect on the induction of defence protein activity by JA. However, SA attenuated the induction of glucosinolates by JA and therefore may explain better the interactive effects of SA and JA on insect performance. This study illustrates that direct and indirect cross-effects of SA on resistance to S. exigua can occur in A. thaliana. Effects of SA may be mediated through effects on plant defence chemistry or other aspects of the suitability of foliage for insect feeding and growth.

Fetal muscle gene transfer is not enhanced by an RGD capsid modification to high-capacity adenoviral vectors

High levels of alpha(v) integrin expression by fetal muscle suggested that vector re-targeting to integrins could enhance adenoviral vector-mediated transduction, thereby increasing safety and efficacy of muscle gene transfer in utero. High-capacity adenoviral (HC-Ad) vectors modified by an Arg-Gly-Asp (RGD) peptide motif in the HI loop of the adenoviral fiber (RGD-HC-Ad) have demonstrated efficient gene transfer through binding to alpha(v) integrins. To test integrin targeting of HC-Ad vectors for fetal muscle gene transfer, we compared unmodified and RGD-modified HC-Ad vectors. In vivo, unmodified HC-Ad vector transduced fetal mouse muscle with four-fold higher efficiency compared to RGD-HC-Ad vector. Confirming that the difference was due to muscle cell autonomous factors and not mechanical barriers, transduction of primary myogenic cells isolated from murine fetal muscle in vitro demonstrated a three-fold better transduction by HC-Ad vector than by RGD-HC-Ad vector. We hypothesized that the high expression level of coxsackievirus and adenovirus receptor (CAR), demonstrated in fetal muscle cells both in vitro and in vivo, was the crucial variable influencing the relative transduction efficiencies of HC-Ad and RGD-HC-Ad vectors. To explore this further, we studied transduction by HC-Ad and RGD-HC-Ad vectors in paired cell lines that expressed alpha(v) integrins and differed only by the presence or absence of CAR expression. The results increase our understanding of factors that will be important for retargeting HC-Ad vectors to enhance gene transfer to fetal muscle.

Negative cross-talk between salicylate- and jasmonate-mediated pathways in the Wassilewskija ecotype of Arabidopsis thaliana

Plants often respond to attack by insect herbivores and necrotrophic pathogens with induction of jasmonate-dependent resistance traits, but respond to attack by biotrophic pathogens with induction of salicylate-dependent resistance traits. To assess the degree to which the jasmonate- and salicylate-dependent pathways interact, we compared pathogenesis-related protein activity and bacterial performance in four mutant Arabidopsis thaliana lines relative to their wild-type backgrounds. We found that two salicylate-dependent pathway mutants (cep1, nim1-1) exhibited strong effects on the growth of the generalist biotrophic pathogen, Pseudomonas syringae pv. tomato, whereas two jasmonate-dependent pathway mutants (fad3-2fad7-2fad8, jar1-1) did not. Leaf peroxidase and exochitinase activity were negatively correlated with bacterial growth, whereas leaf polyphenol oxidase activity and trypsin inhibitor concentration were not. Interestingly, leaf total glucosinolate concentration was positively correlated with bacterial growth. In the same experiment, we also found that application of jasmonic acid generally increased leaf peroxidase activity and trypsin inhibitor concentration in the mutant lines. However, the cep1 mutant, shown previously to overexpress salicylic acid, exhibited no detectable biological or chemical responses to jasmonic acid, suggesting that high levels of salicylic acid may have inhibited a plant response. In a second experiment, we compared the effect of jasmonic acid and/or salicylic acid on two ecotypes of A. thaliana. Application of salicylic acid to the Wassilewskija ecotype decreased bacterial growth. However, this effect was not observed when both salicylic acid and jasmonic acid were applied, suggesting that jasmonic acid negated the beneficial effect of salicylic acid. Collectively, our results confirm that the salicylate-dependent pathway is more important than the jasmonate-dependent pathway in determining growth of P. syringae pv. tomato in A. thaliana, and suggest important negative interactions between these two major defensive pathways in the Wassilewskija ecotype. In contrast, the Columbia ecotype exhibited little evidence of negative interactions between the two pathways, suggesting intraspecific variability in how these pathways interact in A. thaliana.

