PLASTICITY AND GENETIC DIVERSITY MAY ALLOW SALTCEDAR TO INVADE COLD CLIMATES IN NORTH AMERICA
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Abstract:
Two major mechanisms have been proposed to explain the ability of introduced populations to colonize over large habitat gradients, despite significant population bottlenecks during introduction: (1) Broad environmental tolerance—successful invaders possess life history traits that confer superior colonizing ability and/or phenotypic plasticity, allowing acclimation to a wide range of habitats. (2) Local adaptation—successful invaders rapidly adapt to local selective pressures. However, even with bottlenecks, many introduced species exhibit surprisingly high levels of genetic variation and thus the potential for evolutionary increases in invasive traits and plasticity. Here we assess the invasive potential of Tamarix ramosissima, by examining the degree of genetic differentiation within and among populations from the latitudinal extremes of its introduced range. Using growth chamber experiments we examined ecologically important variation in seedlings, both in trait means and their reaction norms across temperature environments. Although we found no genetic variation for gas exchange traits, within or among populations, we did find significant genetic variation for growth traits, both in the trait means and in the degree of plasticity in these traits. Northern ecotypes invested more in roots relative to southern ecotypes but only under low temperatures. Both ecotypes increased shoot investment in warm temperatures. Increased root investment in cold temperatures by northern ecotypes may increase their first winter survival. Genetic differences in seedling root investment may contribute to the ability of this species to successfully tolerate and invade a broader latitudinal range. Our data support a model in which both plasticity and adaptive evolution can contribute to the invasive potential of introduced species.Keywords:
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Local adaptation
Premise of research. Recent increases in global temperature have been shown to adversely affect the reproductive success of certain plant species. It is predicted that plant taxa exhibiting phenotypic plasticity in flowering traits might be able to tolerate increased mean annual temperatures better than less plastic taxa. However, the underlying genetic and developmental mechanisms for phenotypic plasticity in response to high temperatures are only starting to be elucidated.Methodology. We characterized flowering time plasticity in 14 wild ecotypes of Arabidopsis thaliana under 18°C and 26°C and determined whether alternative splicing of the ambient temperature flowering pathway gene FLOWERING LOCUS-M (FLM) and expression of SHORT VEGETATIVE PHASE (SVP) can explain flowering time plasticity in a subset of these ecotypes.Pivotal results. Our results demonstrate intraspecific variation in A. thaliana temperature-mediated phenotypic plasticity and show that potentially stressful high temperatures do not dampen intraspecific variation in flowering time relative to lower temperatures but do reduce variation across the ecotype means. Although average SVP expression is consistently lower in plants grown at 18°C versus 26°C, the ratio of FLM-β to FLM-δ correlates with flowering time plasticity in only three out of five ecotypes. Furthermore, percentage change in the FLM-β:FLM-δ ratio between temperatures does not explain plasticity in flowering time across all populations.Conclusions. Arabidopsis thaliana ecotypes respond plastically in flowering time to changes in temperature, with higher temperatures causing a general shift toward early flowering. Although SVP and FLM-β expression tracks reaction norms, we failed to find evidence supporting a role for plasticity of alternative FLM splicing in intraspecific flowering time variation when ecotypes of A. thaliana were grown under moderate versus high temperatures. This suggests other, as yet unexplored genetic regulators of high-temperature flowering time plasticity across the natural range of A. thaliana.
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Flowering Locus C
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Phenotypic plasticity is the ability of a genotype to produce multiple phenotypes depending on the environmental conditions and it can allow persistence of populations in heterogeneous habitats or under climate change. Therefore, phenotypic plasticity can play a major role in the divergence of populations across habitats. Trade-offs in plant performance in various habitats can give rise to the evolution of specialized ecotypes which are locally adapted (specialized) populations of the same species in distinct environments. According to the specialization hypothesis, specialization of ecotypes to either relatively favorable or unfavorable habitats results in increased or decreased phenotypic plasticity, respectively. The presence of phenotypic plasticity differences among ecotypes can be easily detected by examining their performances at home (native) versus foreign (alien) environments in reciprocal field experiments. In this meta-analysis, I compared phenotypic plasticity of ecotypes specialized in favorable and unfavorable habitats to test the specialization hypothesis by extracting data from 47 empirical studies. Log response ratio (LRR) and plasticity index ( PI v ) were used as effect sizes to detect and quantify significant differences in phenotypic plasticity of ecotypes across habitats. The overall result indicated that it was failed to find an effect of habitat origin on phenotypic plasticity expression of ecotypes. Specialization to either favorable or unfavorable habitats may not alter phenotypic plasticity expression in ecotypes. The interplay between phenotypic plasticity and specialization is quite complex and results of this study may shed light into these two important evolutionary mechanisms in plant ecology which have implications for biodiversity conservation, environmental management, agricultural industry, and ecosystem services.
