Asymmetric reproductive isolation among polymorphic salamanders

The study of reproductive isolation (RI) as a prerequisite to sympatric speciation has been limited by a focus on species that have already experienced isolation. However, a complete understanding of speciation depends on observing taxa before they complete the speciation process. We estimated RI in field populations of the polyphenic mole salamander, Ambystoma talpoideum, by capturing paedomorphic (aquatic) and metamorphic (terrestrial) adults during the breeding season from two natural populations. We found evidence for asymmetric RI between morphs, such that paedomorphic males and metamorphic females had functionally zero RI, whereas metamorphic males and paedomorphic females had substantial RI. Evidence suggests that ecological factors such as the abundance of each morph, timing of rainfall, and water depth of the breeding habitat play a large role in the production of these asymmetries. Spatial aspects of RI had a greater relative impact on overall isolation than temporal differences, in part because metamorphic adults were often captured in shallower water than paedomorphic adults. However, morph separation varied across populations and year, suggesting that environmental heterogeneity likely plays a large role in the potential for RI, particularly between metamorphic males and paedomorphic females. In addition, body‐size variation and behavioural differences could also influence the RI estimates presented here. Although facultative paedomorphosis appears to have played a large role in macroevolutionary change via allopatric speciation in some taxa, our results suggest that there is little potential for sympatric speciation in the future within these populations. However, asymmetric RI creates the opportunity for fitness differences between morphs and sexes that would directly affect the maintenance of this polymorphism. Our results suggest that further studies on this and other polyphenisms may provide valuable insight into the evolution of RI and the role of environmental heterogeneity in the production and maintenance of biological diversity. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 86 , 265?281.     

Genetic structure of lake and stream populations in a Pyrenean amphibian (Calotriton asper) reveals evolutionary significant units associated with paedomorphosis

Differences in environmental conditions such as those between lakes and streams can produce phenotypic variation and ultimately promote evolutionary diversification. Some species of newts and salamanders can occupy these habitats and express alternative phenotypes: metamorphs that lose gills at metamorphosis and paedomorphs that retain them at the adult stage. Whereas this process is facultative in some species, it is obligatory in others, thus suggesting that isolation and environmental pressures may have canalized developmental pathways. In this study, we focused our research on the Pyrenean brook newt, Calotriton asper, which is present in both lakes and streams, but whose fully aquatic paedomorphic individuals are only present in lakes. We aimed to determine the genetic structure and differentiation of two paedomorphic populations, including their surrounding stream and lake metamorphic populations, to test whether populations of paedomorphs can constitute evolutionary significant units. Although gene flow was identified between lakes and nearby stream populations, there was a low percentage of dispersers, and the paedomorphic populations were genetically differentiated from the populations of metamorphs. It is likely that the studied lakes have offered peculiar conditions that have allowed the development of a paedomorphic phenotype. These populations and phenotypes therefore constitute good models to understand local adaptations. As each of these populations of paedomorphs can be considered evolutionary significant units that cannot be replaced by other nearby populations in case of a population crash, conservation actions should be focused directly on them.     

Foraging tactics in alternative heterochronic salamander morphs: trophic quality of ponds matters more than water permanency

1. In lentic freshwater habitats, the composition of animal assemblages shifts along a gradient from temporary to permanent basins. When habitats with different degrees of permanence are at the scale of the home range of species, they constitute alternatives in terms of energy acquisition through feeding. 2. In this context, previous studies showed an advantage of metamorphic over paedomorphic tiger salamanders (Ambystoma tigrinum) in temporary ponds which are only available to metamorphs. The aim of this study was to establish whether salamanders obtain similar benefits in ponds that do not differ in water permanence and whether salamanders shifted from detrimental to advantageous ponds. To this end, we determined the feeding habits, body condition and movement patterns of the two morphs in a complex of four permanent and four temporary ponds. 3. Consistent with previous studies, metamorphs consumed higher‐quality diets than paedomorphs in term of energy intake. However, these differences occurred because metamorphs consumed fairy shrimp in a single temporary pond. Individual movement patterns confirmed that most of the metamorphs used different aquatic habitats both within and between years and that most of them moved from permanent ponds for breeding towards the most profitable temporary pond in terms of foraging. 4. These results indicate that habitat selection by salamanders is optimal in term of energy intake in metamorphs that use high quality ponds independently of hydroperiod. It seems that both spatial and temporal variation can influence the relative foraging success of each morph.     

