Terrestriality as a constraint on vertebral shape in the eastern newt

Area of Science:

• Evolutionary biology
• Developmental biology
• Comparative anatomy

Background:

• Eastern newts (Notophthalmus viridescens) display complex, multiphasic life cycles with distinct terrestrial and aquatic stages.
• Polyphenism in newts allows for alternative life history strategies, including facultative paedomorphosis, leading to diverse phenotypes.
• Habitat shifts across life stages expose newts to varied mechanical forces, making them an ideal model for studying skeletal adaptation.

Purpose of the Study:

• To investigate how different life stages and habitats influence vertebral morphology and disparity in Eastern newts.
• To test the hypothesis that terrestrial life stages impose mechanical constraints on vertebral shape.
• To compare morphological disparity between aquatic phenotypes and explore the role of developmental pathways versus environmental factors.

Main Methods:

• Micro-computed tomography (μCT) was used to obtain high-resolution 3D images of newt skeletons.
• Geometric morphometrics (GMM) quantified vertebral shape and variation across different life stages and axial regions.
• Analysis included terrestrial juveniles, aquatic juveniles, aquatic paedomorphs, and semi-aquatic adults.

Main Results:

• Vertebral morphology in terrestrial life stages was constrained and this pattern persisted across ontogeny, even with niche shifts.
• Aquatic juveniles and paedomorphs showed significant morphological differences, indicating developmental pathways are key, not just shared aquatic habitat.
• Fully aquatic stages exhibited greater vertebral morphological disparity, especially in the caudal region, supporting relaxed gravitational constraints.

Conclusions:

• Vertebral shape in Eastern newts is significantly influenced by early terrestrial experiences, with these constraints carrying over into aquatic phases.
• Developmental trajectory, rather than solely the current environment, plays a crucial role in shaping aquatic newt vertebral morphology.
• The study highlights the interplay between developmental plasticity, environmental pressures, and skeletal evolution in amphibians.