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Local adaptation
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Abstract Background The role of phenotypic plasticity is increasingly being recognized in the field of evolutionary studies. In this paper we look at the role of genetic determination versus plastic response by comparing the protein expression profiles between two sympatric ecotypes adapted to different shore levels and habitats using two-dimensional protein maps. Results We compared qualitative and quantitative differences in protein expression between pools of both ecotypes from different environments (field and laboratory conditions). The results suggested that ecotype differences may affect about 7% of the proteome in agreement with previous studies, and moreover these differences are basically insensitive to environmental changes. Thus, observed differences between wild ecotypes can be mainly attributed to genetic factors rather than phenotypic plasticity. Conclusions These results confirm the mechanism of adaptation already proposed in this species and a minor role of phenotypic plasticity in this ecological speciation process. In addition, this study provides a number of interesting protein spots potentially involved in adaptation, and therefore candidates for a future identification.
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Entomology
Proteome
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Using differences between the alpine and prairie ecotypes of Stellaria longipes, we investigated whether characteristic levels of phenotypic plasticity could be explained in terms of either Grime's C-S-R model or Taylor and Aarssen's specialization hypothesis. Seven populations of S longipes were located along an elevational gradient, and fine-scale differentiation between the alpine and prairie forms was quantified genetically and morphologically. Analysis of eight enzyme systems revealed two distinct clusters separating high-elevation populations from low-elevation populations. Morphologically, means and plasticities of traits previously used to describe the tundra ecotype showed parallel population grouping to that of the isozyme analysis. Analysis of environmental parameters showed that growth change and amount of phenotypic plasticity had significant associations with increased wind and temperature stress at high elevations and high interspecific competition at low elevations. This is consistent with the idea that the parallel patterns of separation observed for isozymes, morphology, and plasticity are a result of selection pressures consistent with the C-S-R model. In reciprocal transplants, tundra plants with low plasticity were selected against in a prairie environment. However, no clear advantage was observed for either ecotype in the alpine environment. The high plasticity of the prairie ecotype is likely adaptive for competitive habitats and does not bear a cost of low performance in the unfavorable alpine habitat, as predicted by the specialization hypothesis. The relatively poor performance of the alpine ecotype may reflect a restriction rather than specialization to stressful alpine environments. However, selection for the tundra ecotype in its native habitat may have been detected in experiments carried out over a longer period than those of this study.
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Forty years ago, Robert Allard and colleagues documented that the slender wild oat, Avena barbata, occurred in California as two multi-locus allozyme genotypes, associated with mesic and xeric habitats. This is arguably the first example of ecotypes identified by molecular techniques. Despite widespread citation, however, the inference of local adaptation of these ecotypes rested primarily on the allozyme pattern. This study tests for local adaptation of these ecotypes using reciprocal transplant and quantitative trait locus (QTL) mapping techniques. Both ecotypes and 188 recombinant inbred lines (RILs) derived from a cross between them were grown in common garden plots established at two sites representative of the environments in which the ecotypes were first described. Across four growing seasons at each site, three observations consistently emerged. First, despite significant genotype by environment interaction, the mesic ecotype consistently showed higher lifetime reproductive success across all years and sites. Second, the RILs showed no evidence of a trade-off in performance across sites or years, and fitness was positively correlated across environments. Third, at QTL affecting lifetime reproductive success, selection favoured the same allele in all environments. None of these observations are consistent with local adaptation but suggest that a single genotype is selectively favoured at both moist and dry sites. I propose an alternative hypothesis that A. barbata may be an example of contemporary evolution--whereby the favoured genotype is spreading and increasing in frequency--rather than local adaptation.
