VARIABILITY IN SURVIVAL, GROWTH AND METAMORPHOSIS IN THE LARVAL FIRE SALAMANDER (SALAMANDRA SALAMANDRA): EFFECTS OF LARVAL BIRTH SIZE, SIBSHIP AND ENVIRONMENT

Food and light availability induce plastic responses in fire salamander larvae from contrasting environments

Phenotypic plasticity has been proposed as a mechanism facilitating the colonisation and adaptation to novel environments, such as caves. However, phenotypic plasticity in subterranean environments remains largely unexplored. Here, we test for plasticity in growth and development of fire salamander larvae (Salamandra salamandra) from subterranean and surface habitats, in response to contrasting food availability and light conditions. We hypothesized that: (i) low food availability and absence of light decrease larval growth and delay metamorphosis, (ii) light conditions mediate the effects of food availability on growth and time to metamorphosis, and (iii) larval response to contrasting light and food conditions is shaped by the habitat of origin. Our study showed that reduced food availability significantly delayed metamorphosis and slowed total length and body mass growth rates, while exposure to constant darkness slowed body mass growth rate. However, larvae slowed growth rates and increased time to metamorphosis without compromising size at metamorphosis. The effect of food availability on growth and time to metamorphosis did not change under different light conditions. Fire salamanders from subterranean and surface habitats responded differently only in relation to contrasting food availability conditions. Specifically, larvae from the surface habitat grew faster in high food conditions, while growth in larvae from the subterranean habitat was not influenced by food availability. Initial size also appeared to be an influential factor, since larger and heavier larvae grew slower, metamorphosed faster, and the size advantage was maintained in newly-metamorphosed juveniles. Overall, the results of our experiment suggest that plasticity and local adaptation favor the exploitation of aquatic subterranean habitats for breeding by fire salamanders, allowing successful development even under food shortage and day-length constraints, without compromising metamorphic size. Our findings have implications for conservation because they confirm that phenotypic plasticity plays a critical role in allowing fire salamanders to overcome altered environmental conditions.

Physical and ecological isolation contribute to maintain genetic differentiation between fire salamander subspecies

Landscape features shape patterns of gene flow among populations, ultimately determining where taxa lay along the continuum between panmixia to complete reproductive isolation. Gene flow can be restricted, leading to population differentiation in two non-exclusive ways: "physical isolation", in which geographic distance in combination with the landscape features restricts movement of individuals promoting genetic drift, and "ecological isolation", in which adaptive mechanisms constrain gene flow between different environments via divergent natural selection. In central Iberia, two fire salamander subspecies occur in parapatry across elevation gradients along the Iberian Central System mountains, while in the adjacent Montes de Toledo Region only one of them occurs. By integrating population and landscape genetic analyses, we show a ubiquitous role of physical isolation between and within mountain ranges, with unsuitable landscapes increasing differentiation between populations. However, across the Iberian Central System, we found strong support for a significant contribution of ecological isolation, with low genetic differentiation in environmentally homogeneous areas, but high differentiation across sharp transitions in precipitation seasonality. These patterns are consistent with a significant contribution of ecological isolation in restricting gene flow among subspecies. Overall, our results suggest that ecological divergence contributes to reduce genetic admixture, creating an opportunity for lineages to follow distinct evolutionary trajectories.

The Role of Plasticity and Adaptation in the Incipient Speciation of a Fire Salamander Population

Phenotypic plasticity and local adaptation via genetic change are two major mechanisms of response to dynamic environmental conditions. These mechanisms are not mutually exclusive, since genetic change can establish similar phenotypes to plasticity. This connection between both mechanisms raises the question of how much of the variation observed between species or populations is plastic and how much of it is genetic. In this study, we used a structured population of fire salamanders (Salamandra salamandra), in which two subpopulations differ in terms of physiology, genetics, mate-, and habitat preferences. Our goal was to identify candidate genes for differential habitat adaptation in this system, and to explore the degree of plasticity compared to local adaptation. We therefore performed a reciprocal transfer experiment of stream- and pond-originated salamander larvae and analyzed changes in morphology and transcriptomic profile (using species-specific microarrays). We observed that stream- and pond-originated individuals diverge in morphology and gene expression. For instance, pond-originated larvae have larger gills, likely to cope with oxygen-poor ponds. When transferred to streams, pond-originated larvae showed a high degree of plasticity, resembling the morphology and gene expression of stream-originated larvae (reversion); however the same was not found for stream-originated larvae when transferred to ponds, where the expression of genes related to reduction-oxidation processes was increased, possibly to cope with environmental stress. The lack of symmetrical responses between transplanted animals highlights the fact that the adaptations are not fully plastic and that some level of local adaptation has already occurred in this population. This study illuminates the process by which phenotypic plasticity allows local adaptation to new environments and its potential role in the pathway of incipient speciation.

