Department Aquatic Ecology

Adaptive divergence in response to environmental stress – amphibians along an acidification gradient

Why this project?  Environmental stress is a major ecological and evolutionary force, driving shifts in species distribution and intra-specific phenotypic divergence. We currently study the role of physiology in adaptive divergence to environmental stress and the genomic basis of physiology and maternal effect mediated adaptation.

Acid stress: Human induced acidification has resulted in severe reductions in pH since the industrial revolution, in both aquatic and terrestrial systems.  Acid stress has strong negative consequences on organismal fitness and thereby causes strong natural selection. Acid stress reduces individual growth rates, survival and reproduction. Furthermore, acidification causes large shifts in ecological communities (e.g. predators, parasites and fungal pathogens) and abiotic factors (e.g. increased metal leeching). These correlated changes can result in multifarious selection in response to acidification. Natural populations inhabiting environments that differ in pH due to natural and/or human induced acidification provide a good model system for testing how organisms adapt to both pH and interacting environmental stressors.

Evolutionary physiology (NEW SNF PROJECT): In vertebrates, corticosterone (CORT) is a key stress hormone. CORT levels are often elevated in short term stress responses - to facilitate adaptive physiological responses - but can come at a cost under long-term stress.
Moreover, the CORT hormonal pathway influences the expression of gene networks related to metabolism, growth, repair, reproduction, the management of resource allocation and immune function. Hence, populations differing in the magnitude or type of environmental stress should be under divergent selection on physiological stress responses.
Of key importance in relation to acidification mediated natural selection in amphibians, is that CORT affects behavioral activity and predator defense traits (e.g. tail depth), that CORT levels are elevated in response to acidic pH and predator presence and that trematode parasites (that use tadpoles as hosts) are lost in acidic environments. As pH, predation and parasites change simultaneously due to acidification we test the predictions that I) acidification causes divergent selection via acid stress tolerance, predator defense and immune defense traits and II) the responses to these correlated selective agents can be mediated through the CORT pathway (providing a “magic” link to multifarious divergent selection).

Egg coats: Maternal effects are often key adaptations during early life-stages, and can allow rapid adaptation or alter direction of evolution. One important, but relatively little studied, source of maternal effects in various taxa are egg coats (maternally derived extracellular matrix that surrounds the embryos). These can have strong effects on reproductive success in natural populations, as they can influence sperm-egg interactions as well embryonic performance under different abiotic and biotic conditions. In amphibians, the egg coats are multilayered structures consisting primarily of glycoproteins. In acidic conditions, egg coats undergo chemical changes and can become a trap for the developing embryo (i.e. so called “curling defect” that results in hatching failure).

Study system: In this core project, we study adaptive divergence of amphibian populations (focusing on the moor frog, Rana arvalis) along an acidification gradient in SW Sweden. In this area, amphibian breeding pond pH can range from 4 to 7. 

Approaches: We apply a combination of empirical approaches, ranging from laboratory common garden experiments, quantitative genetics and standard population genetics approaches to – omics tools (genomics, proteomics, transcriptomics, glycomics).

What have we found so far?

  • R. arvalis (RA) shows adaptive divergence in embryonic acid stress tolerance, larval life-history traits, anti-predator defenses and maternal investment along the acidification gradient.
  • The degree of phenotypic divergence in RA is related to the magnitude of differences in breeding pond pH, indicating that pH reflects variation in the strength of divergent selection. Moreover, RA appears to be responding to multiple selective agents simultaneously (e.g. pH and predators).
  • The mechanisms underlying divergence in RA include both maternal (esp. egg coats and egg size) and direct genetic effects.
  • Egg jelly coats harbour extensive within species molecular variation, whereby jelly coat glycoproteins influence water balance at acidic pH (providing a mechanism for the curling defect).

