POTENTIAL FITNESS TRADE‐OFFS FOR THERMAL TOLERANCE IN THE INTERTIDAL COPEPOD TIGRIOPUS CALIFORNICUS

Thermal adaptation to spatially varying environmental conditions occurs in a wide range of species, but what is less clear is the nature of fitness trade‐offs associated with this temperature adaptation. Here, populations of the intertidal copepod Tigriopus californicus are examined at both local and latitudinal scales to determine whether these populations have evolved differences in their survival under high temperature stress. A clear pattern of increasing high temperature stress tolerance is seen with decreasing latitude, consistent with temperature adaptation. Additionally, there is also evidence for significant variation in thermal tolerance on a smaller scale. The competitive fitness of pairs of northern and southern copepod populations were also examined under a series of lower, more moderate temperatures. These fitness assays show that the southern populations that have the best survival under extreme high temperatures have lowered competitive fitness at the lower temperatures tested, whereas the fitness of the southern populations exceeded that of the northern populations at the highest temperatures tested. Combined, these results suggest that there may be evolutionary trade‐offs between performance at high and stressful temperatures and fitness at moderate temperatures in this species.     

Positive and negative biotic interactions and invasive Triadica sebifera tolerance to salinity: a cross‐continent comparative study

Exotic plant species may exhibit abiotic niche expansions that enable them to persist in a greater variety of habitat types in their introduced ranges than in their native ranges. This may reflect variation in limitation by different abiotic niche dimensions (realized niche shift) or phenotypic effects of biotic interactions that vary among ranges (realized niche expansion). Novel abiotic and biotic environments in the introduced range may also lead to genetic changes in exotic plant traits that enhance their abiotic stress tolerance (fundamental niche expansion). Here, we investigated how biotic interactions (aboveground herbivory and soil organisms) affect plant salinity tolerance using the invasive species Triadica sebifera from China (native range) and US (introduced range) populations grown in common gardens in both ranges. Simulated herbivory significantly reduced survival in saline treatments with reductions especially large at low salinity. Soil sterilization had a negative effect on survival at low salinity in China but had a positive effect on survival at low salinity in the US. Triadica survival and biomass were higher for US populations than for China populations, particularly in China but salinity tolerance did not depend on population origin. On average, arbuscular mycorrhizal (AM) colonization was higher for US populations, US soils and low salinity. These factors had a significant, positive, non‐additive interaction so that clipped seedlings from US populations in low saline US soils had high levels of AM colonization. Overall, our results show that phenotypic biotic interactions shape Triadica's salinity tolerance. Positive and negative biotic interactions together affected plant performance at intermediate stress levels. However, only aboveground damage consistently affected salinity tolerance, suggesting an important role for enemy release in expanding stress tolerance.     

Recovery from hybrid breakdown in a marine invertebrate is faster,stronger and more repeatable under environmental stress

Understanding how environmental stress alters the consequences of hybridization is important, because the rate of hybridization and the likelihood of hybrid speciation both appear elevated in harsh, disturbed or marginal habitats. We assessed fitness, morphometrics and molecular genetic composition over 14 generations of hybridization between two highly divergent populations of the marine copepod Tigriopus californicus. Replicated, experimental hybrid populations in both control and high‐salinity conditions showed a decline in fitness, followed by a recovery. Recovery was faster in the salinity stress treatment, returning to parental levels up to two generations earlier than in the control. This recovery was stable in the high‐salinity treatment, whereas in the control treatment, fitness dropped back below parental levels at the final time point. Recovery in the high‐salinity treatment was also stronger in terms of competitive fitness and heat‐shock tolerance. Finally, consequences of hybridization were more repeatable under salinity stress, where among‐replicate variance for survivorship and molecular genetic composition was lower than in the control treatment. In a system with low effective population sizes (estimates ranged from 17 to 63), where genetic drift might be expected to be the predominate force, strong selection under harsh environmental conditions apparently promoted faster, stronger and more repeatable recovery from depressed hybrid fitness.     

Mating success at high temperature in highland‐ and lowland‐derived populations as well as in heat knock‐down selected Drosophila buzzatii

Thermal‐stress selection can affect multiple fitness components including mating success. Reproductive success is one of the most inclusive measures of overall fitness, and mating success is a major component of reproduction. However, almost no attention has been spent to test how mating success can be affected by thermal‐stress selection. In this study, we examine the mating success in the cactophilic Drosophila buzzatii Patterson & Wheeler (Diptera: Drosophilidae) derived from two natural populations that nearly represent the ends of an altitudinal cline for heat knock‐down resistance. Furthermore, we extended the analysis using laboratory lines artificially selected for high and low heat knock‐down resistance. Mating success at high temperature was found to be higher in the lowland than the highland population after a heat pre‐treatment. Moreover, individuals selected for heat knock‐down resistance showed higher mating success at high temperature than did individuals selected for low knock‐down resistance. These results indicate that adaptation to thermal stress can confer an advantage on fitness‐related traits including mating success and highlight the benefits of earlier heat exposure as an adaptive plastic response affecting mating success under stress of higher temperature.     