Fitness costs of R-gene-mediated resistance in Arabidopsis thaliana

Resistance genes (R-genes) act as an immune system in plants by recognizing pathogens and inducing defensive pathways. Many R-gene loci are present in plant genomes, presumably reflecting the need to maintain a large repertoire of resistance alleles. These loci also often segregate for resistance and susceptibility alleles that natural selection has maintained as polymorphisms within a species for millions of years. Given the obvious advantage to an individual of being disease resistant, what prevents these resistance alleles from being driven to fixation by natural selection? A cost of resistance is one potential explanation; most models require a lower fitness of resistant individuals in the absence of pathogens for long-term persistence of susceptibility alleles. Here we test for the presence of a cost of resistance at the RPM1 locus of Arabidopsis thaliana. Results of a field experiment comparing the fitness of isogenic strains that differ in the presence or absence of RPM1 and its natural promoter reveal a large cost of RPM1, providing the first evidence that costs contribute to the maintenance of an ancient R-gene polymorphism.

Models and data on plant-enemy coevolution

Although coevolution is complicated, in that the interacting species evolve in response to each other, such evolutionary dynamics are amenable to mathematical modeling. In this article, we briefly review models and data on coevolution between plants and the pathogens and herbivores that attack them. We focus on "arms races," in which trait values in the plant and its enemies escalate to more and more extreme values. Untested key assumptions in many of the models are the relationships between costs and benefits of resistance in the plant and the level of resistance, as well as how costs of virulence or detoxification ability in the enemy change with levels of these traits. A preliminary assessment of these assumptions finds only mixed support for the models. What is needed are models that are more closely tailored to particular plant-enemy interactions, as well as experiments that are expressly designed to test existing models.

Diamondback moth compensatory consumption of protease inhibitor-transformed plants

Prior study of the effect of protease inhibitors (PIs) on diamondback moths suggests that moths are resistant to them, so PIs represent an ineffective defence against moths. However, our data suggest that diamondback moths do suffer lower growth rates when they consume plants transformed with potato protease inhibitor (PI2), but that effect is hidden by compensatory consumption. Plants, instead of gaining an advantage by lowering the insect growth rate, suffer a disadvantage as moths consume more tissue to mitigate the effect. Furthermore, PI2, when used in conjunction with another transgenic pesticidal protein, Bt (from Bacillus thuringiensis) counteracts the effectiveness of Bt at protecting plant tissue. Thus, transgenic PIs are not only less effective than previously thought in protecting Brassica plants from diamondback moths, they may actually lead to increased plant damage by the moths.

Human coxsackie-adenovirus receptor is colocalized with integrins alpha(v)beta(3) and alpha(v)beta(5) on the cardiomyocyte sarcolemma and upregulated in dilated cardiomyopathy: implications for cardiotropic viral infections

BACKGROUND

The coxsackievirus and adenovirus receptor (CAR) was identified as a common cellular receptor for both viruses, but its biological and pathogenic relevance is uncertain. Knowledge of CAR localization in the human cardiovascular system is limited but important with respect to CAR-dependent viral infections and gene transfer using CAR-dependent viral vectors.

METHODS AND RESULTS

Explanted failing hearts from 13 patients (8 with dilated cardiomyopathy [DCM] and 5 with other heart diseases [non-DCM]) and normal donor hearts (n=7) were investigated for the expression levels and subcellular localization of CAR and the adenovirus coreceptors alpha(v)beta(3) and alpha(v)beta(5) integrins. CAR immunoreactivity was very low in normal and non-DCM hearts, whereas strong CAR signals occurred at the intercalated discs and sarcolemma in 5 of the 8 DCM hearts (62.5%); these strong signals colocalized with both integrins. In all hearts, CAR was detectable in subendothelial layers of the vessel wall, but not on the luminal endothelial surface, and on interstitial cells. Human CAR (hCAR) expressed in rat cardiomyocytes was targeted to cell-cell contacts, which resembled CAR localization in DCM hearts and resulted in 15-fold increased adenovirus uptake.

CONCLUSIONS

Low hCAR abundance may render normal human myocardium resistant to CAR-dependent viruses, whereas re-expression of hCAR, such as that observed in DCM, may be a key determinant of cardiac susceptibility to viral infections. Asymmetric expression of hCAR in the vessel wall may be an important determinant of adenovirus tropism in humans. hCAR subcellular localization in human myocardium and hCAR targeting to cell-cell contacts in cardiomyocyte cultures suggest that hCAR may play a role in cell-cell contact formation.

