Anssi Laurila

Latitudinal countergradient variation in the common frog (Rana temporaria) development rates – evidence for local adaptation

Adaptive genetic differentiation along a climatic gradient as a response to natural selection is not necessarily expressed at phenotypic level if environmental effects on population mean phenotypes oppose the genotypic effects. This form of cryptic evolution – called countergradient variation – has seldom been explicitly demonstrated for terrestrial vertebrates. We investigated the patterns of phenotypic and genotypic differentiation in developmental rates of common frogs (Rana temporaria) along a ca. 1600 km latitudinal gradient across Scandinavia. Developmental rates in the field were not latitudinally ordered, but displayed large variation even among different ponds within a given latitudinal area. In contrast, development rates assessed in the laboratory increased strongly and linearly with increasing latitude, suggesting a genetic capacity for faster development in the northern than the southern larvae. Experiments further revealed that environmental effects (temperature and food) could easily override the genetic effects on developmental rates, providing a possible mechanistic explanation as to why the genetic differentiation was not seen in the samples collected from the wild. Our results suggest that the higher developmental rates of the northern larvae are likely to be related to selection stemming from seasonal time constrains, rather than from selection dictated by low ambient temperatures per se. All in all, the results provide a demonstration of environmental effects concealing substantial latitudinally ordered genetic differentiation understandable in terms of adaptation to clinal variation in time constrains.

Latitudinal divergence of common frog (Rana temporaria) life history traits by natural selection: evidence from a comparison of molecular and quantitative genetic data

The relative roles of natural selection and direct environmental induction, as well as of natural selection and genetic drift, in creating clinal latitudinal variation in quantitative traits have seldom been assessed in vertebrates. To address these issues, we compared molecular and quantitative genetic differentiation between six common frog (Rana temporaria) populations along an approximately 1600 km long latitudinal gradient across Scandinavia. The degree of population differentiation (QST ≈ 0.81) in three heritable quantitative traits (age and size at metamorphosis, growth rate) exceeded that in eight (neutral) microsatellite loci (FST = 0.24). Isolation by distance was clear for both neutral markers and quantitative traits, but considerably stronger for one of the three quantitative traits than for neutral markers. QST estimates obtained using animals subjected to different rearing conditions (temperature and food treatments) revealed some environmental dependency in patterns of population divergence in quantitative traits, but in general, these effects were weak in comparison to overall patterns. Pairwise comparisons of FST and QST estimates across populations and treatments revealed that the degree of quantitative trait differentiation was not generally predictable from knowledge of that in molecular markers. In fact, both positive and negative correlations were observed depending on conditions where the quantitative genetic variability had been measured. All in all, the results suggest a very high degree of genetic subdivision both in neutral marker genes and genes coding quantitative traits across a relatively recently (< 9000 years) colonized environmental gradient. In particular, they give evidence for natural selection being the primary agent behind the observed latitudinal differentiation in quantitative traits.

ADAPTIVE PHENOTYPIC PLASTICITY AND GENETICS OF LARVAL LIFE HISTORIES IN TWO RANA TEMPORARIA POPULATIONS

Abstract 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 (∼8% faster development in north as compared to south) or a minimum metamorphic size at the expense of phenotypic plasticity.

Predator-induced changes in life history in two anuran tadpoles : effects of predator diet

