Subterranean Fishes of the World

3211

2013

AIM 2013

The vertebrate sclera is an elastic tissue consisting of fibrous and cartilaginous or ossified layers. Despite its importance in visual acuity, little is known about the molecular basis of sclera development. Astyanax mexicanus can help resolve this issue because the surface fish (SF) and cavefish (CF) morphs exhibit different scleral phenotypes. A large sclera with ossified elements is present in SF adults, whereas a smaller and entirely cartilaginous sclera is present in CF (Pachón) adults. Sections of adult eyes show that the SF and CF sclera also differ in thickness, with the CF type almost three times thicker than the SF variety. The differences in Astyanax scleral morphology first appear near the end of the larval stage. The larval SF sclera initially develops a cartilaginous layer that gradually becomes ossified in the adult, whereas the CF larval sclera remains cartilaginous throughout life. Several experiments show that scleral morphology is controlled within the developing eye by the lens. First, removal of the CF eye and its replacement with a SF eye during embryogenesis results in the development of a SF-type ossified sclera in the CF host. Second, removal of the embryonic lens in SF results in the formation of a thick sclera resembling that of CF. Third, transplantation of a SF embryonic lens into a CF optic cup causes the development of a SF-type ossified sclera in the CF host. To understand the genetic basis of scleral diversity, the thickness of the fibrous and cartilaginous/ossified scleral layers was determined in 41 members of a SF x CF F2 hybrid family. The overall thickness of the two scleral layers was found to be continuously and normally distributed in these F2 hybrids, suggesting that multiple genes control the cavefish scleral phenotype. A preliminary QTL scan revealed three regions in the genome (with LOD scores approaching 3): one, on chromosome 2, responsible for fibrous layer thickness, and two, on chromosomes 17 and 24, responsible for cartilaginous/ossified layer thickness. Interestingly, the preliminary QTL on chromosome 24 covers the same genomic region as previously described Astyanax QTL for pupil (lens) area and retinal outer nuclear layer thickness, suggesting that the same or closely linked genes may control the CF lens, retinal, and scleral traits. We conclude that the lens controls phenotypic differences between the SF and CF sclera and that genetic approaches may be useful in revealing the molecular basis of this optic interaction.

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