Ongoing and upcoming: Ongoing work focuses on the molecular and physiological underpinnings of adaptive divergence in relation to pH and correlated selective agents. We now use transcriptomics and SNP genotyping to investigate the molecular genetic basis of adaptive divergence in R. arvalis. In addition to physiological processes, a new dimension is to investigate the role of trematode parasites as agent of natural selection in tadpoles.

Key collaborators: Prof. Anssi Laurila (Uppsala University, Sweden), Dr. Otto Seppälä (Eawag/ETH-Z, Switzerland), Dr. Marc Suter (Eawag, Switzerland), Dr. Longfei Shu (Washington Univ., St Louis, USA).

Related publications (own student papers):

Conceptual reviews:

Shu, L, Suter, M-F & Räsänen, K*. (2015). Evolution of egg coats: linking molecular biologyand ecology. Mol. Ecol., 24, 4052-73 (Invited reviews and syntheses).

Räsänen, K & Hendry, AP. (2008) Disentangling interactions between adaptive divergence
and gene flow when ecology drives diversification. Ecol. Lett. 11, 624-636.

Räsänen, K & Kruuk, LEB. (2007) Maternal effects and evolution at ecological time scales. Func. Ecol. 21, 408-421.

Empirical:

Shu L, Laurila A, Suter M, Räsänen K. (2016). Molecular phenotyping of maternally mediated parallel adaptive divergence within Rana arvalis and Rana temporaria. Mol. Ecol., 25, 4564 -4579

Shu L, Suter M-F, Laurila, A & Räsänen, K. (2015) Environmental stress mediated adaptive divergence in ion channel function during embryogenesis in Rana arvalis, Sci. Rep. Sci. Rep. doi: 10.1038/srep14201.

Shu, L, Suter M-F, Laurila, A & Räsänen, K. (2015). Mechanistic basis of adaptive maternal effects: egg jelly water balance mediates embryonic adaptation to acidity in Rana arvalis. Oecologia 179, 617-28.

Egea-Serrano, A, Hangartner, S, Laurila, A & Räsänen, K. (2014) Multifarious selection through environmental stress: pH and predation driven adaptive divergence of the moor frog (Rana arvalis). Proc. R. Soc. B. 281, 20133266.

Hangartner, S, Laurila, A & Räsänen K. (2012) The quantitative genetic basis of adaptive
divergence in the moor frog (Rana arvalis) and its implications for gene flow. J. Evol. Biol. 66, 867-881.

People

Publications

Conceptual

Evolution of egg coats: linking molecular biology and ecology

One central goal of evolutionary biology is to explain how biological diversity emerges and is maintained in nature. Given the complexity of the phenotype and the multifaceted nature of inheritance, modern evolutionary ecological studies rely heavily on the use of molecular tools. Here, we show how molecular tools help to gain insight into the role of egg coats (i.e. the extracellular structures surrounding eggs and embryos) in evolutionary diversification. Egg coats are maternally derived structures that have many biological functions from mediating fertilization to protecting the embryo from environmental hazards. They show great molecular, structural and functional diversity across species, but intraspecific variability and the role of ecology in egg coat evolution have largely been overlooked. Given that much of the variation that influences egg coat function is ultimately determined by their molecular phenotype, cutting-edge molecular tools (e.g. proteomics, glycomics and transcriptomics), combined with functional assays, are needed for rigorous inferences on their evolutionary ecology. Here, we identify key research areas and highlight emerging molecular techniques that can increase our understanding of the role of egg coats in the evolution of biological diversity, from adaptation to speciation.
Shu, L.; Suter, M. J. -F.; Räsänen, K. (2015) Evolution of egg coats: linking molecular biology and ecology, Molecular Ecology, 24(16), 4052-4073, doi:10.1111/mec.13283, Institutional Repository

Disentangling interactions between adaptive divergence and gene flow when ecology drives diversification

