Subterranean Fishes of the World

4301

2019

AIM 2019

53

Deviations from symmetry are common among cave-dwelling organisms. Laterality in the cranium was among the first characterized aberrations discovered in Mexican cavefish following their characterization in 1936. Early researchers were unclear of the origin of these asymmetries, suggesting they may have arisen from a wound inflicted in life, or during post mortem handling. Several populations (and decades) later, it became clear that certain asymmetries – those associated with bone fragmentation – were a normative component of the cranial skeleton in cavefish. Interestingly, these aberrations are never observed in surface-dwelling fish, enabling comparative analyses in the two morphotypes. Certain components of the cranial skeleton are controlled by asymmetric genetic loci, i.e., QTL detected when analyzing one side of the head. Here, we describe the presence of three subtypes of cranial asymmetry. The first component is a cranial “bend” that appears late in the cavefish life history, nearly always associated with the left side. A second component is facial bone fragmentation, which is mediated by two unusual ossification processes. The first process involves ectopic centers that largely coalesce back into the developing dermal bone. The second process, which is more common, involves the spontaneous dissolution of a bony channel through mature bone. The third component of asymmetry involves the dermal bone positioning of the suborbital series. We demonstrate that primary ossification centers are associated with the positions of early canal neuromasts. The presence/absence of certain neuromasts are associated with presence/absence of associated bones. Moreover, shorter distances between embryonic canal neuromasts appears to predict fusion of neighboring suborbital bones. In conclusion, cranial asymmetry is a complex phenotype in cavefish, which is underpinned by a number of aberrant developmental events. Future studies are devoted towards understanding how sensory-skeletal integration may underlie adaptation to the extreme environment of the cave.

1

Astyanax Interrnational Meeting