Adaptation or exaptation? An experimental test of hypotheses on the origin of salinity tolerance in Bufo calamita

The natterjack toad (Bufo calamita) shows variation in embryonic and larval salinity tolerance across populations in southern Spain. However, its aquatic/terrestrial biphasic life cycle, together with remarkable differences in salinity tolerance between Spanish and UK freshwater populations suggest an alternative hypothesis to local adaptation. Drought resistance during the terrestrial phase and salinity tolerance during the aquatic phase are both related to osmotic stress tolerance, and if there were an association between them, one could have evolved as an exaptation from the other. To test such an association, we reared B. calamita juveniles from three populations known to differ genetically in their salinity tolerance, under either dry or humid conditions. Drought decreased growth rate, enhanced burying behaviour, and decreased foraging activity and efficiency. No significant population x treatment interaction was found for any variable, i.e. populations were equally affected by drought. These results do not support the hypothesis of a genetic association between salinity and drought tolerance.     

Contrasting patterns of quantitative and neutral genetic variation in locally adapted populations of the natterjack toad, Bufo calamita

The relative importance of natural selection and genetic drift in determining patterns of phenotypic diversity observed in nature is still unclear. The natterjack toad (Bufo calamita) is one of a few amphibian species capable of breeding in saline ponds, even though water salinity represents a considerable stress for them. Results from two common-garden experiments showed a pattern of geographic variation in embryonic salinity tolerance among populations from either fresh or brackish environments, consistent with the hypothesis of local adaptation. Full-sib analysis showed increased variation in survival among sibships within population for all populations as osmotic stress was increased (broad-sense heritability increased as salinity raised). Nevertheless, toads native to the brackish water environment had the highest overall survival under brackish conditions. Levels of population genetic differentiation for salinity tolerance were higher than those of neutral genetic differentiation, the latter obtained through the analysis of eight microsatellite loci. Microsatellite markers also revealed little population differentiation, lack of an isolation-by-distance pattern, and moderate gene flow connecting the populations. Therefore, environmental stress tolerance appears to have evolved in absence of geographic isolation, and consequently we reject the null hypothesis of neutral differentiation.     

Variation in morphological traits and trait asymmetry in field Drosophila serrata from marginal populations

Drosophila serrata occurs along the eastern coast of Australia with a southern range boundary near Sydney. To compare levels of phenotypic variation in marginal and central populations, we examined morphological variation in populations of this species from the southern range boundary and two more northerly populations. The populations differed for wing traits and there was an increase in wing size in the marginal locations which persisted under laboratory culture. The means of wing and bristle traits increased under laboratory culture, whereas wing trait coefficients of variation and variances decreased. Heritability estimates for wing size traits tended to be lower in the field compared with the laboratory, whereas bristle and crossvein length heritabilities were similar across environments. There was evidence for heritable variation in wing and bristle traits in both the marginal and more northern populations, suggesting that genetic variation was not limiting in marginal populations. Fluctuating asymmetry (FA) was also assessed as a measure of genomic and environmental stress. There were no consistent differences among populations for the FA of individual traits, or for a total FA score summed across traits. FA levels in field parents and laboratory‐reared progeny were similar. Overall, the results do not support the conjecture that levels of phenotypic and genetic variability differ between central and marginal D. serrata populations.     

Exposure to fluctuating salinity enhances free amino acid accumulation inTigriopus californicus (Copepoda)

Summary Intracellular concentrations of free amino acids (FAA) in the intertidal copepodTigriopus californicus increase in response to hyperosmotic stress and decrease in response to hypo-osmotic stress. The purpose of this study was to determine if exposure to repeated bouts of osmotic stress resulted in changes in FAA accumulation or the degree of FAA retention in subsequent episodes. Five groups ofT. californicus were exposed for 22 days to a fluctuating salinity regime which consisted of 24 h at 100% seawater followed by 24 h at either 90, 80, 70, 60 or 50% seawater (11 cycles). After the tenth exposure to 100% seawater, individuals from each treatment group were analyzed for alanine and proline concentration. Alanine and proline accumulation generally increased in proportion to the osmotic stress up to 60–100% seawater — additional osmotic stress failed to increase total accumulation. Prior exposure to fluctuating salinity increased the extent of alanine and proline retention observed upon transfer to a hypo-osmotic medium. The treatment group which had experienced the most extreme fluctuation (50–100% seawater) retained alanine and proline levels approximately 10- and 20-fold higher, respectively, than controls. A less severe salinity fluctuation was required to elicit this response for alanine (90–100% seawater) than for proline (60–100% seawater). Previous exposure to fluctuating salinity also resulted in increased alanine and proline accumulation in subsequent episodes of hyperosmotic stress. 24 h after transfer from 50 to 100% seawater, alanine and proline levels in the conditioned copepods were approximately 3- and 7-fold higher, respectively, than in copepods which had not been cycled. This facilitation in alanine and proline accumulation occurred after 10 and 11 cycles, respectively. Of the increased accumulation in alanine and proline, 7.0% and 22.5%, respectively, could be accounted for by the higher degree of FAA retention while under hypo-osmotic conditions.Abbreviation FAA free amino acids     

