Stronger Plastic Responses Research Articles

Strong plastic responses in aerenchyma formation in F1 hybrids of Imperata cylindrica under different soil moisture conditions.

Hybrids can express traits plastically, enabling them to occupy environments that differ from parental environments. However, there is insufficient evidence demonstrating how phenotypic plasticity in specific traits mediates hybrid performance. Two parental ecotypes of Imperata cylindrica produce F1 hybrids. The E-type in wet habitats has larger internal aerenchyma than the C-type in dry habitats. This study evaluated relationships between habitat utilisation, aerenchyma plasticity, and growth of I. cylindrica accessions. We hypothesize that plasticity in expressing parental traits explains hybrid establishment in habitats with various soil moisture conditions. Aerenchyma formation was examined in the leaf midribs, rhizomes and roots of two parental ecotypes and their F1 hybrids in their natural habitats. In common garden experiments, we examined plastic aerenchyma formation in leaf midribs, rhizomes and roots of natural and artificial F1 hybrids and parental ecotypes and quantified vegetative growth performance. In the natural habitats where soil moisture content varied widely, the F1 hybrids showed larger variation in aerenchyma formation in rhizomes than their parental ecotypes. In the common garden experiments, F1 hybrids showed high plasticity of aerenchyma formation in rhizomes, and their growth was similar to that of C-type and E-type under drained and flooded conditions, respectively. The results demonstrate that F1 hybrids of I. cylindrica exhibit plasticity in aerenchyma development in response to varying local soil moisture content. This characteristic allows the hybrids to thrive in diverse soil moisture conditions.

High tolerance to zinc but no evidence for local adaptation in the aquatic plant Lemna minor

Duckweeds are a widely distributed and economically important aquatic plant family that have high potential for phytoremediation of polluted water bodies. We collected four ecotypes of the common duckweed Lemna minor from the four corners of Switzerland and assessed how their home versus away environments influenced their growth. Additionally, we investigated their response to a metal pollutant (zinc, Zn) in both their home and away environments. Zn is found in freshwater systems and can become harmful to plants at elevated concentrations. We hypothesized that growing in their home environment would help the plants buffer the negative effect of the metal pollutant. To test this, we measured Lemna growth in a common garden experiment in a glasshouse where the four ecotypes were grown in each of the water environments, as well as in three different concentrations of Zn. To investigate whether interactions between Lemna and its microbial community can enhance or reduce tolerance to heavy metal pollution, we sampled chlorophyll‐a as a proxy for algal biomass. Finally, we measured total nitrogen and total organic carbon to describe the abiotic environment in more detail.The four Lemna ecotypes exhibited significantly different growth rates across the water treatments. This difference in fitness was matched with DNA sequencing revealing genetic differentiation between the four ecotypes. However, the effect of the water and Zn treatment on Lemna growth was the same for all ecotypes. We did not find evidence for local adaptation; instead, we observed strong plastic responses. Lemna growth rates were higher under higher Zn concentrations. This positive effect of Zn on Lemna growth could be in part due to reduced competition with algae. We conclude that L. minor ecotypes may exhibit large differences in growth rate, but that the species overall has a high Zn tolerance and strong plastic adaptive potential in novel environments.

Phenotypic variation of wild radishes Raphanus pugioniformis and R. raphanistrum associated with local conditions in the southeast Mediterranean

The geographical distribution of Raphanus pugioniformis (Brassicaceae), endemic to the southeast Mediterranean, covers distinct and heterogeneous habitats over a slight rainfall and elevation gradients. In contrast, the closely related R. raphanistrum is widely distributed only in well-drained sandy soils along the Mediterranean coastal plain. Here we hypothesize that the allopatric distribution of the two species, assisted by short- and long-seed dispersal of R. pugioniformis and R. raphanistrum, respectively, is also associated with inter- and intra-species phenotypic variation. In a common garden experiment, we assessed various phenological, morphological and fitness-related traits in five populations of each species. The results showed significant differences between the two species, generally in phenological and fitness-related traits. The results point to the role of early flowering and large reproductive output in the wide distribution of R. raphanistrum. In contrast, phenotypic variation in most of the measured traits among populations of R. pugioniformis suggests that short dispersal distance contributed to increased phenotypic differentiation among populations. Moreover, significant correlations between several of the measured traits with rainfall and elevation (e.g., bolting date and duration of flowering) suggest adaptive phenotypic differentiation. In R. raphanistrum, differences in most phenotypic traits were not found between populations, supporting the hypothesis that long distance dispersal of single seeded units contributes to relatively low genetic variation. Finally, we suggest that it is also possible that R. raphanistrum exhibits strong plastic responses, explaining its worldwide distribution, although this remained to be investigated.

