Comparative analysis of the parasphenoid bone development in Mexican tetra ( Astyanax mexicanus)

Abstract

Astyanax mexicanus is a teleost fish that belongs to the order Characiform. Ithas two distinct eye morphs as surface-dwelling, pigmented eyed surface fish population, and an eyeless cavefish population. Accumulation of functional mutations in the genes which are important for eye development led to the loss of eyes in cavefish. Some cranial changes have occurred from the direct effect of eye loss in cavefish including size and position of the circumorbital bones, ossified sclera, and the shape of both suborbital 3 and the supraorbital bone. The maxillary teeth, the positions of suborbital 4 to suborbital 6, and the shape of the opercular bone were found to be unaffected by the eye loss. Unlike higher vertebrates, the teleost eyeball does not cover by a bony socket and it has direct communication with the inner neurocranium. However, the influence of eye development on the inner neurocranial bones has gained little attention in research. In A. mexicanus palate bones are mainly made of a series of bones such as parasphenoid bone, basioccipital, vomer, and ethmoid. Among them, parasphenoid bone makes the largest part of the palate. This study was designed to understand the effect of eye loss in cavefish on parasphenoid bone development, shape determination, and bone remodeling activity. A. mexicanus populations; Surface fish and Pachon cavefish at 5dpf, 10 dpf, 20 dpf, 35 dpf, and 60 dpf subjected to alizarin red bone staining. Adult surface fish and two cave populations Pachon and Tinaja cavefish were subjected to micro-CT imaging and morphometric analysis was carried out using those images to compare the parasphenoid bone anatomy amongst these three morphs. TRAP and ALP staining were used to determine the bone remodeling activity in 60 dpf, A. mexicanus populations of surface and Pachon cavefish. A. mexicanus cavefish and surface are similar to each other in bone development, articulation, and ossification with certain exceptions such as late protrusion of lateral wings in cavefish. The straightened parasphenoid bone observed in cavefish and curved parasphenoid bone observed in surface fish beneath the eye orbit. Bone remodeling activity in cavefish was found to be increased compared to the surface fish. Cavefish have a direct or indirect effect on eye loss on the development of bones in the cranial region. Differential bone remodeling activity observed in cave and surface fish may result from the limited resources in the cave environment and as a long-term adaptation into troglomorphic cave environment.