Facultative paedomorphosis and the pattern of intra- and interspecific variation in cranial skeleton: lessons from European newts (Ichthyosaura alpestris and Lissotriton vulgaris)

Paedomorphosis, the presence of ancestral larval and juvenile traits that occur at the descendent adult stage, is an evolutionary phenomenon that shaped morphological evolution in many vertebrate lineages, including tailed amphibians. Among salamandrid species, paedomorphic and metamorphic phenotypes can be observed within single populations (facultative paedomorphosis). Despite wide interest in facultative paedomorphosis and polymorphism produced by heterochronic changes (heterochronic polymorphism), the studies that investigate intraspecific morphological variation in facultative paedomorphic species are largely missing. By quantifying the cranium size and development (bone development and remodeling), we investigated the variation at multiple levels (i.e., between sexes, populations and species) of two facultatively paedomorphic European newt species: the alpine and the smooth newt. The pattern of variation between paedomorphs (individuals keeping larval traits at the adult stage) and metamorphs (metamorphosed adult individuals) varied between species and among populations within a single species. The patterns of variation in size and skull formation appear to be more uniform in the alpine than in the smooth newt, indicating that developmental constraints differed between species (more pronounced in alpine than in smooth newt). Our study shows that the cranial skeleton provides detailed insight in the pattern of variation and divergence in heterochronic polymorphism within and between species and open new questions related to heterochronic polymorphism and evolution of cranial skeleton.     

Paedomorphosis in <Emphasis Type="Italic">Ambystoma talpoideum</Emphasis>: effects of initial body size variation and density

Facultative paedomorphosis is the ability of a salamander to either metamorphose into a terrestrial, metamorphic adult or retain a larval morphology to become a sexually mature paedomorphic adult. It has been hypothesized that density and initial body size variation within populations are instrumental in cueing metamorphosis or paedomorphosis in salamanders, yet few studies have adequately tested these hypotheses in long-term experiments. Beginning in the spring of 2004, 36 experimental ponds were used to manipulate three body size variation levels (low, medium, high) and two density levels (low, high) of Ambystoma talpoideum larvae. Larvae were individually marked using visible implant elastomers and collected every 2 weeks in order to measure snout–vent length and mass. Bi-nightly sampling was used to collect new metamorphs as they appeared. Analysis revealed significant effects of density, size variation and morph on body size of individuals during the summer. Individuals that metamorphosed during the fall and following spring were significantly larger as larvae than those becoming paedomorphic across all treatments. These results support the Best-of-a-Bad-Lot hypothesis, which proposes that the largest larvae metamorphose in order to escape unfavorable aquatic habitats. Large larvae may metamorphose to leave aquatic habitats, regardless of treatment, due to the colder climate and lower productivity found in Kentucky, which is in the northern-most part of A. talpoideum’s range. By maintaining a long-term experiment, we have provided evidence for the transition of both larvae and paedomorphs into metamorphs during fall and spring metamorphosis events. Furthermore, the production of similar morphs under different environmental conditions observed in this research suggests that the ecological mechanisms maintaining polyphenisms may be more diverse that first suspected. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Population‐specific effects of developmental temperature on body condition and jumping performance of a widespread European frog

All physiological processes of ectotherms depend on environmental temperature. Thus, adaptation of physiological mechanisms to the thermal environments is important for achieving optimal performance and fitness. The European Common Frog, Rana temporaria, is widely distributed across different thermal habitats. This makes it an exceptional model for studying the adaptations to different thermal conditions. We raised tadpoles from Germany and Croatia at two constant temperature treatments (15°C, 20°C), and under natural temperature fluctuations (in outdoor treatments), and tested how different developmental temperatures affected developmental traits, that is, length of larval development, morphometrics, and body condition, as well as jumping performance of metamorphs. Our results revealed population‐specific differences in developmental time, body condition, and jumping performance. Croatian frogs developed faster in all treatments, were heavier, in better body condition, and had longer hind limbs and better jumping abilities than German metamorphs. The populations further differed in thermal sensitivity of jumping performance. While metamorphs from Croatia increased their jumping performance with higher temperatures, German metamorphs reached their performance maximum at lower temperatures. These population‐specific differences in common environments indicate local genetic adaptation, with southern populations being better adapted to higher temperatures than those from north of the Alps.     