Ecological and evolutionary responses of an arctic plant to variation in microclimate and soil

The arctic and alpine regions are predicted to experience some of the highest rates of climate change, and the arctic vegetation is expected to be especially sensitive to such changes. Understanding the ecological and evolutionary responses of arctic plant species to changes in climate is therefore a key objective. Geothermal areas, where natural temperature gradients occur over small spatial scales, and without many of the confounding environmental factors present in latitudinal and other gradient studies, provide a natural experimental setting in which to examine the response of arctic–alpine plants to increasing temperatures. To test the ecological and evolutionary response of the circumpolar alpine bistort Persicaria vivipara to temperature, we collected plant material and soil from areas with low, intermediate and high soil temperatures and grew them at three different temperatures in a three‐factorial growth chamber experiment. At higher experimental soil temperatures, sprouting was earlier and plants had more leaves. Sprouting was earlier in soil originating from intermediate temperature and plants had more leaves when grown in soil originating from low temperatures. We did not find evidence of local adaptation or genetic variation in reaction norms among plants originating from areas with low, intermediate and high soil temperature. Our findings suggest that the alpine bistort has a strong plastic response to warming, but that differences in soil temperature have not resulted in genetic differentiation. The lack of an observed evolutionary response may, for example, be due to the absence of temperature‐mediated selection on P. vivipara, the low rate of sexual recombination, or high levels of gene flow balancing differences in selection. When placed within the context of other studies, we conclude that arctic–alpine plant species often show strong plastic responses to spring warming, while evidence of evolutionary responses varies among species.

Fine‐scale root community structure in the field: Species aggregations change with root density

Abstract Roots are capable of strong plastic responses to environmental signals, but the extent of such responses in the field is essentially unknown. Here, we aimed to identify interspecific root aggregations and segregations as indicators of plastic responses to the presence of other species. Furthermore, we asked whether aggregations change with heterogeneity in overall root density along soil depth and whether root communities show any relationship to species' functional traits. We used quantitative real‐Time PCR to estimate root quantities of nine grassland species in soil blocks at a centimetre scale. We combined these estimates with fine‐scale recording of species' positions above‐ground. We used deviations between measured root density and that expected from the relationship to above‐ground species abundance to correct for different chances of individual species to meet due to the non‐random distribution of their individuals. This approach allows us to identify whether species associations below‐ground are due to plastic response of their roots. We showed that while there were signals of both interspecific aggregation and segregation, prevalence of aggregation over segregation strongly increased with increasing soil depth. In shallow layers, where root density is high and its heterogeneity low, roots are likely to respond to a number of signals which produce both interspecific aggregation and segregation. In contrast, in deeper layers, where root density is low and its heterogeneity high, root distribution is likely to be driven primarily by foraging for resources, producing aggregations. Species composition of the root community did not show any relationship to functional traits (SLA, shoot life span, spreading distance and rooting depth) of the examined species. Synthesis. The results imply that processes that determine interspecific root associations change at a scale of centimetres which is close to the scale of individual fine roots. They also imply that while root foraging might explain root co‐occurrence under low root density, very different processes determine it when root density is high. Our findings also support the notion that roots, while extremely plastic, may not receive sufficient signals to elicit a response in the field if overall root density is high and homogeneous.

Phenotypic plasticity, but not adaptive tracking, underlies seasonal variation in post-cold hardening freeze tolerance of Drosophila melanogaster.

In temperate regions, an organism's ability to rapidly adapt to seasonally varying environments is essential for its survival. In response to seasonal changes in selection pressure caused by variation in temperature, humidity, and food availability, some organisms exhibit plastic changes in phenotype. In other cases, seasonal variation in selection pressure can rapidly increase the frequency of genotypes that offer survival or reproductive advantages under the current conditions. Little is known about the relative influences of plastic and genetic changes in short‐lived organisms experiencing seasonal environmental fluctuations. Cold hardening is a seasonally relevant plastic response in which exposure to cool, but nonlethal, temperatures significantly increases the organism's ability to later survive at freezing temperatures. In the present study, we demonstrate seasonal variation in cold hardening in Drosophila melanogaster and test the extent to which plasticity and adaptive tracking underlie that seasonal variation. We measured the post‐cold hardening freeze tolerance of flies from outdoor mesocosms over the summer, fall, and winter. We bred outdoor mesocosm‐caught flies for two generations in the laboratory and matched each outdoor cohort to an indoor control cohort of similar genetic background. We cold hardened all flies under controlled laboratory conditions and then measured their post‐cold hardening freeze tolerance. Comparing indoor and field‐caught flies and their laboratory‐reared G1 and G2 progeny allowed us to determine the roles of seasonal environmental plasticity, parental effects, and genetic changes on cold hardening. We also tested the relationship between cold hardening and other factors, including age, developmental density, food substrate, presence of antimicrobials, and supplementation with live yeast. We found strong plastic responses to a variety of field‐ and laboratory‐based environmental effects, but no evidence of seasonally varying parental or genetic effects on cold hardening. We therefore conclude that seasonal variation in post‐cold hardening freeze tolerance results from environmental influences and not genetic changes.