Photoperiod affects compensating developmental rate across latitudes in the damselfly Lestes sponsa

1. Although there is a great deal of theoretical and empirical data about the life history responses of time constraints in organisms, little is known about the latitude‐compensating mechanism that enables northern populations' developmental rates to compensate for latitude. To investigate the importance of photoperiod on development, offspring of the obligatory univoltine damselfly Lestes sponsa from two populations at different latitudes (53°N and 63°N) were raised in a common laboratory environment at both northern and southern photoperiods that corresponded to the sites of collection. 2. Egg development time was shorter under northern photoperiod regimes for both populations. However, the northern latitude population showed a higher phenotypic plasticity response to photoperiod compared with the southern latitude population, suggesting a genetic difference in egg development time in response to photoperiod. 3. Larvae from both latitudes expressed shorter larval development time and faster growth rates under northern photoperiod regimes. There was no difference in phenotypic plastic response between northern and southern latitude populations with regard to development time. 4. Data on field collected adults showed that adult sizes decreased with an increase in latitude. This adult size difference was a genetically fixed trait, as the same size difference between populations was also found when larvae were reared in the laboratory. 5. The results suggest phenotypic plasticity responses in life history traits to photoperiod, but also genetic differences between north and south latitude populations in response to photoperiod, which indicates the presence of a latitudinal compensating mechanism that is triggered by a photoperiod.     

Abscisic acid content at defined levels of bud dormancy and frost tolerance in two contrasting populations of Picea abies grown in a phytotron

Seedlings of a southern (Romanian) and a northern (Swedish) population of Picea abies were cultivated under continuous light and 20°C for 10 weeks. To arrest growth, induce terminal bud dormancy and promote frost tolerance the seedlings were then exposed to 16 h nights for 12 weeks, with gradually lower temperature during the last 6 weeks. Samples for estimating the abscisic acid content of the needles were taken just before the onset of the night treatment, at day 3 of the treatment, and then with one, and later 2 week, intervals. From the second week onwards (third week for frost tolerance) bud dormancy and frost tolerance were assessed at the same time as abscisic acid (ABA) determinations. Phosphate-buffered saline extracts were purified on mini-columns (in some cases immunoaffinity colums) and quantified by HPLC. The degree of dormancy was estimated by transferring the seedlings to growth conditions and determining the number of days until growth was resumed. The frost tolerance of the needles exposed to –10°C and –20°C was classified in 6 classes. The frost tolerance of the terminal buds was estimated as the number of seedlings that showed some growth after 6 weeks in growth conditions. The night treatment rapidly induced terminal bud dormancy in both populations, but the release of dormancy occurred earlier in the northern population. The needles and the terminal buds became highly frost tolerant more rapidly in the northern than in the southern population and before the temperature decrease. The degree of dormancy began to decline before full frost tolerance was obtained in the southern population and this decline continued in both populations, while frost tolerance remained at a high level. The southern population showed a transient peak in ABA content at day 3. Although the ABA content of the northern population was lower than in the southern before the 16-h night treatment, it increased in the northern population during the treatment period, in particular after the temperature decrease.     

Mixed support for an alignment between phenotypic plasticity and genetic differentiation in damselfly wing shape

The relationship between genetic differentiation and phenotypic plasticity can provide information on whether plasticity generally facilitates or hinders adaptation to environmental change. Here, we studied wing shape variation in a damselfly (Lestes sponsa) across a latitudinal gradient in Europe that differed in time constraints mediated by photoperiod and temperature. We reared damselflies from northern and southern populations in the laboratory using a reciprocal transplant experiment that simulated time-constrained (i.e. northern) and unconstrained (southern) photoperiods and temperatures. After emergence, adult wing shape was analysed using geometric morphometrics. Wings from individuals in the northern and southern populations differed significantly in shape when animals were reared in their respective native environment. Comparing wing shape across environments, we found evidence for phenotypic plasticity in wing shape, and this response differed across populations (i.e. G × E interactions). This interaction was driven by a stronger plastic response by individuals from the northern population and differences in the direction of plastic wing shape changes among populations. The alignment between genetic and plastic responses depended on the specific combination of population and rearing environment. For example, there was an alignment between plasticity and genetic differentiation under time-constrained, but not under non-time-constrained conditions for forewings. We thus find mixed support for the hypothesis that environmental plasticity and genetic population differentiation are aligned. Furthermore, although our laboratory treatments mimicked the natural climatic conditions at northern and southern latitudes, the effects of population differences on wing shape were two to four times stronger than plastic effects. We discuss our results in terms of time constraints and the possibility that natural and sexual selection is acting differently on fore- and hindwings.     