Adaptive diversification is driven by selection in ecologically different environments. In absence of geographical barriers to dispersal, this adaptive divergence (AD) may be constrained by gene flow (GF). And yet the reverse may also be true, with AD constraining GF (i.e. 'ecological speciation'). Both of these causal effects have frequently been inferred from the presence of negative correlations between AD and GF in nature - yet the bi-directional causality warrants caution in such inferences. We discuss how the ability of correlative studies to infer causation might be improved through the simultaneous measurement of multiple ecological and evolutionary variables. On the one hand, inferences about the causal role of GF can be made by examining correlations between AD and the potential for dispersal. On the other hand, inferences about the causal role of AD can be made by examining correlations between GF and environmental differences. Experimental manipulations of dispersal and environmental differences are a particularly promising approach for inferring causation. At present, the best studies find strong evidence that GF constrains AD and some studies also find the reverse. Improvements in empirical approaches promise to eventually allow general inferences about the relative strength of different causal interactions during adaptive diversification.
Räsänen, K.; Hendry, A. P. (2008) Disentangling interactions between adaptive divergence and gene flow when ecology drives diversification, Ecology Letters, 11(6), 624-636, doi:10.1111/j.1461-0248.2008.01176.x, Institutional Repository

Maternal effects and evolution at ecological time-scales

1. Genetic and environmental maternal effects can play an important role in the evolutionary dynamics of a population: they may have a substantial impact on the rate and direction of genetic change in response to selection, and they may generate immediate phenotypic change via phenotypic plasticity. Because of this potential to generate rapid phenotypic change in a population, maternal effects may be particularly important for evolution at ecological time-scales.
2. Despite an increased interest in the prevalence, composition and adaptive benefits of maternal effects, little is still known of their impact on ecological and evolutionary processes in natural populations. We consider here the insights that a quantitative genetic framework provides into the pathways by which maternal effects can influence trait evolution in wild populations. Widespread evidence for a genetic basis of a range of maternal effects traits reinforces the notion that they cannot be treated as purely environmental sources of variation. We also provide an overview of the impact of environmental conditions on the expression and impact of maternal effects, and describe empirical evidence for their impact on evolution at ecological time-scales.
3. We emphasize the need for empirical work to quantify the associations between maternal and offspring phenotype and genotype, and the suite of selection pressures generated by maternal effects, as well as the relationship between maternal effects and environmental variation. Future work should aim to identify the conditions under which maternal effects are likely to play a role in evolution, as well as explicitly test the contribution of maternal effects to evolutionary responses.
Räsänen, K.; Kruuk, L. E. B. (2007) Maternal effects and evolution at ecological time-scales, Functional Ecology, 21(3), 408-421, doi:10.1111/j.1365-2435.2007.01246.x, Institutional Repository

Empirical

Mechanistic basis of adaptive maternal effects: egg jelly water balance mediates embryonic adaptation to acidity in Rana arvalis

Environmental stress, such as acidification, can challenge persistence of natural populations and act as a powerful evolutionary force at ecological time scales. The ecological and evolutionary responses of natural populations to environmental stress at early life-stages are often mediated via maternal effects. During early life-stages, maternal effects commonly arise from egg coats (the extracellular structures surrounding the embryo), but the role of egg coats has rarely been studied in the context of adaptation to environmental stress. Previous studies on the moor frog Rana arvalis found that the egg coat mediated adaptive divergence along an acidification gradient in embryonic acid stress tolerance. However, the exact mechanisms underlying these adaptive maternal effects remain unknown. Here, we investigated the role of water balance and charge state (zeta potential) of egg jelly coats in embryonic adaptation to acid stress in three populations of R. arvalis. We found that acidic pH causes severe water loss in the egg jelly coat, but that jelly coats from an acid-adapted population retained more water than jelly coats from populations not adapted to acidity. Moreover, embryonic acid tolerance (survival at pH 4.0) correlated with both water loss and charge state of the jelly, indicating that negatively charged glycans influence jelly water balance and contribute to embryonic adaptation to acidity. These results indicate that egg coats can harbor extensive intra-specific variation, probably facilitated in part via strong selection on water balance and glycosylation status of egg jelly coats. These findings shed light on the molecular mechanisms of environmental stress tolerance and adaptive maternal effects.
Shu, L.; Suter, M. J. -F.; Laurila, A.; Räsänen, K. (2015) Mechanistic basis of adaptive maternal effects: egg jelly water balance mediates embryonic adaptation to acidity in Rana arvalis, Oecologia, 179(3), 617-628, doi:10.1007/s00442-015-3332-4, Institutional Repository