Precipitation, not air temperature, drives functional responses of trees in semi‐arid ecosystems

Summary Model scenarios of climate change predict that warming and drought will occur simultaneously in the future in many regions. The capacity of woody species to modify their physiology and morphology in response to environmental conditions is widely recognized, but little is known about the responses of trees to reduced precipitation and increased temperature acting simultaneously. In a semi‐arid woodland, we assessed the responses in physiological (needle emergence, maximum photosynthesis, stomatal conductance, water use efficiency (WUE) and shoot elongation) and morphological (needle length and thickness, and leaf mass per area (LMA)) foliar traits of piñon pine (Pinus edulis) in response to three years of a 45% reduction in precipitation, a 4.8 °C increase in air temperature and their simultaneous effects. A strong change in physiological and morphological traits in response to reduced precipitation was observed. Precipitation reduction delayed needle emergence, decreased photosynthesis and stomatal conductance, increased WUE, decreased shoot elongation and induced shorter needles with a higher LMA. Trees subjected to simultaneous reductions in precipitation and warming demonstrated a similar response. However, atmospheric warming did not induce a response in any of the measured traits. Physiological and morphological traits of trees in this semi‐arid climate were more responsive to changes in soil moisture than air temperature. Long‐term exposure to seasonal drought stress in arid sites may have resulted in strong plastic responses to this first stressor. However, atmospheric warming probably was not experienced as a stress for trees in this warm and dry climate. Overall, our results indicate that in semi‐arid ecosystems where tree functioning is already highly limited by soil water availability, atmospheric warming as anticipated with climate change may have less impact on foliar trait responses than previously thought.

Evolution of growth by genetic accommodation in Icelandic freshwater stickleback

Classical Darwinian adaptation to a change in environment can ensue when selection favours beneficial genetic variation. How plastic trait responses to new conditions affect this process depends on how plasticity reveals to selection the influence of genotype on phenotype. Genetic accommodation theory predicts that evolutionary rate may sharply increase when a new environment induces plastic responses and selects on sufficient genetic variation in those responses to produce an immediate evolutionary response, but natural examples are rare. In Iceland, marine threespine stickleback that have colonized freshwater habitats have evolved more rapid individual growth. Heritable variation in growth is greater for marine full-siblings reared at low versus high salinity, and genetic variation exists in plastic growth responses to low salinity. In fish from recently founded freshwater populations reared at low salinity, the plastic response was strongly correlated with growth. Plasticity and growth were not correlated in full-siblings reared at high salinity nor in marine fish at either salinity. In well-adapted lake populations, rapid growth evolved jointly with stronger plastic responses to low salinity and the persistence of strong plastic responses indicates that growth is not genetically assimilated. Thus, beneficial plastic growth responses to low salinity have both guided and evolved along with rapid growth as stickleback adapted to freshwater.

Adaptation at range margins: common garden trials and the performance ofArabidopsis lyrataacross its northwestern European range

Widely distributed species, such as the perennial plant Arabidopsis lyrata, face a range of environmental conditions across space, creating selective pressures for local evolutionary adaptation. The species' fragmented distribution may reduce gene flow, which could either reduce or increase adaptive potential. The substantial variation in phenotypic traits observed across this species' northwestern European range may reflect a combination of plastic responses to environmental conditions, evolutionary adaptation and nonadaptive genetic differentiation. We conducted multi-site common garden experiments to study differences in plant performance in core and marginal areas. Plants from eight source populations representing the species' full geographic and altitudinal range in northwestern Europe were planted out in Iceland, Sweden, Scotland and Wales. We found evidence of both strong plastic responses and apparently adaptive differentiation in performance. Most evidence for local adaptation was found at range margins, with the strongest effects on reproductive output. Both biotic and abiotic factors affected performance, especially at range margins. Performance of most plants was best in the Scottish and Swedish common garden sites, in the core of the species' distribution. Despite adaptations at range margins, the performance of the species declines at distributional limits, with extreme southern populations looking particularly vulnerable.

THE UNUSUAL LIFE HISTORY OFALSEIS BLACKIANA: A SHADE-PERSISTENT PIONEER TREE?