Evolutionary dynamics of plant R-genes

Plant R-genes involved in gene-for-gene interactions with pathogens are expected to undergo coevolutionary arms races in which plant specificity and pathogen virulence continually adapt in response to each other. Lending support to this idea, the solvent-exposed amino acid residues of leucine-rich repeats, a region of R-genes involved in recognizing pathogens, often evolve at unusually fast rates. But within-species polymorphism is also common in R-genes, implying that the adaptive substitution process is not simply one of successive selective sweeps. Here we document these features in available data and discuss them in light of the evolutionary dynamics they likely reflect.

Plant density and nutrient availability constrain constitutive and wound-induced expression of trypsin inhibitors in Brassica napus

We investigated the effects of plant density on plant size, leaf total soluble protein content, and constitutive and wound-induced levels of proteinaceous trypsin inhibitors in pot-grown Brassica napus seedlings in two greenhouse studies. We manipulated plant density by varying the number of intraspecific neighbors surrounding a target plant in the center of each pot. In general, constitutive and induced levels of trypsin inhibitors were significantly reduced by competition in a density-dependent manner, to the extent that induction was greatly reduced or abolished in target plants surrounded by six neighbors. To investigate whether the effects of plant density on inhibitor production were mediated by nutrient availability, we manipulated the concentration of a complete fertilizer applied to target plants surrounded by six neighbors in two greenhouse studies. In general, constitutive and wound-induced levels of inhibitors in plants surrounded by six neighbors were increased by nutrient addition in a dose-dependent manner, such that wound-induction was completely restored in competing plants under conditions of high nutrient availability. Leaf total soluble protein content, measured only in the second trial of each experiment, was not affected by any of the treatments. The effects of plant density, nutrient addition, and wounding on inhibitor levels in all experiments were independent of their effects on above-ground plant size at the time of wounding. Overall, our results suggest that decreasing nutrient availability mediates the density-dependent reductions in inhibitor levels in B. napus seedings.

Evolutionary ecology of the tropane alkaloids of Datura stramonium L. (Solanaceae)

Although insect herbivory is frequently assumed to be responsible for the maintenance of plant secondary metabolites such as alkaloids, the assumption is controversial and experimental evidence for this assumption is sparse. We examined natural selection on the two major alkaloids present in the leaves of Datura stramonium and found that both alkaloids came under active selection. We found negative directional selection for scopolamine (natural selection acting to reduce scopolamine levels) and stabilizing selection for hyoscyamine (natural selection acting to maintain an intermediate level of hyoscyamine). We also present evidence that insect herbivores act as the agents of selection on these alkaloids. Finally, we show that there were no trade-offs in resistance to different species of insects.

Evolutionary ecology of the tropane alkaloids of Datura stramonium L. (Solanaceae)

Although insect herbivory is frequently assumed to be responsible for the maintenance of plant secondary metabolites such as alkaloids, the assumption is controversial and experimental evidence for this assumption is sparse. We examined natural selection on the two major alkaloids present in the leaves of Datura stramonium and found that both alkaloids came under active selection. We found negative directional selection for scopolamine (natural selection acting to reduce scopolamine levels) and stabilizing selection for hyoscyamine (natural selection acting to maintain an intermediate level of hyoscyamine). We also present evidence that insect herbivores act as the agents of selection on these alkaloids. Finally, we show that there were no trade-offs in resistance to different species of insects.

The evolution of compensation to herbivory in scarlet gilia, Ipomopsis aggregata: herbivore-imposed natural selection and the quantitative genetics of tolerance