Multifarious selection through environmental change: acidity and predator-mediated adaptive divergence in the moor frog (Rana arvalis)

Environmental change can simultaneously cause abiotic stress and alter biological communities, yet adaptation of natural populations to co-changing environmental factors is poorly understood. We studied adaptation to acid and predator stress in six moor frog (Rana arvalis) populations along an acidification gradient, where abundance of invertebrate predators increases with increasing acidity of R. arvalis breeding ponds. First, we quantified divergence among the populations in anti-predator traits (behaviour and morphology) at different rearing conditions in the laboratory (factorial combinations of acid or neutral pH and the presence or the absence of a caged predator). Second, we evaluated relative fitness (survival) of the populations by exposing tadpoles from the different rearing conditions to predation by free-ranging dragonfly larvae. We found that morphological defences (relative tail depth) as well as survival of tadpoles under predation increased with increasing pond acidity (under most experimental conditions). Tail depth and larval size mediated survival differences among populations, but the contribution of trait divergence to survival was strongly dependent on prior rearing conditions. Our results indicate that R. arvalis populations are adapted to the elevated predator pressure in acidified ponds and emphasize the importance of multifarious selection via both direct (here: pH) and indirect (here: predators) environmental changes.
Egea-Serrano, A.; Hangartner, S.; Laurila, A.; Räsänen, K. (2014) Multifarious selection through environmental change: acidity and predator-mediated adaptive divergence in the moor frog (Rana arvalis), Proceedings of the Royal Society B: Biological Sciences, 281(1780), 20133266 (10 pp.), doi:10.1098/rspb.2013.3266, Institutional Repository

The quantitative genetic basis of adaptive divergence in the moor frog (Rana arvalis) and its implications for gene flow

Knowledge on the relative contribution of direct genetic, maternal and environmental effects to adaptive divergence is important for understanding the drivers of biological diversification. The moor frog (Rana arvalis) shows adaptive divergence in embryonic and larval fitness traits along an acidification gradient in south-western Sweden. To understand the quantitative genetic basis of this divergence, we performed reciprocal crosses between three divergent population pairs and reared embryos and larvae at acid and neutral pH in the laboratory. Divergence in embryonic acid tolerance (survival) was mainly determined by maternal effects, whereas the relative contributions of maternal, additive and nonadditive genetic effects in larval life-history traits differed between traits, population pairs and rearing environments. These results emphasize the need to investigate the quantitative genetic basis of adaptive divergence in multiple populations and traits, as well as different environments. We discuss the implications of our findings for maintenance of local adaptation in the context of migrant and hybrid fitness.
Hangartner, S.; Laurila, A.; Räsänen, K. (2012) The quantitative genetic basis of adaptive divergence in the moor frog (Rana arvalis) and its implications for gene flow, Journal of Evolutionary Biology, 25(8), 1587-1599, doi:10.1111/j.1420-9101.2012.02546.x, Institutional Repository

Adaptive divergence in moor frog (Rana arvalis) populations along an acidification gradient: inferences from QST-FST correlations

