Variation in thermal plasticity of larval morphology among crested newt species and their reciprocal hybrids (Salamandridae: Triturus)

hybrids. Larvae were raised under experimental conditions at 19 °C and, from the late larval phase (stage 62), were exposed to either 19 °C or 24 °C. Geometric morphometrics was applied to quantify and analyse size and shape variation and its relationship with the rate of metamorphosis. Our results revealed that the four analysed genotypes (two species and two reciprocal hybrids) displayed divergent responses to elevated temperature. Triturus ivanbureschi and hybrids with T. macedonicus mitochondrial DNA were the most thermally sensitive, showing the fastest metamorphosis and marked shape plasticity in both body regions. In contrast, T. macedonicus exhibited the slowest metamorphosis but increased growth. Temperature-induced allometric changes had genotypic-specific patterns. Plastic response was partially explained by a change in allometric trajectories only for head shape. Generally, changes in head shape were more pronounced than those in tail shape. Overall, our findings indicate that accelerated metamorphosis is the main factor contributing to a plastic response. Hybridization contributes to the level of plasticity, shedding light on potential mechanisms underlying the dynamic interaction between species within their natural hybrid zone, and highlighting the possible impact of rising temperatures on species distribution and interactions.