Plasticity and evolutionary divergence in gene expression associated with alternative habitat use in larvae of the European Fire Salamander

Transcriptomes of organisms reveal differentiation associated with the use of different habitats. However, this leaves open how much of the observed differentiation can be attributed to genetic differences or to transcriptional plasticity. In this study, we disentangle causes of differential gene expression in larvae of the European fire salamander from the Kottenforst forest in Germany. Larvae inhabit permanent streams and ephemeral ponds and represent an example of a young evolutionary split associated with contrasting ecological conditions. We hypothesized that adaptation towards differences in water temperature plays a role because the thermal regime between stream and pond habitats differs notably. Tissue samples from tail fins of larvae were collected to study gene expression using microarrays. We found ample evidence for differentiation among larvae occupying different habitats in nature with 2,800 of 11,797 genes being differentially expressed. We then quantified transcriptional plasticity towards temperature and genetic differentiation based on controlled temperature laboratory experiments. Gene-by-environment interactions modelling revealed that 28% of the gene expression divergence observed among samples in nature could be attributed to plasticity related to water temperature. Expression patterns of only a small number of 101 genes were affected by the genotype. Our analysis demonstrates that effects of environmental factors must be taken into account to explain variation of gene expression in salamanders in nature. Notwithstanding, it provides first evidence that genetic factors determined gene expression divergence between pond and stream ecotypes and could be involved in adaptive evolution.

Plasticity of metamorphic traits in a high-altitude toad: interactive effects of food level and temperature

In organisms with complex life cycles, such as amphibians, morphological variation itself is strongly influenced by environmental factors and maternal effects. Although temperature and food level exert a strong influence on larval growth and development, little is known about the interacting effects of these factors on age and size at metamorphosis. In this study, plasticity in growth rates, larval mass, larval period, and body size at metamorphosis were experimentally examined for a high-altitude toad (Bufo minshanicus) under different combinations of temperature and food level. Larval period and mass at metamorphosis were sensitive to food level, and varied with temperature. At high food level, tadpoles reared at 29.8°C had shorter larval period lengths and larger mass at metamorphosis than those reared at 25.8 and 22.6°C, but not between 25.8 and 22.6°C. Interestingly, tadpoles at 29.8°C that were offered with a higher level of food supply achieved a larger size than those with a lower level of food supply; however, food supply did not affect body mass at the two lower temperature settings. Thus, the effects of food level were dependent on water temperature. Although there was high mortality at 29.8°C, surviving tadpoles have been much stronger to achieve faster growth and reach a larger mass at metamorphosis, which were positively correlated with juvenile survival and adult fecundity. Thus, under varied environmental conditions, we could say that there is more plasticity in development and growth of larvae in high altitude populations than in the same species or other species from low altitude populations.

Long-term consequences of early nutritional conditions on the behaviour and growth of fire salamanders

Early developmental conditions can have a strong influence on the life history. Fire salamanders represent an interesting system due to their biphasic life cycle consisting of an aquatic larval stage and a subsequent terrestrial stage. The environment experienced as larvae affects growth, age at metamorphosis and behaviour. In particular, diet restrictions can cause developmental stress. However, the impact of such developmental stress on the growth and behaviour in a long-lived amphibian are not yet well studied. We manipulated the early diet of sibling pairs of fire salamander larvae. One sibling was raised under food-restriction and the other was raised under conditions of abundant food. We report long-term effects on growth and exploratory behaviour; exploratory behaviour was tested in two sessions, first as young juveniles and later as sub-adults. We found that food-restricted conditions during early development affected exploratory behaviour. Early deficits in body mass and body size were compensated later in life, by approximately 18 months of age. When morphological differences were compensated, the initial differences in exploratory behaviour also disappeared. Thus, compensation not only led to an alignment of body parameters but was also accompanied by an adjustment in exploratory behaviour. No cost of compensation was detected, but future research will need to verify whether the potential costs of compensation are negligible in slow-growing salamanders or whether they are shifted to later life stages. Our study provides novel insights into the life history of fire salamanders and reveals that early larval conditions have effects on individuals long after metamorphosis.