Growth and foraging consequences of facultative paedomorphosis in the tiger salamander,Ambystoma tigrinum nebulosum

Summary Facultative paedomorphosis in salamanders occurs when larvae respond to varying environmental conditions by either metamorphosing into terrestrial metamorphic adults or retaining their larval morphology to become sexually mature paedomorphic adults. Several hypotheses have been proposed for the evolutionary maintenance of this environmentally induced dimorphism, but few data are available to assess them adequately. We studied a montane population of the tiger salamander,Ambystoma tigrinum nebulosum, and measured the adult growth rate and body condition across three growing seasons to assess the relative costs and benefits of each morph. Metamorphic adults grew more than paedomorphic adults in terms of snout—vent length across years and in weight within years. Dietary analyses and foraging experiments revealed some of the proximate factors that may underlie these differential growth patterns. Across all prey, metamorphs had significantly higher biomass and calories per stomach sample than paedomorphs. Metamorphic diets primarily consisted of the fairy shrimpBranchinecta coloradensis, whereas paedomorphic diets contained a variety of benthic and terrestrial invertebrates. Foraging experiments revealed that both morphs are more successful at capturing fairy shrimp relative to other prey types and both show high electivity toward this prey. However, fairy shrimp occurred only in non-permanent ponds and thus are inaccessible to paedomorphs, which can survive only in permanent ponds. Paedomorphs also experience higher levels of intraspecific competition with large larvae in permanent ponds than metamorphs do in non-permanent ponds. Thus, metamorphs obtain a growth advantage over paedomorphs by foraging in non-permanent ponds that contain fairy shrimp and have reduced intraspecific competition. These results suggest that paedomorphs should have decreased fitness relative to metamorphs, primarily because metamorphs can move into the best habitats for growth. The net fitness effect of morph-specific differences in dispersal depend on whether there are trade-offs with other life history traits. Nonetheless, because the relative benefit of metamorph dispersal will change with environmental conditions in permanent ponds and the surrounding habitat, the relative fitness payoff to each morph should track changes in the environment. Thus, facultative paedomorphosis may be maintained in part by variable, environmentally-specific fitness payoffs to each morph.     

Metamorphic duration: an under-studied variable in frog life histories

Over a narrow temperature range (25.5–28 °C), metamorphic duration in 14 taxonomically diverse frog species ranged from 2.0 to 7.3 days. For 12 species, duration was significantly related to tadpole body-size parameters, especially tail length. For the remainder, duration was longer than expected. Two patterns of tail regression were seen with either rapid or slow regression at the start. Most species lost about 30% of body wet weight during metamorphosis, but two were exceptional: X. laevis lost 58% and F. fitzgeraldi 13%. The results are discussed in the context of Wassersug & Sperry's (1977 ) hypothesis that selection acts to minimize metamorphic duration because metamorphosis is a highly vulnerable stage. Several other factors are outlined as possible determinants of metamorphic duration: developmental constraints, functional trade-offs, phylogeny, predator abundance, adaptive de-coupling, body condition and metabolic and water costs. There is evidence from the data for the operation of several of these factors, especially in species utilizing specialized habitats, and a plea is made for further study of metamorphic duration as an ecologically and evolutionarily important variable in amphibian life histories. Finally, the effects of different temperatures on metamorphic duration are presented for several species, and discussed in terms of local adaptation and reaction norms.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 261–272.     

