Absence of heterosis in hybrid crested newts

A staging table of Balkan crested newt embryonic development to serve as a baseline in evolutionary developmental studies

There is an increased interest in the evolution and development of newts from the genus Triturus because: (1) morphological differentiation among the nine constituent species largely corresponds to different ecological preferences, (2) hybridization between different species pairs has various evolutionary outcomes in terms of life history traits and morphology, and (3) the genus expresses a balanced lethal system that causes arrested growth and death of half of the embryos. These features provide natural experimental settings for molecular, morphological, and life-history studies. Therefore, we produce a staging table for the Balkan crested newt (T. ivanbureschi). We provide detailed descriptions of 34 embryonic stages based on easily observable and interpretable external morphological characters, to ensure reproducibility. Compared with previous staging tables for Triturus, we include a vastly increased sample size and provide high-resolution photographs in lateral, ventral, and dorsal view, complemented by videos of specific developmental periods, and accompanied by detailed explanations on how to delineate the specific stages. Our staging table will serve as a baseline in comparative studies on Triturus newts: an emerging model system in evolutionary and developmental studies.

Higher temperature induces oxidative stress in hybrids but not in parental species: A case study of crested newts

Ectotherms are particularly sensitive to global warming due to their limited capacity to thermoregulate, which can impact their performance and fitness. From a physiological standpoint, higher temperatures often enhance biological processes that can induce the production of reactive oxygen species and result in a state of cellular oxidative stress. Temperature alters interspecific interactions, including species hybridization. Hybridization under different thermal conditions could amplify parental (genetic) incompatibilities, thus affecting a hybrid's development and distribution. Understanding the impact of global warming on the physiology of hybrids and particularly their oxidative status could help in predicting future scenarios in ecosystems and in hybrids. In the present study, we investigated the effect of water temperature on the development, growth and oxidative stress of two crested newt species and their reciprocal hybrids. Larvae of Triturus macedonicus and T. ivanbureschi, and their T. macedonicus-mothered and T. ivanbureschi-mothered hybrids were exposed for 30 days to temperatures of 19°C and 24°C. Under the higher temperature, the hybrids experienced increases in both growth and developmental rates, while parental species exhibited accelerated growth (T. macedonicus) or development (T. ivanbureschi). Warm conditions also had different effects on the oxidative status of hybrid and parental species. Parental species had enhanced antioxidant responses (catalase, glutathione peroxidase, glutathione S-transferase and SH groups), which allowed them to alleviate temperature-induced stress (revealed by the absence of oxidative damage). However, warming induced an antioxidant response in the hybrids, including oxidative damage in the form of lipid peroxidation. These findings point to a greater disruption of redox regulation and metabolic machinery in hybrid newts, which can be interpreted as the cost of hybridization that is likely linked to parental incompatibilities expressed under a higher temperature. Our study aims to improve mechanistic understanding of the resilience and distribution of hybrid species that cope with climate-driven changes.

Changes in thyroid histomorphology and thyroglobulin immunostaining upon exposure to thiourea in Triturus newts

Amphibians are useful bioindicators for monitoring aquatic health and the influence of xenobiotics such as endocrine disrupting chemicals. Because aquatic ecosystems experience the majority of global pollution, aquatic organisms are most exposed and vulnerable to endocrine disruptors. Furthermore, penetration of endocrine disruptors into aquatic organisms especially in amphibians is even easier because of more permeable skin, resulting in high bioavailability and bioaccumulation of chemicals. One of the most potent endocrine disruptors is thiourea, which chemically blocks the synthesis of thyroid hormones and prevents metamorphosis in amphibians. We investigated the influence of thiourea on histomorphology of the thyroid gland in Triturus newts at the metamorphic stage, when thyroid hormone concentrations should reach their maximum level. Chronic exposure to thiourea induced hypertrophy and hyperplasia of follicular cells as well as a significant reduction of interstitial tissue. The intensity of the thyroglobulin immunostaining signal significantly decreases upon chronic exposure to thiourea. Successful cross-reactivity of human primary antibody in immunochemical detection of thyroglobulin in Urodela confirms potential homology in thyroglobulin structure throughout the vertebrates.

