Milena Cvijanović

The ontogenetic origins of skull shape disparity in the Triturus cristatus group

Comparative studies of ontogenies of closely related species provide insights into the mechanisms responsible for morphological diversification. Using geometric morphometrics, we investigated the ontogenetic dynamics of postlarval skull shape and disparity in three closely related crested newt species. The skull shapes of juveniles just after metamorphosis (hereafter metamorphs) and adult individuals were sampled by landmark configurations that describe the shape of the dorsal and ventral side of the newt skull, and analyzed separately. The three species differ in skull size and shape in metamorphs and adults. The ontogenies of dorsal and ventral skull differ in the orientation but not lengths of the ontogenetic trajectories. The disparity of dorsal skull shape increases over ontogeny, but that of ventral skull shape does not. Thus, modifications of ontogenetic trajectories can, but need not, increase the disparity of shape. In species with biphasic life‐cycles, when ontogenetic trajectories for one stage can be decoupled from those of another, increases and decreases in disparity are feasible, but our results show that they need not occur.

The distributions of the six species constituting the smooth newt species complex ( Lissotriton vulgaris sensu lato and L. montandoni ) – an addition to the New Atlas of Amphibians and Reptiles of Europe

The ‘smooth newt’, the taxon traditionally referred to as Lissotriton vulgaris , consists of multiple morphologically distinct taxa. Given the uncertainty concerning the validity and rank of these taxa, L. vulgaris sensu lato has often been treated as a single, polytypic species. A recent study, driven by genetic data, proposed to recognize five species, L. graecus , L. kosswigi , L. lantzi , L. schmidtleri and a more restricted L. vulgaris . The Carpathian newt L. montandoni was confirmed to be a closely related sister species. We propose to refer to this collective of six Lissotriton species as the smooth newt or Lissotriton vulgaris species complex. Guided by comprehensive genomic data from throughout the range of the smooth newt species complex we 1) delineate the distribution ranges, 2) provide a distribution database, and 3) produce distribution maps according to the format of the New Atlas of Amphibians and Reptiles of Europe, for the six constituent species. This allows us to 4) highlight regions where more research is needed to determine the position of contact zones.

The study of larval tail morphology reveals differentiation between two Triturus species and their hybrids

In amphibians, morphological differentiation and disparity at the larval and post-metamorphic ontogenetic stages can diverge, owing to various contrasting environments and different selective pressures. In the monophyletic clade of nine Triturus newt species, five different morphotypes can be recognized, but information on larval morphology is limited. Here we explore divergence of larval morphology in Triturus ivanbureschi , T. macedonicus , and their F1 hybrids. These two genetically and morphologically distinct crested newt species hybridize in nature and form a relatively wide hybrid zone in the central part of the Balkan Peninsula. Using a geometric morphometric approach and multivariate statistics, we evaluated differences of tail size and shape, colouration pattern, and the presence of a tail filament at the mid-larval stage in larvae reared under controlled laboratory conditions. We chose the tail as the main propulsive organ crucial for locomotion, feeding, and escaping predators. We found that Triturus ivanbureschi and T. macedonicus larvae differ in tail shape, but not in tail size. Two groups of F1 hybrid larvae (obtained from reciprocal crossing) were similar to each other, but differed from the parental species in size and shape of the tail, colouration pattern, and the presence of a tail filament. Our results indicate that, like adults, larvae diverge morphologically and hybrid larvae do not exhibit intermediate morphology of the parental species.

The Roles of Phylogeny and Climate in Shaping Variation in Life-History Traits of the Newt Genus Triturus (Caudata, Salamandridae)

Assessing the origin of trait variation during evolutionary history is an important first step in understanding evolutionary diversification. Here, we tested the influence of shared ancestry and climate, and the interplay of both, on the variation of ten life history traits in Triturus newts. We showed that (1) climate alone has driven the evolution of variation in five life history traits, (2) phylogenetic signal partly explains the variation in two traits (vitellus diameter and snout—vent length of larvae at metamorphosis), and (3) the interplay of shared ancestry and climate explains the variation in one trait (snout—vent length of larvae at metamorphosis). This study highlights the coarse-grained influence of shared ancestry and climate on the structure of phenotypic trait variation in Triturus and provides a handle for more detailed, fine grained studies on the evolution of phenotypic trait variation.

Trunk elongation and ontogenetic changes in the axial skeleton of Triturus newts

Body elongation in vertebrates can be achieved by lengthening of the vertebrae or by an increase in their number. In salamanders, longer bodies are mostly associated with greater numbers of vertebrae in the trunk or tail region. However, studies on the relative contribution of the length of single vertebra to body elongation are lacking. In this study, we focus on evolutionary and ontogenetic changes in differentiation of the trunk vertebrae and the relative contribution of individual vertebrae to trunk lengthening in Triturus newts, a monophyletic group of salamanders that shows remarkable disparity in body shape. We compared juveniles and adults of the most elongated T. dobrogicus, which has 17 trunk vertebrae, with juveniles and adults of two closely related species (T. ivanbureschi and T. anatolicus belonging to the T. karelinii species complex) representing a stout and robust morphotype with thirteen trunk vertebrae. We show that trunk vertebrae are uniform in size at the juvenile stage of both analyzed morphotypes. In adults, the trunk vertebrae of the elongated T. dobrogicus are largely uniform, while in those of T. anatolicus, the first two vertebrae differ from the remaining trunk vertebrae. There was no difference in the relative contribution of individual vertebrae to body lengthening between species or stages. We conclude that body elongation in Triturus newts is achieved by increasing the number of vertebrae but not their length.

Morphological Integration and Alternative Life History Strategies: A Case Study in a Facultatively Paedomorphic Newt

Tetrapod limbs are serially homologous structures that represent a particularly interesting model for studies on morphological integration, i.e. the tendency of developmental systems to produce correlated variation. In newts, limbs develop at an early larval stage and grow continuously, including after the habitat transition from water to land following metamorphosis. However, aquatic and terrestrial environments impose different constraints and locomotor modes that could affect patterns of morphological integration and evolvability. We hypothesize that this would be the case for alternative heterochronic morphs in newts, i.e. aquatic paedomorphs that keep gills at the adult stage and adult metamorphs that are able to disperse on land. To this end, we analyzed patterns and strengths of correlations between homologous skeletal elements of the fore- and hindlimbs as well as among skeletal elements within limbs in both phenotypes in the alpine newt, Ichthyosaura alpestris. Our results showed that metamorphs and paedomorphs had similar, general patterns of limb integration. Partial correlations between homologous limb elements and within limb elements were higher in paedomorphs when compared to metamorphs. A decrease in partial correlation between homologous limb elements in metamorphs is accompanied with a higher evolvability of the terrestrial morph. All these results indicate that environmental demands shaped the patterns of morphological integration of alpine newt limbs and that the observed diversity in correlation structure could be related to a qualitative difference in the modes of locomotion between the morphs.

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.