Distinctive developmental variability of genital parts in the sexually dimorphic beetle, Prosopocoilus inclinatus (Coleoptera: Lucanidae)

Recent comparative studies have revealed that the rapid diversity of genitalia is closely related to sexual selection and that genital development interacts with the development of different body parts. Hypotheses about developmental stability due to selection to genital parts were tested by estimating allometric relations in a sexually dimorphic stag beetle Prosopocoilus inclinatus . All genital parts of males scaled to body size with a slope of less than 1 and all but the median lobe (male intromittent organ) showed smaller variability than other body parts. This supported the 'one-size-fits-all' hypothesis, which suggests broad copulation opportunity by males of any size with females within a population. Nevertheless, we found large variation among different genital parts in coefficients of variation and in values of the switch point where the allometric relations varied significantly. These results strongly support the view that developmental trajectories of genital traits are not necessarily integrated. Among the genitalic traits, male intromittent organ and female genitalia exhibited large variability, suggesting a high responsiveness to the selective regimes and physical interaction during copulation. This may account for rapid diversification of genital morphology, even in closely-related populations in beetle species.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 573–581.     

Geographical variation in asymmetry in Gasterosteus aculeatus

We tested whether fluctuating asymmetry (FA) in undisturbed populations is associated with several natural environmental factors and whether FA is negatively correlated with fitness in the wild. We compared the FA of multiple bony structural defences among 87 endemic populations of threespine stickleback ( Gasterosteus aculeatus L.) inhabiting pristine freshwater habitats on the islands of Haida Gwaii, British Columbia, Canada. Multi-trait FA for adults and juvenile fish varied extensively among populations, but only in adults did it correlate with geography and two habitat characteristics (pH and water colour). Mean FA among individual traits varied concordantly among populations but was not correlated within individuals. While asymmetrical fish showed slightly higher levels of parasitism as predicted, selection differentials based on age class comparisons suggested that asymmetrical fish had the same or marginally higher survival than symmetrical fish. Selection differentials of FA varied significantly among traits and may reflect variability in their functional importance and in the strength of selection on their developmental stability. The data imply that FA/fitness associations are heterogeneous and character-specific.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 9–22.     

Effects of habitat heterogeneity and local adaptation on the body condition of a forest passerine at the edge of its distributional range

We studied fluctuating asymmetry and feather growth rates as indicators of fitness of blackcaps ( Sylvia atricapilla ) breeding at the border of their distribution range in the Iberian Peninsula. Iberian blackcaps increase their abundance with increasing rainfall and ground cover of brambles. In border habitats, they are sedentary and morphologically different from migrants, suggesting that they could be adapted to peripheral conditions. We tested whether juvenile body condition depends on (1) distance from the centre of the range, (2) mean precipitation or (3) bramble cover. Controlling for environmental variation, we tested for differences between migratory and sedentary populations. Body condition varied across the Iberian gradient in parallel with changes in precipitation. Controlling for this effect, sedentary populations were in better body condition than migratory populations. Our results support the idea that environmental heterogeneity causes fitness to fluctuate across species' ranges, and also that local adaptation may mean that peripheral populations are more than a 'tail end' of the species.  © 2003 The Linnean Society of London . Biological Journal of the Linnean Society , 2003, 78 , 479–488.     

Morphological and pollen differentiation in Solanum cardiophyllum ssp. cardiophyllum and S. cardiophyllum ssp. ehrenbergii

A multivariate analysis to differentiate morphologically the populations of wild potatoes Solanum cardiophyllum ssp. cardiophyllum and S. cardiophyllum ssp. ehrenbergii was carried out. An analysis of the morphology and the viability of pollen of these potato populations was also made. The results of the morphometric analysis indicate that both subspecies are phenetically different. The pollen grain shape and size in ssp. cardiophyllum are different in northern and southern populations. Pollen diameter is significantly different between subspecies. Based on these results we propose that these taxa should be considered as two separate species.  © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 2002, 140 , 415–426.     