The Reproductive Success of Triturus ivanbureschi × T. macedonicus F1 Hybrid Females (Amphibia: Salamandridae)

Simple Summary

Two moderately related large-bodied newt species endemic to the Balkan Peninsula, the Balkan crested newt (Triturus ivanbureschi) and the Macedonian crested newt (T. macedonicus), coexist and hybridize in central Serbia. Many generations of mutual hybrid crossings and backcrossings with parental species shaped the genetic composition of hybrid populations. Natural populations have admixed nuclear DNA (nuDNA) of parental species and T. ivanbureschi mitochondrial DNA (mtDNA), which is usually maternally inherited. The mechanisms that direct gene flow and shape the first generations of hybrids could explain the formation of hybrid zones and their maintenance in nature. We followed and compared life history traits related to reproduction of the first generation of reciprocal hybrids obtained by experimental crossing. Our results suggested that possible incompatibilities between mitochondrial and nuclear genomes, which could lead to the exclusion of T. macedonicus mtDNA in natural populations, most likely act at later stages of development or subsequent hybrid generations. Results from this study add to the growing knowledge of Triturus hybrid biology and ecology, which is the baseline for conservation programs necessary to protect these highly endangered amphibians.

Abstract

Two large-bodied newt species, Triturus ivanbureschi and T. macedonicus, hybridize in nature across the Balkan Peninsula. Consequences of hybridization upon secondary contact of two species include species displacement and asymmetrical introgression of T. ivanbureschi mtDNA. We set an experimental reciprocal cross of parental species and obtained two genotypes of F₁ hybrids (with T. ivanbureschi or T. macedonicus mtDNA). When hybrids attained sexual maturity, they were engaged in mutual crossings and backcrossing with parental species. We followed reproductive traits over two successive years. Our main aim was to explore the reproductive success of F₁ females carrying different parental mtDNA. Additionally, we tested for differences in reproductive success within female genotypes depending on the crossing with various male genotypes (hybrids or parental species). Both female genotypes had similar oviposition periods, number of laid eggs and hatched larvae but different body and egg sizes. Overall reproductive success (percentage of egg-laying females and viability of embryos) was similar for both genotypes. The type of crossing led to some differences in reproductive success within female genotypes. The obtained results suggest that processes that led to exclusion of T. macedonicus mtDNA in natural populations may be related to the survival at postembryonic stages of F₂ generation or reproductive barriers that emerged in subsequent hybrid generations.

Effects of thiourea on the skull of Triturus newts during ontogeny

Background

In amphibians, thyroid hormone (TH) has a profound role in cranial development, especially in ossification of the late-appearing bones and remodeling of the skull. In the present study, we explored the influence of TH deficiency on bone ossification and resulting skull shape during the ontogeny of Triturus newt hybrid larvae obtained from interspecific crosses between T. ivanbureschi and T. macedonicus.

Methods

Larvae were treated with two concentrations of thiourea (an endocrine disruptor that chemically inhibits synthesis of TH) during the midlarval and late larval periods. Morphological differences of the cranium were assessed at the end of the midlarval period (ontogenetic stage 62) and the metamorphic stage after treatment during the late larval period.

Results

There was no difference in the ossification level and shape of the skull between the experimental groups (control and two treatment concentrations) at stage 62. During the late larval period and metamorphosis, TH deficit had a significant impact on the level of bone ossification and skull shape with no differences between the two treatment concentrations of thiourea. The most pronounced differences in bone development were: the palatopterygoid failed to disintegrate into the palatal and pterygoid portions, retardation was observed in development of the maxilla, nasal and prefrontal bones and larval organization of the vomer was retained in thiourea-treated larvae.

Conclusions

This implies that deficiency of TH caused retardation in development and arrested metamorphic cranium skeletal reorganization, which resulted in divergent cranial shape compared to the control group. Our results confirmed that skull remodeling and ossification of late-appearing bones is TH–dependent, as in other studied Urodela species. Also, our results indicate that TH plays an important role in the establishment of skull shape during the ontogeny of Triturus newts, especially during the late larval period and metamorphosis, when TH concentrations reach their maximum.