Comparative germination responses to water potential across different populations of Aegilops geniculata and cultivar varieties of Triticum durum and Triticum aestivum

• Crop Wild Relatives are often used to improve crop quality and yields because they contain genetically important traits that can contribute to stress resistance and adaptation.
• Seed germination of different populations of Aegilops geniculata Roth collected along a latitudinal gradient was studied under different drought stress in order to find populations suitable for improving drought tolerance in wheat. Different accessions of Aegilops neglecta Req. ex Bertol., Triticum aestivum L. and T. durum Desf. were used as comparison.
• Under full hydration, germination was high in all populations, but increasing drought stress led to reduced and delayed germination. Significant differences in final germination and mean time to germinate were detected among populations. Wheat, durum wheat and the southern population of Ae. geniculata were not significantly affected by drought stress, germinating similarly under all treatments. However, seed germination of the northern populations of Ae. geniculata was significantly reduced under high water stress treatment. Differences between populations of the same species could not be explained by annual rainfall across populations’ distributions, but by rainfall during seed development and maturation.
• Differences in the germination responses to drought found here highlight the importance of source populations as criteria for genotype selection for pre‐breeders.

     

Developmental acclimation affects clinal variation in stress resistance traits in Drosophila buzzatii

Patterns of clinal genetic variation in Drosophila are often characterized after rearing at constant temperatures. However, clinal patterns might change after acclimation if populations differ in their plastic response to fluctuating environments. We studied longevity, starvation and heat knock‐down resistance after development at either constant or fluctuating temperatures in nine Drosophila buzzatii populations collected along an altitudinal gradient in Tenerife, Spain. Flies that developed at fluctuating temperatures had higher stress resistance despite experiencing a slightly lower average temperature than those at constant temperatures. Genetic variation along the gradient was found in both stress‐resistance traits. Because Q_ST values greatly exceeded F_ST values, genetic drift could not explain this diversification. In general, differences among populations were larger after rearing at fluctuating temperatures, especially in heat knock‐down, for which clinal patterns disappeared when flies were reared at constant temperatures. This result emphasizes the importance of determining whether populations originating from different environments differ in their plastic responses to stress.     

Adaptive phenotypic plasticity and genetics of larval life histories in two Rana temporaria populations

Phenotypic plasticity provides means for adapting to environmental unpredictability. In terms of accelerated development in the face of pond-drying risk, phenotypic plasticity has been demonstrated in many amphibian species, but two issues of evolutionary interest remain unexplored. First, the heritable basis of plastic responses is poorly established. Second, it is not known whether interpopulational differences in capacity to respond to pond-drying risk exist, although such differences, when matched with differences in desiccation risk would provide strong evidence for local adaptation. We investigated sources of within- and among-population variation in plastic responses to simulated pond-drying risk (three desiccation treatments) in two Rana temporaria populations originating from contrasting environments: (1) high desiccation risk with weak seasonal time constraint (southern population); and (2) low desiccation risk with severe seasonal time constraint (northern population). The larvae originating from the environment with high desiccation risk responded adaptively to the fast decreasing water treatment by accelerating their development and metamorphosing earlier, but this was not the case in the larvae originating from the environment with low desiccation risk. In both populations, metamorphic size was smaller in the high-desiccation-risk treatment, but the effect was larger in the southern population. Significant additive genetic variation in development rate was found in the northern and was nearly significant in the southern population, but there was no evidence for genetic variation in plasticity for development rates in either of the populations. No genetic variation for plasticity was found either in size at metamorphosis or growth rate. All metamorphic traits were heritable, and additive genetic variances were generally somewhat higher in the southern population, although significantly so in only one trait. Dominance variances were also significant in three of four traits, but the populations did not differ. Maternal effects in metamorphic traits were generally weak in both populations. Within-environment phenotypic correlations between larval period and metamorphic size were positive and genetic correlations negative in both populations. These results suggest that adaptive phenotypic plasticity is not a species-specific fixed trait, but evolution of interpopulational differences in plastic responses are possible, although heritability of plasticity appears to be low. The lack of adaptive response to desiccation risk in northern larvae is consistent with the interpretation that selection imposed by shorter growing season has favored rapid development in north (approximately 8% faster development in north as compared to south) or a minimum metamorphic size at the expense of phenotypic plasticity.