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Local adaptation
Ecological genetics
Avena
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The ability to express phenotypically plastic responses to environmental cues might be adaptive in changing environments. We studied phenotypic plasticity in mating behaviour as a response to population density and adult sex ratio in a freshwater isopod (Asellus aquaticus). A. aquaticus has recently diverged into two distinct ecotypes, inhabiting different lake habitats (reed Phragmites australis and stonewort Chara tomentosa, respectively). In field surveys, we found that these habitats differ markedly in isopod population densities and adult sex ratios. These spatially and temporally demographic differences are likely to affect mating behaviour. We performed behavioural experiments using animals from both the ancestral ecotype ("reed" isopods) and from the novel ecotype ("stonewort" isopods) population. We found that neither ecotype adjusted their behaviour in response to population density. However, the reed ecotype had a higher intrinsic mating propensity across densities. In contrast to the effects of density, we found ecotype differences in plasticity in response to sex ratio. The stonewort ecotype show pronounced phenotypic plasticity in mating propensity to adult sex ratio, whereas the reed ecotype showed a more canalised behaviour with respect to this demographic factor. We suggest that the lower overall mating propensity and the phenotypic plasticity in response to sex ratio have evolved in the novel stonewort ecotype following invasion of the novel habitat. Plasticity in mating behaviour may in turn have effects on the direction and intensity of sexual selection in the stonewort habitat, which may fuel further ecotype divergence.
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Divergence (linguistics)
Functional divergence
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Abiotic factors can act as barriers to colonization and drive local adaptation. During colonization, organisms may cope with changes in abiotic factors using existing phenotypic plasticity, but the role of phenotypic plasticity in assisting or hindering the process of local adaptation remains unclear. To address these questions, we explore the role of winter conditions in driving divergence during freshwater colonization and the effects of plasticity on local adaptation in ancestral marine and derived freshwater ecotypes of threespine stickleback (Gasterosteus aculeatus). We found that freshwater-resident stickleback had greater tolerance of acute exposure to low temperatures than marine stickleback, but these differences were abolished after acclimation to simulated winter conditions (9L:15D photoperiod at 4 °C). Plasma chloride levels differed between the ecotypes, but showed a similar degree of plasticity between ecotypes. Gene expression of the epithelial calcium channel (ECaC) differed between ecotypes, with the freshwater ecotype demonstrating substantially greater expression than the marine ecotype, but there was no plasticity in this trait under these conditions in either ecotype. In contrast, growth (assessed as final mass) and the expression of an isoform of the electroneutral Na(+)/H(+) exchanger (NHE3) exhibited substantial change with temperature in the marine ecotype that was not observed in the freshwater ecotype under the conditions tested here, which is consistent with evolution of these traits by a process such as genetic assimilation. These data demonstrate substantial divergence in many of these traits between freshwater and marine stickleback, but also illustrate the complexity of possible relationships between plasticity and local adaptation.
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Few surveys have concentrated on studying the adaptive value of phenotypic plasticity within genetically-distinct conspecific ecotypes. Here, we conduct a test to assess the adaptive value that partial phenotypic plasticity may have for survival in the marine gastropod Littorina saxatilis. This species has evolved canalized ecotypes but, nevertheless, the ecotypes show some phenotypic plasticity for the traits under divergent selection between wave-exposed and high-predation habitats. We exposed juveniles of each ecotype to several environmental treatments under laboratory conditions in order to produce shape variation associated with plasticity. The two ecotypes from different treatments were then transplanted to the wave-exposed habitat and the survival rate was monitored. Ecotype explained the largest distinction in survival rate while treatment caused variation in survival rate within the ecotype released into its parental habitat which was correlated with plastic changes in shell shape. Snails that had experienced a treatment mimicking the environment of the transplantation location survived with the highest rate, while individuals from the contrary experimental treatment had lower survivorship. We conclude that the partial plastic response shown in Littorina saxatilis has a significant impact on fitness, although this remains small compared to the overall adaptive difference between ecotypes.
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Adaptive value
Entomology
Value (mathematics)
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One of the most promising directions in the study of phenotypic plasticity is its detailed analysis in organisms that are also well-studied in other aspects. Also, conclusions based on plasticity studies in environmental gradients that closely mimic natural variation are shown to be the most relevant. Following those directions, we conducted this study of phenotypic plasticity on the currently best available model system in flowering plants--Arabidopsis thaliana, and utilized one of the most common variations experienced in the wild--variation in density. Four Arabidopsis thaliana commonly used inbred lines (ecotypes) were grown in densities from one to seven plants per pot. Both phenotypic plasticity and its genetic variability were detected for almost all of 11 analyzed traits, with analyzed ecotypes responding strongly to density of just two plants per pot. Density had small effect on life history and moderate effect on size traits, while vegetative and reproductive traits responded strongly. Mortality of plants during the experiment was almost absent, showing that all densities corresponded to the medium density phase in which "carrying capacity" is not yet reached. Genetic variability for phenotypic plasticity was in most cases the result of profound deviation of only one ecotype from the response of others. In the case of reproductive output, however, G x E interaction was the result of greater between-ecotype variability at lower densities. If we reasonably assume that dense stands are more common in the wild, this difference between ecotypes (populations) closely resembles the cases of so-called potential variability within populations.
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