Tolerance is the ability of plants to maintain fitness after experiencing herbivore damage. We investigated scarlet gilia tolerance to browsing in the framework of phenotypic plasticity using both an operational and candidate trait approach. Individuals from full-sib families were split into an artificial clipping treatment, a natural-damage treatment, or left as controls. We tested for genetic variation in tolerance by evaluating family x herbivory treatment interactions on fitness in a mixed model analysis of variance. In addition, we used selection analyses to assess the function of flowering phenology and compensatory regrowth (via branch production) as candidate tolerance traits. We found a strong detrimental fitness effect of browsing and considerable variation among sire half-sib families in levels of tolerance (25% to 63% of the fitness of controls). There was no evidence of overcompensation at either the population or family level and no additive genetic variation in operationally defined tolerance. Phenotypic selection analyses provide evidence that early flowering and compensatory regrowth function as tolerance characters. We found strong linear and correlational selection for early flowering and increased branch production for damaged plants and linear selection for apical dominance (reduced branchiness) and early flowering in control plants. Moreover, reduced phenological delay and increased plasticity in branch production were correlated with tolerance. We detected significant additive genetic variation in flowering phenology in both treatments and a positive genetic correlation between the phenology of control and damaged plants. We found significant additive genetic variation in branch production in undamaged and naturally damaged plants, but not in clipped plants. Damaged plants exhibited marginally significant additive genetic variance in fitness, although its heritability was very low (approximately 3.6%). We failed to find additive genetic variation in the fitness of control plants. Our results suggest that tolerance traits are under herbivore-imposed natural selection in this population, but that responses to selection are limited by available genetic variation and selective constraints.

Dynamics of disease resistance polymorphism at the Rpm1 locus of Arabidopsis

The co-evolutionary 'arms race' is a widely accepted model for the evolution of host-pathogen interactions. This model predicts that variation for disease resistance will be transient, and that host populations generally will be monomorphic at disease-resistance (R-gene) loci. However, plant populations show considerable polymorphism at R-gene loci involved in pathogen recognition. Here we have tested the arms-race model in Arabidopsis thaliana by analysing sequences flanking Rpm1, a gene conferring the ability to recognize Pseudomonas pathogens carrying AvrRpm1 or AvrB. We reject the arms-race hypothesis: resistance and susceptibility alleles at this locus have co-existed for millions of years. To account for the age of alleles and the relative levels of polymorphism within allelic classes, we use coalescence theory to model the long-term accumulation of nucleotide polymorphism in the context of the short-term ecological dynamics of disease resistance. This analysis supports a 'trench warfare' hypothesis, in which advances and retreats of resistance-allele frequency maintain variation for disease resistance as a dynamic polymorphism.

Temporally regulated expression patterns following in utero adenovirus-mediated gene transfer

Developmental patterns of gene expression were determined following intravascular administration of adenovirus in utero, during sequential stages of murine development. Replication-deficient adenovirus (AdCMV.LacZ) was injected into yolk sac vessels of mouse embryos 12, 13, 15 and 18 days post-conception (d.p.c.). beta-Galactosidase (beta-gal) expression was evaluated 24-48 h after injection, at birth, and 5 weeks following normal delivery. Gene expression was detected in myocardial cells, endothelial cells of heart, lung, kidney, adrenal, gut, and in hepatocytes. The patterns of expression were distinct for each stage of virus administration and time-point of analysis. Intensity of individual organ expression varied with injection time-point, with the largest number of organs express- ing the transgene when embryos were injected at 15 d.p.c. beta-Gal activity was detected in only a subset of cells expressing the murine coxsackievirus and adenovirus receptor (CAR), indicating factors other than receptor distribution were responsible for the pattern of transgene expression observed. These studies begin to define critical parameters affecting intravascular gene delivery in utero and indicate that intrinsic developmental regulatory mechanisms may control exogenous gene expression. Intravenous administration of adenovirus provides a unique approach for in utero gene transduction and will be a useful adjunct in evaluating genes which have early lethal mutations.

The effect of seed and rosette cold treatment on germination and flowering time in some Arabidopsis thaliana (Brassicaceae) ecotypes

The germination and flowering responses to cold treatment were investigated in 32 ecotypes of Arabidopsis thaliana. A month-long cold treatment at the seed stage decreased the time until flowering in all but one strain, whereas a 3-d cold treatment had little, or the opposing effect. A month-long cold treatment at the rosette stage also decreased the time until flowering, but was less effective than seed cold treatment. Seed and rosette cold treatments did not have an additive effect on time until flowering. Cold treatment usually increased the speed of germination, however no clear response patterns for the probability of germination were detected. These findings are discussed in relation to the life cycle of the plant.