We studied effects of the non-lethal presence of a predator on behaviour and larval life history of two species of anuran tadpoles, common frog, Rana temporaria, and common toad, Bufo bufo. We manipulated predator diet to study whether differences in chemically perceived predation risk affect the tadpoles. Tadpoles of both species were raised at two food levels either in the absence of the predator or in the presence of an insect-, frog- or toad-fed larval dragonfly Aeshna juncea, R. temporaria tadpoles lowered their activity in the presence of a predator and also avoided the predator spatially. Furthermore, they showed the strongest response to tadpole-fed predators. The number of days elapsed since the last provisioning of food affected the behaviour of both species. In R. temporaria, differences in activity level between predator treatments vanished as the food resources were depleted. In B. bufo, activity was lower in the presence of a toad-fed predator shortly after food had been added to the containers. Spatial avoidance of predators by B. bufo was stronger in the presence of a toad-fed predator than when an insect- or frog-fed predator was present. In both tadpole species growth rate was higher at the high food level but remained unaffected by the predator treatments. In both species individuals at higher food level metamorphosed earlier and at larger size. The metamorphic responses to predator treatments differed between the species. R. temporaria metamorphosed later and at larger size in the presence of tadpole-fed predators than in the control treatment or in the presence of an insect-fed predator. B. bufo metamorphosed earlier in the presence of a toad-fed predator, but this was only slightly correlated to metamorphic size. Manipulations of predator diet affected tadpole life history in both species. However, predator effects on larval life history were not mediated by tadpole behaviour in either species.

Reinventing the wheel : analysis of sexual dimorphism in body size

Sexual dimorphism in body size has received increasing scientific interest during the past decade. In most of the papers on the subject, the magnitude of sexual size dimorphism is expressed as some conversion of body size ratio. There are several possible flaws in using such ratios, however, the most serious one concers the scaling of body size and size differences between sexes. Using ratios may easily lead to wrong conclusions. In this paper we emphasize some fallacies related to ratios and present a less biased method, based on residuals of linear regression. This method is suggested to be used in comparative analyses of sexual size dimorphism.

High degree of population subdivision in a widespread amphibian

In general, amphibians are known to exhibit a higher degree of population subdivision than any other major animal taxa, but large‐scale population genetic surveys of widely distributed species are still scarce, especially in the Eurasian continent. Using microsatellite markers and mitochondrial DNA sequences, we investigated the large‐scale population genetic structure of the common frog (Rana temporaria) — one of the most widespread amphibians of the Palearctic region. Analyses of cytochrome b sequences revealed evidence for two distinct lineages inhabiting western and eastern parts of Europe. The separation of these lineages c. 700 000 years ago may have been induced by the onset of the Middle Pleistocene continental glaciations. Analyses of the variability of microsatellite loci within each of the clades revealed evidence for evolution of a high degree of population subdivision (FST ∼ 0.23) even in northern Fennoscandia, colonized less than 10 000 years ago. The high level of substructuring is puzzling in the face of an apparently high dispersal capacity, as evidenced by the rather rapid recolonization of northern Europe. This suggests that processes other than restricted dispersal capacity need to be explored as explanations for the high degree of population subdivision in amphibians. The colonization of northern Europe has been accompanied by loss of genetic variability as evidenced by decreasing levels of intrapopulational genetic variability in microsatellite loci from south to north across Europe.

ANTIPREDATOR DEFENSES ALONG A LATITUDINAL GRADIENT IN RANA TEMPORARIA

Antipredator defenses are expected to decrease toward higher latitudes because predation rates are predicted to decrease with latitude. However, latitudinal variation in predator avoidance and defense mechanisms has seldom been studied. We studied tadpole antipredator defenses in seven Rana temporaria populations collected along a 1500-km latitudinal gradient across Sweden, along which previous studies have found increasing tadpole growth and development rates. In a laboratory common garden experiment, we measured behavioral and morphological defenses by raising tadpoles in the presence and absence of a predator (Aeshna dragonfly larva) in two temperature treatments. We also estimated tadpole survival in the presence of free-ranging predators and compared predator densities between R. temporaria breeding ponds situated at low and high latitudes. Activity and foraging were generally positively correlated with latitude in the common garden experiment. While all populations responded to predator presence by decreasing activity and foraging, high-latitude populations maintained higher activity levels in the presence of the predator. All populations exhibited defensive morphology in body and tail shape. However, whereas tail depth tended to increase with latitude in the presence of predator, it did not change with latitude in the absence of the predator. Predator presence generally increased larval period and decreased growth rate. In the southern populations, predator presence tended to have a negative effect on metamorphic size, whereas in the northern populations predators had little or a positive effect on size. Latitude of origin had a strong effect on survival in the presence of a free-ranging predator, with high-latitude tadpoles experiencing higher mortality than those from the low latitudes. In the wild, predator densities were significantly lower in high-latitude than in mid-latitude breeding ponds. Although the higher activity level in the northern populations seems to confer a significant survival disadvantage under predation risk, it is probably needed to maintain the high growth and development rates. However, the occurrence of R. temporaria at high latitudes may be facilitated by the lower predator densities in the north.