Microevolutionary responses to spatial variation in the environment seem ubiquitous, but the relative role of selection and neutral processes in driving phenotypic diversification remain often unknown. The moor frog (Rana arvalis) shows strong phenotypic divergence along an acidification gradient in Sweden. We here used correlations among population pairwise estimates of quantitative trait (PST or QST from common garden estimates of embryonic acid tolerance and larval life-history traits) and neutral genetic divergence (FST from neutral microsatellite markers), as well as environmental differences (pond pH, predator density, and latitude), to test whether this phenotypic divergence is more likely due to divergent selection or neutral processes. We found that trait divergence was more strongly correlated with environmental differences than the neutral marker divergence, suggesting that divergent natural selection has driven phenotypic divergence along the acidification gradient. Moreover, pairwise PSTs of embryonic acid tolerance and QSTs of metamorphic size were strongly correlated with breeding pond pH, whereas pairwise QSTs of larval period and growth rate were more strongly correlated with geographic distance/latitude and predator density, respectively. We suggest that incorporating measurements of environmental variation into QST–FST studies can improve our inferential power about the agents of natural selection in natural populations.
Hangartner, S.; Laurila, A.; Räsänen, K. (2012) Adaptive divergence in moor frog (Rana arvalis) populations along an acidification gradient: inferences from QST-FST correlations, Evolution, International Journal of Organic Evolution, 66(3), 867-881, doi:10.1111/j.1558-5646.2011.01472.x, Institutional Repository

Geographic variation in maternal investment: acidity affects egg size and fecundity in Rana arvalis

Environmental-stress-mediated geographic variation in reproductive parameters has been little studied in natural vertebrate populations outside the context of climatic variation. Based on life-history theory, an increase in the degree of environmental stress experienced by a population should lead to (1) a shift in reproductive allocation from fecundity to offspring quality, (2) stronger trade-offs between reproductive parameters, and (3) changes in the relationship between female phenotype and maternal investment. To test these predictions, we investigated geographic variation in maternal investment of moor frogs (Rana arvalis) in relation to breeding site acidity (pH 4–8). We found that mean egg size increased and clutch size and total reproductive output (TRO) decreased with increasing acidity among 19 Swedish moor frog populations. Tests for variation and co-variation in maternal investment and female size and age in 233 females from a subset of four acid origin (AO) and four neutral origin (NO) populations revealed that clutch size and TRO increased with female size in both acid and neutral environments. However, in AO populations, egg size also increased with female size, and clutch size and TRO with female age, whereas in NO populations, egg size increased with female age. The strength of the egg-size–clutch-size trade-off tended to be stronger in AO than in NO females as expected if the former experience stronger environmental constraints. All in all, these results suggest that environmental acidification selects for investment in larger eggs at a cost to fecundity, imposes negative effects on reproductive output, and alters the relationship between female phenotype and maternal investment.
Rasanen, K.; Söderman, F.; Laurila, A.; Merilä, J. (2008) Geographic variation in maternal investment: acidity affects egg size and fecundity in Rana arvalis, Ecology, 89(9), 2553-2562, doi:10.1890/07-0168.1, Institutional Repository

Maternally determined adaptation to acidity in Rana arvalis: are laboratory and field estimates of embryonic stress tolerance congruent?

Geographic variation indicating local adaptation, as well as its quantitative genetic basis, is commonly investigated in common garden experiments in the laboratory. However, the applicability of laboratory results to the complex conditions experienced by populations in the wild may be limited. Our previous laboratory experiments showed maternally determined local adaptation in embryonic acid-stress tolerance (viz. survival) of the moor frog, Rana arvalis Nilsson, 1842. Here we tested whether this laboratory finding holds even when embryos are exposed to acid stress in the wild. We conducted reciprocal crosses between an acid-origin population and a neutral-origin population of R. arvalis and transplanted the embryos to an acid site (pH ~ 4) in the field. Embryonic survival was Much lower in the field experiment than in previous laboratory experiments, but, consistent with laboratory work, embryos from acid-origin females had threefold higher survival than embryos from neutral-origin females. These results suggest that laboratory tests can provide appropriate estimates of among population variation, as well as the quantitative genetic basis of acid-stress tolerance in amphibians.
Persson, M.; Räsänen, K.; Laurila, A.; Merilä, J. (2007) Maternally determined adaptation to acidity in Rana arvalis: are laboratory and field estimates of embryonic stress tolerance congruent?, Canadian Journal of Zoology, 85(7), 832-838, doi:10.1139/Z07-064, Institutional Repository