Modification of the genetic program of human alveolar macrophages by adenovirus vectors in vitro is feasible but inefficient, limited in part by the low level of expression of the coxsackie/adenovirus receptor

Robust expression of genes transferred by adenovirus (Ad) vectors depends upon efficient entry of vectors into target cells. Cells deficient in the coxsackie/adenovirus receptor (CAR) are difficult targets for Ad-mediated gene transfer. We hypothesized that low levels of CAR expression may be responsible, in part, for the relative inefficiency of Ad-mediated gene transfer to human alveolar macrophages (AMs). CAR gene expression was detected in human AMs by reverse transcription-polymerase chain reaction and at low levels by Northern analysis. Indirect immunofluorescence showed specific, low-intensity surface staining for CAR, but at levels below those found on the positive-control A549 human lung epithelial cell line. Consistent with this, AMs expressed Ad vector transgenes 100 to 1,000-fold less efficiently than A549 cells, as assessed using the beta-galactosidase reporter (chemiluminescence assay) and green fluorescent protein (fluorescence microscopy and flow cytometry). At high multiplicity of infection, AMs from an HIV+ individual could be transduced with an AdIFNgamma vector to secrete detectable human interferon-gamma. Ad transgene expression by AMs was blocked by capsid fiber protein, suggesting that CAR is required in the pathway for productive Ad entry into alveolar macrophages. To confirm that Ad transgene expression by AMs is limited by low levels of CAR expression, cells were infected with an Ad vector containing the CAR complementary DNA (cDNA). Enhanced expression of CAR protein was demonstrated by indirect immunofluorescence, and the CAR cDNA-transduced cells showed 5-fold enhancement of subsequent Ad transgene expression. These observations demonstrate that human AMs can be targets for Ad-mediated gene transfer, but that efficiency of transgene expression is limited, at least in part, by low levels of CAR expression.

Genetic variation within and among populations of Arabidopsis thaliana

We investigated levels of nucleotide polymorphism within and among populations of the highly self-fertilizing Brassicaceous species, Arabidopsis thaliana. Four-cutter RFLP data were collected at one mitochondrial and three nuclear loci from 115 isolines representing 11 worldwide population collections, as well as from seven commonly used ecotypes. The collections include multiple populations from North America and Eurasia, as well as two pairs of collections from locally proximate sites, and thus allow a hierarchical geographic analysis of polymorphism. We found no variation at the mitochondrial locus Nad5 and very low levels of intrapopulation nucleotide diversity at Adh, Dhs1, and Gpa1. Interpopulation nucleotide diversity was also consistently low among the loci, averaging 0.0014. gst, a measure of population differentiation, was estimated to be 0.643. Interestingly, we found no association between geographical distance between populations and genetic distance. Most haplotypes have a worldwide distribution, suggesting a recent expansion of the species or long-distance gene flow. The low level of polymorphism found in this study is consistent with theoretical models of neutral mutations and background selection in highly self-fertilizing species.

Fitness consequences of genetically engineered herbicide and antibiotic resistance in Arabidopsis thaliana

Researchers have often invoked the concept of metabolic drain to explain the lower growth rates of bacteria containing plasmids that confer antibiotic resistance. This idea posits that the energetic input needed to produce detoxifying enzymes diverts resources from clonal reproduction. In this paper we examine whether the concept of metabolic drain can be applied successfully to plants that differ from bacteria in several key aspects including their relative genome size and reproductive rate. We have conducted a field experiment using mutant and transgenic Arabidopsis thaliana that allows the comparison of genotypes differing by a single gene conferring resistance to either the herbicide chlorsulfuron or the antibiotic kanamycin. In addition to testing whether these traits reduce fitness, this experiment was conducted at two levels of resource availability to examine whether costs of resistance are sensitive to environmental quality. We found that herbicide-resistant individuals produced 26% fewer seeds than susceptible counterparts. However, contrasting published results in bacterial systems, the fecundity of individuals was completely unaffected by the expression of an introduced antibiotic resistance gene. The fitness cost associated with chlorsulfuron resistance was greater in nutrient-poor conditions relative to nutrient-rich conditions for comparisons involving mutant, but not transgenic, genotypes.

