Facial bone fragmentation in blind cavefish arises through two unusual ossification processes

Abstract

The precise mechanisms underlying cranial bone development, evolution and patterning remain incompletely characterised. This poses a challenge to understanding the etiologies of craniofacial malformations evolving in nature. Capitalising on natural variation, “evolutionary model systems” provide unique opportunities to identify underlying causes of aberrant phenotypes as a complement to studies in traditional systems. Mexican blind cavefish are a prime evolutionary model for cranial disorders since they frequently exhibit extreme alterations to the skull and lateral asymmetries. These aberrations occur in stark contrast to the normal cranial architectures of closely related surface-dwelling fish, providing a powerful comparative paradigm for understanding cranial bone formation. Using a longitudinal and in vivo analytical approach, we discovered two unusual ossification processes in cavefish that underlie the development of ‘fragmented’ and asymmetric cranial bones. The first mechanism involves the sporadic appearance of independent bony elements that fail to fuse together later in development. The second mechanism involves the “carving” of channels in the mature bone, a novel form of post-ossification remodeling. In the extreme cave environment, these novel mechanisms may have evolved to augment sensory input, and may indirectly result in a trade-off between sensory expansion and cranial bone development.

Craniofacial skeleton of MEXICAN tetra (Astyanax mexicanus): As a bone disease model

“…SO3 bone fragmentation in adult CF is associated with the above remodeling mechanisms. This resembles human osteoporosis condition to some extent, although human osteoporosis typically occurs in long bones (Powers et al, ).…”

“…). The size and shape of the upper and lower jaws were documented in several studies among different CF populations (Mitchell et al, ; Yamamoto et al, ; Kowalko et al, ; Powers et al, ). The cartilaginous elements of the developing jaw structures of this species can be observed as early as 2.5 dpf in both morphs (Hinaux et al, ).…”

“…Alizarin red can work as a fluorescent dye and in vivo application of fluorescent marker, Calcein and Alizarin red can easily penetrate the skeleton and bind with calcified tissues. These dyes can be traced during the development of the animal to track the pattern of normal and abnormal bone mineralization (Javidan and Schilling, ; Powers et al, ). These techniques have been used successfully to understand various mineralized tissue dynamics in the teleost skeleton including bone remodeling (Bruneel and Witten, ; Bensimon‐Brito et al, ), tooth regeneration (Bruneel et al, ; Ellis et al, ), and in different Mexican tetra morphs to trace the facial bone fragmentation (Powers et al, ).…”

“…For a comprehensive description of SO3 variation, please refer to Chapter 10 in Biology and Evolution in Mexican Cavefish (Gross and Powers, ). In their elegant Calcein tracing experiment, Power et al (2018) found two temporally distinct developmental mechanisms of SO3 fragmentation in Pachon CF (Powers et al, ). A small percentage of bone fragments arose through the persistence of ectopic ossification centers in CF, as was proposed by Yamamoto et al (), but the majority of bone fragmentation occurred in juvenile SO3 bone, as a result of bone remodeling of ossified bone, through focal areas of resorption.…”

“…A small percentage of bone fragments arose through the persistence of ectopic ossification centers in CF, as was proposed by Yamamoto et al (), but the majority of bone fragmentation occurred in juvenile SO3 bone, as a result of bone remodeling of ossified bone, through focal areas of resorption. At this stage, canals form in the SO3 bone, which are eventually carved out to fragmented elements (Powers et al, ). The same group found that differential gene expression induced craniofacial changes associated with cave adaptation (Gross et al, ).…”