Classifications of tree species into regeneration guilds or functional groups are often based on a limited number of life history characteristics indicative of requirements for early establishment, with the assumption that these traits are correlated with growth requirements of larger individuals. We tested the validity of this assumption for Alseis blackiana, a common canopy tree in central Panama, by examining its seed germination, seedling growth, and sapling distribution, growth, and mortality. We found that the early life history of Alseis is characteristic of pioneer species: the seed germination rate was much higher in forest gaps than in the forest understory, small seedlings were absent from the understory, and seedlings showed strong plastic responses to variation in light. Seedling relative growth rate, leaf area production rate, and maximum carbon assimilation rates of Alseis are much more similar to those of the pioneer tree Miconia argentea than to those of the non-pioneer tree Tetragastris panamensis. However, in contrast to Miconia, estab- lished seedlings of Alseis are capable of remarkable persistence in the understory; in a 1- ha plot on Barro Colorado Island, the mortality rate for 722 saplings ,1 cm dbh was ,4% per year between 1986 and 1998. We suspect that shade tolerance in Alseis is achieved by both physiological and morphological adaptations, e.g., efficient light interception by thin, nonoverlapping leaves, tolerance to litterfall damage, and a high resprouting capacity. These results show that existing functional group classifications do not account for the full range of seedling trait combinations found in tropical trees and indicate that a gap requirement for seedling establishment need not limit the abundance of adult individuals in a stand.

Phenotypic plasticity in hybridizing Carpobrotus spp. (Aizoaceae) from coastal California and its role in plant invasion

Carpobrotus edulis (L.) N. E. Br., an exotic perennial that invades the coastal habitats of California, hybridizes with the presumed native Carpobrotus chilensis (Mol.) N. E. Br. To investigate the potential role of hybrids in the invasion process by C. edulis, we compared the growth and plasticity of clones of hybrids and parental species. Ramets of three hybrid classes and both parental species were grown in a common garden with four different environments. Both the parental species and the hybrids exhibited strong plastic responses to the test environments for characters describing clonal growth and physiology. Carpobrotus edulis consistently had the largest biomass, but C. chilensis produced longer branches. Carbon isotope ratios (<$QFD0E00000010446D80BFFEFF88A6750DA8D9C8282ADCC6210090483DA6ECD7658F80>13C) showed only weak differences among morphotypes. Hybrids were mostly intermediate relative to parental species in both growth and physiological characters. The intermediacy of hybrids in their response to environmental variation indicates that they do not have inferior growth compared with parental species. The high amount of plasticity of the hybrids and C. edulis, as well as their overall similarity, may account for their invasion success in different habitats of coastal California.

Phenotypic plasticity in hybridizing <i>Carpobrotus</i> spp. (Aizoaceae) from coastal California and its role in plant invasion

Carpobrotus edulis (L.) N. E. Br., an exotic perennial that invades the coastal habitats of California, hybridizes with the presumed native Carpobrotus chilensis (Mol.) N. E. Br. To investigate the potential role of hybrids in the invasion process by C. edulis, we compared the growth and plasticity of clones of hybrids and parental species. Ramets of three hybrid classes and both parental species were grown in a common garden with four different environments. Both the parental species and the hybrids exhibited strong plastic responses to the test environments for characters describing clonal growth and physiology. Carpobrotus edulis consistently had the largest biomass, but C. chilensis produced longer branches. Carbon isotope ratios (ƒ 13 C) showed only weak differences among morphotypes. Hybrids were mostly intermediate relative to parental species in both growth and physiological characters. The intermediacy of hybrids in their response to environmental variation indicates that they do not have inferior growth compared with parental species. The high amount of plasticity of the hybrids and C. edulis, as well as their overall similarity, may account for their invasion success in different habitats of coastal California.

Chapter 30 Responses of monkey basal forebrain neurons during visual discrimination task

Publisher Summary The chapter discusses the results of a study of organization in the monkey basal forebrain of visual information concerning real objects. It was found that basal forebrain neurons selectively coded information about reinforcement and novelty-familiarity relations, and have strong plastic neuronal responses that are related to learning. Monkeys with basal forebrain deficits had impairment of some kind of learning and memory, such as new visual object learning, relearning object discrimination, and serial reversal learning. The basal forebrain receives neuronal inputs from many areas, including the inferotemporal and entorhinal cortices, amygdala, hippocampus, and lateral hypothalamic area. Detailed experimental procedures are described. Single unit activity from the monkey basal forebrain, including the nucleus basalis of Meynert and the diagonal band of Broca was recorded during a visual discrimination task to study the contribution of the basal forebrain to visual learning and memory related to novel and familiar objects. The task included discrimination of rewarding or aversive objects, novel and familiar objects, and association or dissociation with stimulus-reinforcement relations.