Testing the evolutionary constraints of metamorphosis: The ontogeny of head shape in Triturus newts

In vertebrates with complex, biphasic, life cycles, larvae have a distinct morphology and ecological preferences compared to metamorphosed juveniles and adults. In amphibians, abrupt and rapid metamorphic changes transform aquatic larvae to terrestrial juveniles. The main aim of this study is to test whether, relative to larval stages, metamorphosis (1) resets the pattern of variation between ontogenetic stages and species, (2) constrains intraspecific morphological variability, and (3) similar to the "hour-glass" model reduces morphological disparity. We explore postembryonic ontogenetic trajectories of head shape (from hatching to completed metamorphosis) of two well-defined, morphologically distinct Triturus newts species and their F1 hybrids. Variation in head shape is quantified and compared on two levels: dynamic (across ontogenetic stages) and static (at a particular stage). Our results show that the ontogenetic trajectories diverge early during development and continue to diverge throughout larval stages and metamorphosis. The high within-group variance and the largest disparity level (between-group variance) characterize the metamorphosed stage. Hence, our results indicate that metamorphosis does not canalize head shape variation generated during larval development and that metamorphosed phenotype is not more constrained relative to larval ones. Therefore, metamorphosis cannot be regarded as a developmental constraint, at least not for salamander head shape.

Flesh and bone: An integrative approach towards sexual size dimorphism of a terrestrial salamander (genus Salamandrina)

Males and females face different selection pressures due to a sexually biased investment into reproduction. This often results in different morphologies. Sexual size dimorphisms (SSD) can give us important hints on the evolution and biology of a species. Salamanders are a perfectly suited system for investigating SSD, including a diversity of reproductive modes and behaviors, and patterns of SSD combined with life history traits in a phylogenetic context help us to understand the evolution of these processes. Because spectacled salamanders (genus Salamandrina) are the phylogenetically most basal taxon of the Salamandridae, they play a key role in reconstructing the evolutionary pattern of SSD. Combining extensive external and skeletal measurements of the cranium, limbs, and the pelvic girdle using high-resolution micro Computer Tomography (μCT) yielded an integrative analysis of expressed SSD of morphology and osteology of Salamandrina perspicillata. Multivariate analysis of external characters showed that males generally had larger cloacae, heads, and limbs relative to body size, while females had larger trunks. Analysis of osteology confirmed this pattern but also revealed new dimorphic characters in the cranium and the pelvic girdle. Dimorphic characters in external morphology and osteology are likely linked to the different reproductive roles of the sexes and support sexual rather than ecological selection as the primary force acting on the phenotype of the phylogenetically basal salamandrids.

Robustness of newt heads in condition of co-existence: a case of the Carpathian newt and the alpine newt

Co-existence between potentially competing newt species can lead to niche differentiation (e.g., in terms of diet shifts). This may cause adaptive responses involving changes in head shape. Here, we tested the hypothesis: the head shape of Lissotriton montandoni is different in conditions of co-occurrence with Ichthyosaura alpestris than in conditions in which other newt species are absent. We analysed images depicting head shape of specimens of I. alpestris and L. montandoni from a museum collection. All specimens of I. alpestris originated in a habitat where L. montandoni also occurred, whereas specimens of L. montandoni derived from populations that cohabited with I. alpestris and populations in which the presence of another newt species was not recorded. In each image, landmarks and semilandmarks were digitised. Females of L. montandoni from the population where I. alpestris also occurred were characterised by more massive heads and longer mouths in lateral views than females from sites where no other newt species occurred. Significant differences in head shape were also found when analysing ventral views between these species when they occupied the same habitat. We confirmed that the head shape of female L. montandoni differs between conditions of co-occurrence and absence of I. alpestris; no differences were found for males. A differently shaped head may be an adaptation to diet; L. montandoni females with longer mouths and more robust basal parts of the head can feed on larger invertebrates and compete more effectively with I. alpestris. The co-existence of newt species should be taken into account in future ecomorphological studies.