Re‐epithelialization of large wound in paedomorphic and metamorphic axolotls

Axolotls (Ambystoma mexicanum ) may heal their skin wounds scar‐free in both paedomorphs and metamorphs. In previous studies on small punch skin wounds, rapid re‐epithelialisation was noted in these two axolotl morphs. However, large wound size in mammals may affect wound healing. In this study, large circumferential full thickness excision wounds on the hind limbs were created on juvenile paedomorphic and metamorphic axolotls. The results showed re‐epithelialisation was more quickly initiated in paedomorphs than in metamorphs after wounding. The migrating rate of epidermis on the wound bed was faster in paedomorphs than in metamorphs and thus completion of re‐epithelialisation was faster in paedomorphs than in metamorphs. Within these re‐epithelialisation periods, neither basement membrane nor dermis was reformed. Epidermal cell proliferation was detected by EdU‐labelling technique. In the normal unwounded skin, epidermal proliferation rate was higher in paedomorphs than in metamorphs. After wounding, the epidermal proliferation rate was significantly lower in the migrating front on the wound bed than in the normal skin in paedomorphs. The EdU‐labelling rate between normal skin and migration front was not different in metamorphs. Lacking of more proliferating epidermal cells on the wound bed indicated that the new epidermis here derived rather from migrating epidermal cells than from cell proliferation in situ. In conclusion, re‐epithelialisation in the large wound might be fully completed in both morphs despite it was initiated earlier and with faster rate in paedomorphs than in metamorphs. The new epidermis on the wound bed derived mainly from cell migration than by cell proliferation in the re‐epithelialisation period. J. Morphol. 278:228–235, 2017. © 2016 Wiley Periodicals,Inc.     

Variance in developmental event timing is greatest at low biological levels: implications for heterochrony

Intraspecific variation in developmental event timing is common and may be the raw material from which heterochronies (altered timing of developmental events between ancestors and descendants) arise. However, our understanding of how variance in intraspecific developmental event timing is distributed across different hierarchical, biological levels is poor, despite recent evidence suggesting a genetic basis for such inter‐individual differences. In the present study, we used high (temporal and spatial) resolution bio‐imaging of the entire embryonic development of the pond snail, Radix balthica, to investigate how variance was partitioned between the biological levels of interpopulation, inter‐egg mass and individual, with respect to the relative timing of four key developmental events (four‐cell division, a discrete heart beat, capsule rupture, and hatching), as well as egg volume (an index of maternal investment), hatchling size and shape (to compare embryonic with post‐hatch variance). We found that the timing of all but one (four cell division) of the developmental events, together with all measures of hatchling size and shape, had most variance partitioned at the individual level; variance at the interpopulation level was surprisingly low despite sampling populations from across the geographical range of R. balthica. These patterns highlight the importance of studying the distribution of variance in developmental event timing with the aim of understanding how it might drive organismal ecology and evolution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 581–590.     

Maternal and genetic contributions to geographical variation in Rana temporaria larval life-history traits

The relative importance of genetic, environmental, and maternal effects as determinants of geographical variation in vertebrate life-histories has not often been explored. We examined the role of genetic and maternal effects as determinants of population divergence in survival and three important larval life-history traits (growth rate, age, and size at metamorphosis) using reciprocal crosses between two latitudinally separated populations of the common frog ( Rana temporaria Linnaeus). Genetic effects were important in all three traits as indicated by the significant effect of male origin, but there was also evidence for nonadditive genetic contributions on metamorphic size and growth rate. Likewise, maternal effect contributions to population divergence were large, partially environment dependent, and apparently acting primarily through egg size in two of three traits. These results suggest that both genetic and maternal effects are important determinants of geographical variation in amphibian life-histories, and that much of the differentiation resulting from maternal effects is mediated through variation in egg size. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 61–70.     

Hybridization, developmental stability, and functionality of morphological traits in the ground beetle Carabus solieri (Coleoptera, Carabidae)

The assessment of developmental stability in hybrids can provide valuable information in the study of species formation because it allows an evaluation of the degree of incompatibility of genetic systems that control developmental processes. The present study assessed the impact of two hybridization events, assumed to have occurred at different times, on developmental instability in the ground beetle Carabus solieri . Developmental instability was estimated in 678 individuals from 27 populations from the fluctuating asymmetry (FA) levels of four morphological traits: the tibia length of middle and hind legs, which are functional structures, and the length and the proximal width of the hind wings, which are vestigial and thus nonfunctional structures. Significant variations of FA levels between populations were shown only for the wing width. For this trait, FA levels in hybrids were higher than in their parental entities for both hybridization events, indicating a significant divergence of the gene systems controlling development between the parental entities in the two hybridization cases. As expected, wing traits exhibited FA levels at least three times higher than leg trait. Finally, the potential interest of vestigial traits in the particular context of hybridization is discussed.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 151–158.     