Carry–over effects of ultraviolet–B radiation on larval fitness in Rana temporaria

A number of studies have failed to find evidence for negative effects of ultraviolet–B radiation (UVBR) on amphibian early–embryonic performance, leading to the conclusions, first, that the embryonic stages of many species are tolerant to UVBR, and second, that the increased amount of UVBR reaching the Earths surface is not likely to have any direct negative effects on many amphibian populations. However, possible carry–over effects of exposure to UVBR in the embryonic stages to the larval stages have received less attention. We studied the effects of UVBR experienced during the embryonic stages (age less than 11 days) on the later performance (age 11–75 days) of common frog, Rana temporaria, larvae. In a factorial laboratory experiment, newly fertilized embryos were divided into three different UVBR treatments (no UVBR (control), 1.25 kJm−2 (normal) and 1.58 kJm−2 (26% enhanced)), after which the individual larvae were raised until metamorphosis in the absence of UVBR. No effects of UVBR on embryonic survival rates, frequency of developmental anomalies or hatchling size were found, corroborating the earlier results indicating that R. temporaria embryos are tolerant to UVBR. However, analyses of larval performance revealed that larvae exposed to enhanced levels of UVBR as embryos suffered from an increased frequency of developmental anomalies and metamorphosed later and at a smaller size than larvae that had been protected from UVBR as embryos. These results suggest, in contrast to the earlier studies, that UVBR has direct negative effects on R. temporaria embryos, but these effects are expressed mostly or only during the later life stages. To this end, our results support the contention that carry–over effects from one life stage to another may be an important source of phenotypic variation in fitness.

Predator-induced plasticity in early life history and morphology in two anuran amphibians

Abstract. Predation pressure during early life stages is often high, but few studies have examined antipredator responses at these stages. We studied the effects of an egg predator (leech, Haemopis sanguisuga) and two tadpole predators (dragonfly larvae, Aeshna spp.; and threespine stickleback, Gasterosteus aculeatus) on the timing of hatching and morphology of hatchlings and young tadpoles in two anuran amphibians [Rana arvalis (RA) and R. temporaria (RT)] in a factorial laboratory experiment. We also compared the responses of two geographically separated RA populations on the Baltic island of Gotland and in Uppland on the Swedish mainland. We found inconsistent evidence for the predictions that the presence of an egg predator induces earlier hatching, and the presence of a larval predator delays hatching. RT hatched later in the presence of stickleback than in the control treatment, but RA hatched earlier, less developed and at smaller size in the leech, dragonfly, and stickleback treatments. There was no indication of predator-induced morphology in hatchlings of either of the species. However, young RA tadpoles had shorter tails and deeper bodies in the stickleback treatment and RT had shorter tails in the leech, dragonfly and stickleback treatments. Irrespective of treatment, RA from Gotland hatched with relatively longer bodies than Uppland individuals and had relatively deeper and short tails as young tadpoles. Our results highlight the diversity of induced responses to predators in anuran amphibians: predator presence affects the timing of hatching and morphology of young tadpoles, but these responses vary depending on the species and predator considered.

Impacts of Warming on the Structure and Functioning of Aquatic Communities : Individual-to Ecosystem-Level Responses

Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.