The effect of short-term fasting on the oxidative status of larvae of crested newt species and their hybrids

In nature, animals often face periods without food caused by seasonal fluctuations and/or prey scarcity. An organism's physiological response to imposed energetic limitations is followed by changes in mitochondrial functioning (adjustment of energy metabolism) and a reduction of non-essential processes. However, this energy-saving strategy can have its costs. In this study, we examined oxidative stress as one of the possible physiological costs of short-term, two-week-long food deprivation on developing amphibian larvae of the crested newts Triturus macedonicus and Triturus ivanbureschi and their hybrids. We investigated whether this exogenous factor additionally affected the oxidative status (fitness-related trait) of hybrid individuals. The fasting treatment led to lower growth and a lower body mass and body condition index of individuals. The results revealed that the antioxidant system (AOS) of food-deprived larvae could not cope in a proper manner with reactive oxygen species production under limited energy availability, leading to higher lipid oxidative damage. The lowest AOS response was observed for H₂O₂ scavenging parameters (catalase, glutathione peroxidase, and total glutathione), which together with the elevated activity of superoxide dismutase suggested increased H₂O₂ concentrations. Comparison between parental species and their hybrids showed that hybrid individuals suffered greater oxidative damage (as demonstrated by higher concentrations of lipid peroxides), indicating that they were more susceptible to fasting-induced oxidative stress. Overall, this study illustrates that: (i) an oxidative event is one of the costs amphibian larvae face during short-term periods of fasting, (ii) hybrids are less capable of dealing with this stressful condition, which can lower their chances of survival in a changing environment.

The Effect of Shelter on Oxidative Stress and Aggressive Behavior in Crested Newt Larvae (Triturus spp.)

Shelters are important for animal survival. Provision of adequate hiding places allow animals to express their natural sheltering behavior and it can have different positive effects on cortisol levels, physiological processes and mental performance. Although the absence of a refuge activates some stress response, its effect on oxidative stress has not been adequately examined. This study investigated whether the presence/absence of a shelter modifies the oxidative status (the antioxidant system and oxidative damage) and aggressive behavior of crested newt larvae (Triturus macedonicus and its hybrid with T. ivanbureschi). Our results show that individuals reared with shelters had lower values of the tested antioxidant parameters (catalase, glutathione peroxidase, glutathione S-transferase and glutathione), indicating a lower production of reactive species than individuals reared without shelter. The same pattern was observed in both T. macedonicus and its hybrid. Contrary to the activation of some physiological pathways, shelter availability did not significantly affect the rate of intraspecific aggressive behavior. The physiological benefits of shelter use can be manifested as a lower requirement for investment in the energy necessary for the maintenance of the upregulated antioxidant defenses, activation of repair systems and synthesis of endogenous antioxidants. This study highlights the importance of shelter provision, which may be valuable in habitat restoration and animal conservation studies.

Species-specific habitat preferences do not shape the structure of a crested newt hybrid zone (Triturus cristatus x T. carnifex)

Reproductive isolation barriers maintain the integrity of species by preventing interspecific gene flow. They involve temporal, habitat or behavioral isolation acting before fertilization, and postzygotic isolation manifested as hybrid mortality or sterility. One of the approaches of how to study reproductive isolation barriers is through the analysis of hybrid zones. In this paper, we describe the structure of a hybrid zone between two crested newt species (Triturus cristatus and T. carnifex) in the southern part of the Czech Republic using morphological, microsatellite, and mitochondrial (mtDNA) markers. Specifically, we tested the hypothesis that the structure of the hybrid zone is maintained by species-specific habitat preferences. Comparing the genetic structure of populations with geographical and ecological parameters, we found that the hybrid zone was structured primarily geographically, with T. cristatus-like populations occurring in the northeast and T. carnifex-like populations in the southwest. Despite T. cristatus tending to occur in deeper ponds and T. carnifex on localities with more shading, the effect of both ecological parameters on the structure of the zone was minimal. Next, we corroborated that T. carnifex individuals and some hybrids possess mtDNA of T. dobrogicus, whose nuclear background was not detected in the studied hybrid zone. Hybridization between T. carnifex and T. dobrogicus (resulting in unidirectional mtDNA introgression) had to predate subsequent formation of the hybrid zone between T. cristatus and T. carnifex. Populations of crested newts in the southern part of the Czech Republic thus represent a genetic mosaic of nuclear and mitochondrial genomes of three species.

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.