Building blocks of a fish head: Developmental and variational modularity in a complex system

Area of Science:

• Developmental biology
• Evolutionary morphology
• Comparative anatomy

Background:

• Vertebrate skull evolution is influenced by interactions between anatomical systems.
• The relationship between dermal bones and lateral line canals is a long-standing debate.
• Morphological integration between these systems has not been empirically tested.

Purpose of the Study:

• To investigate the patterning and modularity of sensory lateral line canals in Amia calva.
• To test the integration between these canals and their supporting cranial bones.
• To understand the developmental constraints on skull evolution.

Main Methods:

• Analysis of an ontogenetic series of 97 Amia calva specimens.
• Examination of the branching sequence of lateral line pores to assess developmental modules.
• Geometric morphometrics applied to digitized landmarks on 25 specimens to test modularity hypotheses.

Main Results:

• The otic canal and supratemporal commissure form distinct developmental modules.
• Significant positive correlations in pore appearance suggest bilateral symmetry in canal patterning.
• While canals and dermal bones showed insufficient integration to form a single module, they were not entirely independent.
• Groups of pores acted as quasi-independent units, suggesting a hierarchical modular organization.

Conclusions:

• Sensory lateral line canals possess a hierarchical modular organization.
• This modularity can explain species-specific variations in canal patterns.
• The findings provide insights into the developmental constraints shaping vertebrate skull evolution.