# Exploring Larval Axolotl Brain Development: Insights Into Developmental and Functional Constraints

## TL;DR

This study examines brain development in axolotl larvae, revealing how brain structures change and the constraints influencing their growth.

## Contribution

The study provides new insights into the interplay of functional and developmental constraints in axolotl brain ontogeny.

## Key findings

- The telencephalon-diencephalon boundary and hypothalamus show low morphological variation during larval development.
- Sensory structures like olfactory bulbs and optic tectum undergo allometric growth followed by isometric growth.
- Both mosaic and concerted evolution models are supported, indicating a complex developmental process.

## Abstract

Brain evolution in vertebrates has been conceptualized through two major hypotheses: the mosaic and concerted evolution models. The mosaic evolution model suggests that brain structures are primarily shaped by functional constraints, whereas the concerted evolution model emphasizes the role of developmental constraints. Our objectives in this study were (1) to describe brain shape and volume changes during Mexican axolotl (Ambystoma mexicanum) larvae development, and (2) to interpret possible functional and developmental constraints during post‐hatching brain maturation. A total of 77 larvae, spanning four developmental stages, were examined using 3D geometric morphometrics and volumetric measurements derived from iodine micro‐CT imaging. To understand the relationships among brain regions, we employed morphological integration and modularity analyses, providing a comprehensive assessment of changes in shape covariation patterns during post‐hatching development. Our results reveal that the telencephalon‐diencephalon boundary and the hypothalamus region exhibit a low level of morphological variation throughout larval development. This stability may influence the positioning of the coronal suture, a key feature in tetrapod skull morphogenesis. In contrast, sensory structures undergo significant changes. The olfactory bulbs and optic tectum display positive allometric growth during early post‐hatching development, transitioning to isometric growth at later stages. These shifts suggest an early developmental emphasis on sensory‐related brain areas, potentially driven by functional constraints. Results also revealed a general correspondence between brain region volume and total brain volume, which aligns with the concerted model. Modular, morphological integration, and volumetric analyses suggest that an interplay of functional and developmental constraints might be involved in axolotl brain development.

We studied brain shape and volume variations during larval development of the Mexican axolotl. Modularity, morphological integration, and allometric regressions supported both mosaic and concerted evolution models, highlighting a complex interplay of constraints during axolotl ontogeny.

Six independent brain regions have been found in larval axolotls.Telencephalon–diencephalon boundary is stable in development.Evidence supports both concerted and mosaic brain evolution.

Six independent brain regions have been found in larval axolotls.

Telencephalon–diencephalon boundary is stable in development.

Evidence supports both concerted and mosaic brain evolution.