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.

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.

BackgroundCave-dwelling animals evolve various traits as a consequence of life in darkness. Constructive traits (e.g., enhanced non-visual sensory systems) presumably arise under strong selective pressures. The mechanism(s) driving regression of features, however, are not well understood. Quantitative trait locus (QTL) analyses in Astyanax mexicanus Pachón cave x surface hybrids revealed phenotypic effects associated with vision and pigmentation loss. Vision QTL were uniformly associated with reductions in the homozygous cave condition, however pigmentation QTL demonstrated mixed phenotypic effects. This implied pigmentation might be lost through both selective and neutral forces. Alternatively, in this report, we examined if a pleiotropic interaction may exist between vision and pigmentation since vision loss has been shown to result in darker skin in other fish and amphibian model systems.ResultsWe discovered that certain members of Pachón x surface pedigrees are significantly darker than surface-dwelling fish. All of these “hypermelanic” individuals demonstrated severe visual system malformations suggesting they may be blind. A vision-mediated behavioral assay revealed that these fish, in stark contrast to surface fish, behaved the same as blind cavefish. Further, hypermelanic melanophores were larger and more dendritic in morphology compared to surface fish melanophores. However, hypermelanic melanophores responded normally to melanin-concentrating hormone suggesting darkening stemmed from vision loss, rather than a defect in pigment cell function. Finally, a number of genomic regions were coordinately associated with both reduced vision and increased pigmentation.ConclusionsThis work suggests hypermelanism in hybrid Astyanax results from blindness. This finding provides an alternative explanation for phenotypic effect studies of pigmentation QTL as stemming (at least in part) from environmental, rather than exclusively genetic, interactions between two regressive phenotypes. Further, this analysis reveals persistence of background adaptation in Astyanax. As the eye was lost in cave-dwelling forms, enhanced pigmentation resulted. Given the extreme cave environment, which is often devoid of nutrition, enhanced pigmentation may impose an energetic cost. Such an energetic cost would be selected against, as a means of energy conservation. Thus, the pleiotropic interaction between vision loss and pigmentation may reveal an additional selective pressure favoring the loss of pigmentation in cave-dwelling animals.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0716-y) contains supplementary material, which is available to authorized users.

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Non‐traditional model systems can provide insight to the developmental and genetic basis of fundamental biological processes. One such process is the origin and patterning of the facial skeleton. Here, we present evidence that the intramembranous bones comprising the circumorbital complex in a teleost model are marked by the positions of large mechanosensory structures, canal neuromasts. Neuromasts comprise the lateral line system, and provide pressure and proprioceptive information to enable a number of key behaviors, such as predator avoidance, foraging, schooling and shoaling. Canal neuromasts appear to set the stage for facial patterning, since they colocalize to ossification sites destined to give rise to the six constituent bones of the circumorbital complex. Here, we demonstrate that closely related surface‐dwelling morphs demonstrate an invariant pattern of development that gives rise to a symmetric and stereotypical circumorbital complex. In cavefish, however, this pattern is frequently altered ‐ giving rise to ossification centers that are set closer, or more distant, from one another. Moreover, these alterations in position can vary across the left‐right axis. Interestingly, in situations where two ossification centers are more closely set towards one another, they result in an inter‐bone fusion (synostotic) event. Cavefish demonstrate at least three forms of cranial asymmetry, and we discuss how facial bone fusions may impact global craniofacial shape. These three‐dimensional changes may prefigure, or facilitate, the lateral swimming preferences observed in cavefish. Moreover, the ‘predictive’ nature of circumorbital bone fusion will enable deeper insight to the connection between the earliest developmental events, and adult bone patterning and morphology. Collectively, the Mexican tetra is poised to provide new insight to the developmental basis for craniofacial aberrations shared deeply among the vertebrate subphylum.