Adaptive plasticity in stressful environments: acidity constrains inducible defences in Rana arvalis

Questions: How do environmental stressors affect the expression of adaptive phenotypic plasticity? Is there inter-population variation in these effects?
Hypothesis: Acid stress constrains the expression of inducible defences by decreasing investment in defences or by increasing the costs of investment. Organisms originating from neutral environments suffer more from acid stress than organisms originating from acid environments.
Organism: Tadpoles of Rana arvalis, originating from two different populations (acid and neutral). This species displays inducible defences in response to insect predators (here dragonfly larvae).
Methods: A laboratory experiment with a factorial design crossing two factors: predator presence (present vs. absent) and acidity (neutral vs. acid). We tested the effects of experimental treatment on tadpole morphology as well as age and size at metamorphosis.
Results: Tadpoles from the neutral origin population invested less in inducible defences (tail fin depth) in the acid than in the neutral treatment. In contrast, tadpoles from the acid origin population were able to respond equally well to predators in both pH treatments. pH-related costs differed between populations: while tadpoles from the neutral origin population suffered from acid stress in terms of reduced developmental rate, those from the acid origin population seemed to suffer from neutral stress in terms of reduced size at metamorphosis.
Teplitsky, C.; Räsänen, K.; Laurila, A. (2007) Adaptive plasticity in stressful environments: acidity constrains inducible defences in Rana arvalis, Evolutionary Ecology Research, 9(3), 447-458, Institutional Repository

Maternal investment in egg size: environment- and population-specific effects on offspring performance

Geographic variation in maternal investment in offspring size can be adaptive if differences in investment translate into improved offspring performance in the given environments. We compared two moor frog, Rana arvalis, populations in the laboratory to test the hypothesis that investment in large eggs in populations originating from stressful (acid) environments improves offspring performance when reared in stressful (acid) conditions. We found that large initial size (hatchling mass) had moderate to strong, environment-dependent positive effects on larval and metamorphic traits in the acidic origin population, but only weak effects in the neutral origin population. Our results suggest that interactions between environmental conditions and initial size can be important determinants of individual performance, and that investment in large eggs is adaptive in acid environments. These findings emphasize the role of maternal effects as adaptations to environmental stress.
Räsänen, K.; Laurila, A.; Merilä, J. (2005) Maternal investment in egg size: environment- and population-specific effects on offspring performance, Oecologia, 142(4), 546-553, doi:10.1007/s00442-004-1762-5, Institutional Repository

Local adaptation and genetics of acid-stress tolerance in the moor frog, Rana arvalis

As potential to adapt to environmental stress can be essential for population persistence, knowledge on the genetic architecture of local adaptation is important for conservation genetics. We investigated the relative importance of additive genetic, dominance and maternal effects contributions to acid stress tolerance in two moor frog (Rana arvalis) populations originating from low and neutral pH habitats. Experiments with crosses obtained from artificial matings revealed that embryos from the acid origin population were more tolerant to low pH than embryos from the neutral origin population in embryonic survival rates, but not in terms of developmental stability, developmental and growth rates. Strong maternal effect and small additive genetic contributions to variation were detected in all traits in both populations. In general, dominance contributions to variance in different traits were of similar magnitude to the additive genetic effects, but dominance effects outweighed the additive genetic and maternal effects contributions to early growth in both populations. Furthermore, the expression of additive genetic variance was independent of pH treatment, suggesting little additive genetic variation in acid stress tolerance. The results suggest that although local genetic adaptation to acid stress has taken place, the current variation in acid stress tolerance in acidified populations may owe largely to non-genetic effects. However, low but significant heritabilities (h2 ≈ 0.07-0.22) in all traits – including viability itself – under a wide range of pH conditions suggests that environmental stress created by low pH is unlikely to lower moor frog populations' ability to respond to selection in the traits studied. Nevertheless, acid conditions could lower populations' ability to respond to selection in the long run through reduction in effective population size.
Merilä, J.; Söderman, F.; O'Hara, R.; Räsänen, K.; Laurila, A. (2004) Local adaptation and genetics of acid-stress tolerance in the moor frog, Rana arvalis, Conservation Genetics, 5(4), 513-527, doi:10.1023/B:COGE.0000041026.71104.0a, Institutional Repository

