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Compounding heterochrony shapes the salamander visual system across adaptive zones
Ronald M Bonett1, Emily L Bierbaum1, Alexander J Hess1
1Department of Biological Science, University of Tulsa, Tulsa, OK 74104, USA.
Proceedings. Biological Sciences
|September 23, 2025
Summary
Changes in developmental timing (heterochrony) and cave adaptation explain salamander eye and retina variation. Thyroid hormone (TH) signaling
Area of Science:
- Evolutionary biology
- Developmental biology
- Comparative anatomy
Background:
- Amphibian metamorphosis involves aquatic-to-terrestrial transitions, influenced by developmental timing (heterochrony).
- Thyroid hormone (TH) signaling regulates metamorphic tissue transformation and developmental diversification.
- Heterochrony has led to diverse life cycle modes, including aquatic-only and terrestrial-only forms.
Purpose of the Study:
- To investigate how life cycle mode and cave adaptation (troglomorphy) explain variation in salamander visual systems.
- To understand the role of heterochrony in reducing visual system investment.
- To examine the impact of thyroid hormone responsiveness on eye size evolution.
Main Methods:
- Comparative analysis of visual system traits across different salamander life cycle modes and environmental adaptations.
- Examination of developmental trajectories and the rate of visual system investment decline (following Haller's Rule).
- Assessment of thyroid hormone responsiveness in paedomorphic and troglomorphic salamanders.
Main Results:
- Life cycle mode and troglomorphy, driven by heterochrony, are primary factors in salamander eye and retina variation.
- Heterochrony causes serial reductions in the visual system of paedomorphic salamanders that have lost metamorphosis.
- Visual system investment declines faster in paedomorphs and even faster in troglomorphs, with reduced or reversed thyroid hormone responsiveness impacting eye size.
Conclusions:
- Alterations in hormone-mediated transformation can shift developmental trajectories, leading to compounded phenotypic modifications in divergent environments.
- Heterochrony and environmental adaptation interact to shape the evolution of complex traits like the visual system.
- Salamander visual system variation provides a model for understanding how developmental plasticity and environmental pressures drive evolutionary diversification.
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