An invasive species imposes selection on life‐history traits of a native frog

As well as their direct ecological impacts on native taxa, invasive species can impose selection on phenotypic attributes (morphology, physiology, behaviour, etc.) of the native fauna. In anurans, body size at metamorphosis is a critical life‐history trait: for most challenges faced by post‐metamorphic anurans, larger size at metamorphosis probably enhances survival. However, our studies on Australian frogs (Limnodynastes convexiusculus) show that this pattern can be reversed by the arrival of an invasive species. When metamorph frogs first encounter invasive cane toads (Bufo marinus), they try to eat the toxic invader and, if they are able to do so, are likely to die from poisoning. Because frogs are gape‐limited predators, small metamorphs cannot ingest a toad and thus survive long enough to disperse away from the natal pond (and thus from potentially deadly toads). These data show that larger size at metamorphosis can reduce rather than increase anuran survival rates, because larger metamorphs are more easily able to ingest (and thus be poisoned by) metamorph cane toads. Our results suggest that patterns of selection on life‐history traits of native taxa (such as size and age at metamorphosis, seasonal timing of breeding and duration of pondside aggregation prior to dispersal) can be modified by the arrival of an invasive species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 329–336.     

Procrustean analysis of fluctuating asymmetry in the bulb mite Rhizoglyphus robini Claparede (Astigmata: Acaridae)

We studied fluctuating asymmetry (FA) in two generations of the bulb mite Rhizoglyphus robini . We used Procrustes analyses, which allow the comparison of dimensionless shapes of body sides. We found little (<4%) directional asymmetry in either sex. Of the two morphs occurring in this species, fighters, which possess a thickened third pair of legs, exhibited higher FA than did scramblers, the morph with unmodified legs; this may reflect the costliness of the fighter developmental pathway. There was a negative relationship between FA and female fecundity, but the regression slope of mid-offspring on mid-parent FA was not significantly greater than zero. We propose that heritability estimates can be biased downwards if highly asymmetric individuals produce fewer viable offspring. However, we found no significant association between parental FA and the proportion of viable embryos in their broods. Furthermore, we hypothesized that parental FA might indicate the presence of largely recessive mutations deleterious to developmental homeostasis that would cause increased embryo mortality under inbreeding. However, we found no significant association between FA of parents that were mated to their full sibs and the proportion of viable embryos in their inbred progeny.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 499–505.     

Developmental plasticity of limb bone microstructural organization in Apateon: histological evidence of paedomorphic conditions in branchiosaurs

SUMMARY Apateon, a key genus among Branchiosauridae from the Carboniferous—Permian of Europe, is often considered closely related to salamanders on the basis of developmental similarities, anatomical features, and life history. The current work deals with recognition of heterochronies among three “time‐averaged populations” of Apateon based on inference from histological features already studied in extant urodeles. Our study is performed on the long bones of 22 specimens of Apateon pedestris and Apateon caducus. Histological observations show that diaphyseal and epiphyseal ossification patterns of Apateon are similar to those of urodeles. From skeletochronological analysis, the identification of the age of sexual maturity allows us to discriminate juveniles from adults and to confirm the previous hypothesis of a paedomorphic condition based on anatomical data among these species. The current study also suggests a paedomorphic condition of each “population” at the histological level. This heterochrony may have been linked to peculiar ecological conditions such as hypoxic and fresh water environment. Functional reasons may also be invoked to explain differences of ossification between fore‐ and hindlimbs of the “populations” from Odernheim and Niederkirchen because paleoecological conditions are very different from one locality to another. This study illustrates the role that the acquisition of heterochronic features plays at the microevolutionary scale.