Effects of Tail Clipping on Larval Performance and Tail Regeneration Rates in the Near Eastern Fire Salamander, Salamandra infraimmaculata

Tail-tip clipping is a common technique for collecting tissue samples from amphibian larvae and adults. Surprisingly, studies of this invasive sampling procedure or of natural tail clipping--i.e., bites inflicted by predators including conspecifics--on the performance and fitness of aquatic larval stages of urodeles are scarce. We conducted two studies in which we assessed the effects of posterior tail clipping (~30 percent of tail) on Near Eastern fire salamander (Salamandra infraimmaculata) larvae. In a laboratory study, we checked regeneration rates of posterior tail-tip clipping at different ages. Regeneration rates were hump-shaped, peaking at the age of ~30 days and then decreasing. This variation in tail regeneration rates suggests tradeoffs in resource allocation between regeneration and somatic growth during early and advanced development. In an outdoor artificial pond experiment, under constant larval densities, we assessed how tail clipping of newborn larvae affects survival to, time to, and size at metamorphosis. Repeated measures ANOVA on mean larval survival per pond revealed no effect of tail clipping. Tail clipping had correspondingly no effect on larval growth and development expressed in size (mass and snout-vent length) at, and time to, metamorphosis. We conclude that despite the given variation in tail regeneration rates throughout larval ontogeny, clipping of 30% percent of the posterior tail area seems to have no adverse effects on larval fitness and survival. We suggest that future use of this imperative tool for the study of amphibian should take into account larval developmental stage during the time of application and not just the relative size of the clipped tail sample.

The more the better - polyandry and genetic similarity are positively linked to reproductive success in a natural population of terrestrial salamanders (Salamandra salamandra)

Although classically thought to be rare, female polyandry is widespread and may entail significant fitness benefits. If females store sperm over extended periods of time, the consequences of polyandry will depend on the pattern of sperm storage, and some of the potential benefits of polyandry can only be realized if sperm from different males is mixed. Our study aimed to determine patterns and consequences of polyandry in an amphibian species, the fire salamander, under fully natural conditions. Fire salamanders are ideal study objects, because mating, fertilization and larval deposition are temporally decoupled, females store sperm for several months, and larvae are deposited in the order of fertilization. Based on 18 microsatellite loci, we conducted paternity analysis of 24 female-offspring arrays with, in total, over 600 larvae fertilized under complete natural conditions. More than one-third of females were polyandrous and up to four males were found as sires. Our data clearly show that sperm from multiple males is mixed in the female's spermatheca. Nevertheless, paternity is biased, and the most successful male sires on average 70% of the larvae, suggesting a 'topping off' mechanism with first-male precedence. Female reproductive success increased with the number of sires, most probably because multiple mating ensured high fertilization success. In contrast, offspring number was unaffected by female condition and genetic characteristics, but surprisingly, it increased with the degree of genetic relatedness between females and their sires. Sires of polyandrous females tended to be genetically similar to each other, indicating a role for active female choice.

Interfamily variation in amphibian early life-history traits: raw material for natural selection?

The embryonic development and time to hatching of eggs can be highly adaptive in some species, and thus under selective pressure. In this study, we examined the underlying interfamily variation in hatching timing and embryonic development in a population of an oviparous amphibian, the rough-skinned newt (Taricha granulosa). We found significant, high variability in degree of embryonic development and hatching timing among eggs from different females. Patterns of variation were present regardless of temperature. We also could not explain the differences among families by morphological traits of the females or their eggs. This study suggests that the variation necessary for natural selection to act upon is present in the early life history of this amphibian.