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

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.