Subterranean Fishes of the World

3265

2017

AIM 2017

Cave-dwelling organisms frequently evolve asymmetric features, including differences in eye size across the left-right axis. In Astyanax cavefish, the circumorbital bones demonstrate ossification abnormalities such as ‘fragmentations’ and ‘fusions’ of dermal bone elements. The developmental basis for these abnormalities has long remained mysterious. To better understand how early ossification may impact on the adult cranial phenotype, we employed an intra-individual live-staining approach coupled with comprehensive longitudinal fluorescence imaging. We discovered that cave-dwelling forms develop secondary ossification centers that periodically fuse with – or remain distinct from - the larger growing bony element. In the adult, these distinct fragments resemble the same ultrastructure as a normal bone-to-bone boundary. Since the positions of sensory structures can be influenced by bony margins, we next evaluated whether superficial neuromasts may “mirror” the patterns observed in an asymmetric circumorbital bones. We quantified the position of neuromasts and used a novel method to determine the degree of similarity in their patterns, across the left-right axis. We discovered that cave-dwelling forms show significantly less symmetric neuromast distributions, reflecting their robust asymmetry in cranial form. Taken together, this work illuminates the developmental basis for cranial bone asymmetry and provides the opportunity to explore if fragmentation evolves neutrally, or whether it may be linked to the adaptive process of expanding non-visual sensation in complete darkness.

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