Geographic variation in acid stress tolerance of the moor frog, Rana arvalis. I. Local adaptation

Spatially varying directional selection together with restricted gene flow among populations is expected to lead to local adaptation. One environmental factor that potentially causes strong directional selection, but is little explored in evolutionary terms, is naturally and anthropogenically induced acidity. We studied local adaptation to acidity in four Swedish populations (two originating from areas that have suffered from severe anthropogenic acidification during the 1900s and two from areas which have remained neutral due to higher buffering capacity) of the moor frog Rana arvalis in a laboratory experiment by investigating whether differences in acid tolerance correspond to population origin. Embryos were raised from fertilization to hatching at three different pH levels (pH 4.0, 4.25 and 7.5), corresponding to levels experienced by these populations in nature, and acid stress tolerance was measured in terms of embryonic survival, hatchling size, and age. Evidence for local adaptation in all of these traits was found, the acid origin embryos having higher survival and less impaired growth performance under acid conditions than the neutral origin embryos. Our estimated rates of divergence (0.007-0.102 haldanes) suggest a rapid adaptation process in response to anthropogenic environmental change, and that the different traits have evolved at relatively similar rates.
Räsänen, K.; Laurila, A.; Merilä, J. (2003) Geographic variation in acid stress tolerance of the moor frog, Rana arvalis. I. Local adaptation, Evolution, International Journal of Organic Evolution, 57(2), 352-362, doi:10.1111/j.0014-3820.2003.tb00269.x, Institutional Repository

Carry-over effects of embryonic acid conditions on development and growth of Rana temporaria tadpoles

1. Conditions experienced during the early stages of development may have carry-over effects on performance during later life. The egg laying period and embryonic development of temperate and boreal zone amphibians often coincides with peak acidity resulting from spring snow-melt, but the effects of acid conditions during embryonic stage on subsequent performance are unknown.
2. We investigated the potential carry-over effects of acidity during the embryonic stage on performance up to metamorphosis in the common frog (Rana temporaria) tadpoles. There were four combinations of acid (4.5) and neutral (7.5) pH treatments applied to the egg and larval stages in a factorial laboratory experiment. In addition, we studied the difference in embryonic and larval tolerance of acidity between two populations originating from circumneutral (pH 6.6) and acidic conditions (pH 4.8).
3. The effects of acid conditions during the embryonic stage were sublethal, as indicated by delayed development and reduced size. Under acid conditions, tadpoles that had been raised in neutral water as embryos at first grew more slowly than tadpoles raised under acid conditions as embryos. At metamorphosis, no effects of embryonic acidity were detectable indicating that tadpoles were able to compensate fully for the initial reduction in growth.
4. Acid conditions during the larval period had a strongly negative effect on survival, size and age at metamorphosis. The amount of food consumed was lower under acid conditions, suggesting that reduced food consumption was at least partly responsible for the negative effects.
5. Although the two populations differed in the length of larval period, there was no indication of a differential response to the treatments in any of the metamorphic traits studied.
6. These results suggest that, although moderate acid conditions during embryonic development affect growth and development negatively, this influence does not persist after conditions have returned to normal. However, even moderately acid conditions during the larval period may have a strong negative influence on survival and performance of the tadpoles.
Räsänen, K.; Laurila, A.; Merilä, J. (2002) Carry-over effects of embryonic acid conditions on development and growth of Rana temporaria tadpoles, Freshwater Biology, 47(1), 19-30, doi:10.1046/j.1365-2427.2002.00777.x, Institutional Repository

