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.

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.