Cranial asymmetry arises later in the life history of the blind Mexican cavefish, Astyanax mexicanus

Abstract

As a consequence of adaptation to the cave environment, the blind Mexican cavefish, Astyanax mexicanus, has evolved several cranial aberrations including changes to bone sizes, shapes and presence of numerous lateral asymmetries. Prior studies of cranial asymmetry in cavefish focused strictly on adult specimens. Thus, the extent to which these asymmetries emerge in adulthood, or earlier in the life history of cavefish, was unknown. We performed a geometric morphometric analysis of shape variation in the chondrocranium and osteocranium across life history in two distinct cavefish populations and surface-dwelling fish. The cartilaginous skull in juveniles was bilaterally symmetric and chondrocranial shape was conserved in all three populations. In contrast, bony skull shapes segregated into significantly distinct groups in adults. Cavefish demonstrated significant asymmetry for the bones surrounding the collapsed eye orbit, and the opercle bone posterior to the eye orbit. Interestingly, we discovered that cavefish also exhibit directional “bends” in skull shape, almost always biased to the left. In sum, this work reveals that asymmetric craniofacial aberrations emerge later in the cavefish life history. These abnormalities may mirror asymmetries in the lateral line sensory system, reflect a ‘handedness’ in cavefish swimming behavior, or evolve through neutral processes.

A geographical cline in craniofacial morphology across populations of Mesoamerican lake‐dwelling fishes

“…A total of 24 homologous landmarks were collected from dorsal, lateral, and ventral orientations inclusive of the premaxilla, maxilla, mandible, supraorbital, lateral ethmoid, and opercle bones in 3D for 36 specimens (Table S1). Landmark collection and shape analyses were conducted, as described in Powers, Davis, Kaplan, and Gross (). Since allometry and size could affect the patterns of variation, we corrected for the possible effects of allometry in our datasets, computing the residuals from the regression of shape on centroid size and used those residuals in the subsequent analyses (Ornelas‐García, Bautista, Herder, & Doadrio, ).…”

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

“…This is common in the teleost craniofacial skeleton. Unlike mammals, ossification of teleost skeleton progresses up to adulthood and ossification occurs by means of both endochondral and intramembranous ossification (Cubbage and Mabee, ; Witten and Hall, ; Powers et al, ).…”

“…These skeletal staining techniques have helped in identifying the natural and induced skeletal phenotypes. Recent morphometric studies on stained craniofacial skeleton include: a comparison of the chondrocranium and osteocranium among cave morphs and SF (Powers et al, ). An analysis of the eye‐dependent and eye independent craniofacial patterning in different cave populations, SF, lens removed SF, and lens transplanted CF (Yamamoto and Jeffery, ; Yamamoto et al, ); and the long‐term effects of lens removal at early stages of life on the cranial bones (Atukorala et al, ; Dufton and Franz‐Odendaal, ).…”

“…The distinct craniofacial characteristics arise from eye loss associated space shift and altered behavioral adaptation. The CF was found to have large opercular bones, jaw bones, increased number of teeth, and altered number of circumorbital bones (Yamamoto et al, ; Atukorala et al, ; Powers et al, ). The F1and F2 hybrids of this species exhibit a wide array of craniofacial variations in above‐mentioned structures.…”

“…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, ).…”