Costs of resistance: a test using transgenic Arabidopsis thaliana

Evolutionary biologists have long attributed polymorphisms in resistance status to fitness costs of resistance traits. Nevertheless, pleiotropic fitness costs of resistance have been notoriously difficult to detect. We have transformed Arabidopsis thaliana with a mutant acetolactate synthase gene that confers resistance to the herbicide, chlorsulfuron. Our experiment revealed a 34% reduction in the lifetime seed production of transgenic, herbicide resistant Arabidopsis thaliana relative to their susceptible null segregants. Our experimental design allows us to conclude that this fitness cost of resistance is caused by the pleiotropic effect of the introduced acetolactate synthase gene rather than other potential costs associated with the plasmid or mutational changes induced by plant transformation. In addition, we can attribute the cost of resistance to the presence of the resistance gene rather than an increase in gene dosages. The implications of these results for the risk assessment of transgenic crops are discussed.

Complex dynamics in a carbon-nitrogen model of a grass-legume pasture

A physiologically based model of a grass-legume pasture is used to study the dynamics of these competing species. In our model, we consider carbon and nitrogen pools and fluxes, incorporating competition for light and soil mineral nitrogen, and including the processes of nitrogen fixation, nitrogen losses and dry matter allocation. First, the steadystate responses of each species to nitrogen deposition, to leaching rate, and to other nitrogen losses are examined. We then consider the dynamic behaviour of these species when there is no time delay for nitrogen cycled through the soil organic matter pool. Next, the effects of various time delays associated with the soil organic matter nitrogen pool on the system dynamics are examined: the behaviour becomes complex, non-linear and exhibits lightly or heavily damped oscillations at two frequencies. The high sensitivity of the system both to the initial value of the soil organic matter nitrogen pool, and to any photosynthetic competitive advantage, is investigated. The implications of these results in relation to observations and experiments on grass-legume pastures are discussed.

Statistical genetics of an annual plant, Impatiens capensis. I. Genetic basis of quantitative variation

Analysis of quantitative genetics in natural populations has been hindered by computational and methodological problems in statistical analysis. We developed and validated a jackknife procedure to test for existence of broad sense heritabilities and dominance or maternal effects influencing quantitative characters in Impatiens capensis. Early life cycle characters showed evidence of dominance and/or maternal effects, while later characters exhibited predominantly environmental variation. Monte Carlo simulations demonstrate that these jackknife tests of variance components are extremely robust to heterogeneous error variances. Statistical methods from human genetics provide evidence for either a major locus influencing germination date, or genes that affect phenotypic variability per se. We urge explicit consideration of statistical behavior of estimation and testing procedures for proper biological interpretation of statistical results.

Statistical genetics of an annual plant, Impatiens capensis. II. Natural selection

Measurement of natural selection on correlated characters provides valuable information on fitness surfaces, patterns of directional, stabilizing, or disruptive selection, mechanisms of fitness variation operating in nature, and possible spatial variation in selective pressures. We examined effects of seed weight, germination date, plant size, early growth, and late growth on individual fitness. Path analysis showed that most characters had direct or indirect effects on individual fitness, indicating directional selection. For most early life-cycle characters, indirect effects via later characters exceed the direct causal effect on fitness. Selection gradients were uniform across the experimental site. There was no evidence for stabilizing or disruptive selection. We discuss several definitions of stabilizing and disruptive selection. Although early events in the life of an individual have important causal effects on subsequent characters and fitness, there is no detectable genetic variance for most of these characters, so little or no genetic response to natural selection is expected.

Statistical genetics of an annual plant, Impatiens capensis. II. Natural selection

Measurement of natural selection on correlated characters provides valuable information on fitness surfaces, patterns of directional, stabilizing, or disruptive selection, mechanisms of fitness variation operating in nature, and possible spatial variation in selective pressures. We examined effects of seed weight, germination date, plant size, early growth, and late growth on individual fitness. Path analysis showed that most characters had direct or indirect effects on individual fitness, indicating directional selection. For most early life-cycle characters, indirect effects via later characters exceed the direct causal effect on fitness. Selection gradients were uniform across the experimental site. There was no evidence for stabilizing or disruptive selection. We discuss several definitions of stabilizing and disruptive selection. Although early events in the life of an individual have important causal effects on subsequent characters and fitness, there is no detectable genetic variance for most of these characters, so little or no genetic response to natural selection is expected.