Early Lens Ablation Causes Dramatic Long-Term Effects on the Shape of Bones in the Craniofacial Skeleton of Astyanax mexicanus

Abstract

The Mexican tetra, Astyanax mexicanus, exists as two morphs of a single species, a sighted surface morph and a blind cavefish. In addition to eye regression, cavefish have an increased number of taste buds, maxillary teeth and have an altered craniofacial skeleton compared to the sighted morph. We investigated the effect the lens has on the development of the surrounding skeleton, by ablating the lens at different time points during ontogeny. This unique long-term study sheds light on how early embryonic manipulations on the eye can affect the shape of the adult skull more than a year later, and the developmental window during which time these effects occur. The effects of lens ablation were analyzed by whole-mount bone staining, immunohistochemisty and landmark based morphometric analyzes. Our results indicate that lens ablation has the greatest impact on the skeleton when it is ablated at one day post fertilisation (dpf) compared to at four dpf. Morphometric analyzes indicate that there is a statistically significant difference in the shape of the supraorbital bone and suborbital bones four through six. These bones expand into the eye orbit exhibiting plasticity in their shape. Interestingly, the number of caudal teeth on the lower jaw is also affected by lens ablation. In contrast, the shape of the calvariae, the length of the mandible, and the number of mandibular taste buds are unaltered by lens removal. We demonstrate the plasticity of some craniofacial elements and the stability of others in the skull. Furthermore, this study highlights interactions present between sensory systems during early development and sheds light on the cavefish phenotype.

Adaptive evolution of the lower jaw dentition in Mexican tetra (Astyanax mexicanus)

“…The significant increase in caudal tooth number on the lens-removed side of the mandible was described in our previous study [12]. The lens was removed at four different time points.…”

“…This is particularly interesting, given that a large difference in tooth number was observed in this region of the mandible after the surgery [12]. …”

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

“…Lens removal and lens transplantation experiments in A. mexicanus have demonstrated the role of the lens in determining the final morphology of these SO bones (Yamamoto et al, ; Dufton et al, ). Dufton et al () performed lens removal surgery on SF at early development stages, from 24 hr post fertilization (hpf) to 4 days post fertilization (dpf), and found shape changes of SO bone and expansion of the SO4 and SO5 bones into the orbital and SO6 areas (Dufton et al, ). The same group investigated the effect of eye reduction on the shape of the surrounding bones of the different A. mexicanus morphs (SF, Tinaja CF, and F1 intermediates) as compared to the outgroup, zebrafish that had undergone unilateral lens removal and in the zebrafish bumper mutant (bmp ‐/‐ ).…”

“…However, the zebrafish has limitations as a model for this disease, as the zebrafish lacks oral teeth. The lens ablated phenotype of A. mexicanus exhibits degeneration of the anterior eye segment, cranial bone abnormalities, and enhanced dentition in the oral jaws (Yamamoto and Jeffery, ; Yamamoto et al, ; Dufton et al, ). We also found increased levels of Pitx2 expression in the surgery eye and low expression of Pitx2 in the forebrain (Atukorala and Franz‐Odendaal, ); thus, the Astyanax system will be an ideal model with which to identify the putative molecular mechanisms of ARS (Atukorala and Franz‐Odendaal, ).…”

“…The craniofacial skeleton of the CF and SF have been subjected to several morphogenetic studies (Yamamoto and Jeffery, , ; Soares et al, ; Jeffery, ; Ma et al, ; Stemmer et al, ). These include investigating the effects of the developing eye on the surrounding soft and hard tissues (Yamamoto et al, ; Soares et al, ; Retaux et al, ; Dufton et al, ; Gross et al, ; Powers et al, ), jaw and tooth development (Yamamoto et al, ; Dufton et al, ; Atukorala et al, ), development of the taste buds (Protas et al, ; Varatharasan et al, ; Yamamoto et al, ) and the lateral line system (Gross et al, ; Powers et al, ).…”