Lethal and sublethal effects of UV-B/pH synergism on common frog embryos

Although the negative effects of ultraviolet-B (UV-B) radiation on the development of many amphibian species have been demonstrated, some species—such as the common frog (Rana temporaria)—seem to be tolerant of UV-B radiation. The amount of UV-B radiation received is likely to vary among populations of the same species, but little is known about geographic variation in UV-B tolerance. Similarly, although UV-B radiation can have synergistic effects with other stressors, no studies have focused on geographic variation of these effects on amphibians. We investigated the synergistic effects of UV-B radiation and low pH on hatchability and early development of R. temporaria embryos in a factorial laboratory experiment with animals originating from southern and northern Sweden. Newly fertilized eggs were exposed to three different UV-B treatments (no UV-B [control], 1.254 k/J/m2 [normal] and 1.584 k/J/m2 [26% enhanced]) and two pH treatments (4.5 [low] and 7.6 [neutral]). Ultraviolet-B radiation in combination with low pH lead to markedly (approximately 50%) reduced survival rates and increased (approximately 30%) frequency of developmental anomalies in the northern but not in the southern population. The UV-B-exposed embryos hatched at smaller size in the southern population, whereas low pH reduced hatchling size in both populations. In both populations and pH treatments, embryos in the normal UV-B treatment developed significantly faster than embryos in the enhanced or control UV-B treatments. No interaction between pH and UV-B on developmental rates or hatchling size was detected. The results demonstrate—contrary to earlier belief—that R. temporaria embryos are not insensitive to increased levels of UV-B radiation. The lethal effects of UV-B radiation may, however, become manifested only in combination with other stressors, such as low pH, and the effects of this synergism may differ among different populations of the same species.
Pahkala, M.; Räsänen, K.; Laurila, A.; Johanson, U.; Björn, L. O.; Merilä, J. (2002) Lethal and sublethal effects of UV-B/pH synergism on common frog embryos, Conservation Biology, 16(4), 1063-1073, doi:10.1046/j.1523-1739.2002.00527.x, Institutional Repository

Book chapters

Räsänen,K.; Green,D.M.; (2009) Acidification and its effects on amphibian populations. In Amphibian Biology, Volume 8. Decline: Diseases, Parasites, Maladies and Pollution. H. Heatwole (ed.), Surrey Beatty and Sons, Chipping Norton, Australia; 3244-3267.

Related student projects

Ph.D. theses

Shu Longfei (2014) The Molecular Basis of Embryonic Adaptation to Acid Stress in Amphibians. Diss. ETH Nr. 22319. Dept. of Aquatic Ecology, Eawag/Institute of Integrative Biology, ETH -Zurich, Switzerland.

Hangartner Sandra (2010) Adaptive divergence of the moor frog (Rana arvalis) along an acidification gradient. Diss ETH nr: 19384. Dept. of Aquatic Ecology, Eawag/Institute of Integrative Biology, ETH-Zurich, Switzerland.

M.Sc. theses

Schenkel Corinne (2013) The role of egg capsules in resistance to Saprolegnia in Rana arvalis and R. temporaria. Univ. of Zurich, Switzerland.

Krähenbühl Kim (2011) Among population divergence in embryonic response of Rana arvalis to interactions of pH and Saprolegnia infections. Dept. of Biology, ETH-Zurich, Switzerland.

Riedener Eliane (2010) Adaptive divergence in embryonic acid stress tolerance of Rana arvalis: the role of microhabitat variation in a reciprocal transplant experiment”. Dept.of Environmental sciences, ETH-Zurich, Switzerland.

Brunold Claudio (2009). Acid stress tolerance and adaptive maternal effects of Rana arvalis and Rana temporaria. Dept. of Biology, ETH-Zurich, Switzerland.

Persson Mikael (2003) Local adaptation to acid stress in the moor frog - a field experiment. Uppsala University, Sweden.

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