Kelly A. Tomins

Vestigial structures are key indicators of evolutionary descent, but the mechanisms underlying their development are poorly understood. This study examines vestigial eye formation in the teleost Astyanax mexicanus, which consists of a sighted surface-dwelling morph and multiple populations of blind cave morphs. Cavefish embryos initially develop eyes, but they subsequently degenerate and become vestigial structures embedded in the head. The mutated genes involved in cavefish vestigial eye formation have not been characterized. Here we identify cystathionine ß-synthase a (cbsa), which encodes the key enzyme of the transsulfuration pathway, as one of the mutated genes responsible for eye degeneration in multiple cavefish populations. The inactivation of cbsa affects eye development by increasing the transsulfuration intermediate homocysteine and inducing defects in optic vasculature, which result in aneurysms and eye hemorrhages. Our findings suggest that localized modifications in the circulatory system may have contributed to the evolution of vestigial eyes in cavefish.

Coding and non-coding mutations in DNA contribute significantly to phenotypic variability during evolution. However, less is known about the role of epigenetics in this process. Although previous studies have identified eye development genes associated with the loss of eyes phenotype in the Pachón blind cave morph of the Mexican tetra Astyanax mexicanus 1-6 , no inactivating mutations have been found in any of these genes 2,3,7-10 . Here we show that excess DNA methylation-based epigenetic silencing promotes eye degeneration in blind cave Astyanax mexicanus. By performing parallel analyses in Astyanax mexicanus cave and surface morphs and in the zebrafish Danio rerio, we have discovered that DNA methylation mediates eye-specific gene repression and globally regulates early eye development. The most significantly hypermethylated and down-regulated genes in the cave morph are also linked to human eye disorders, suggesting the function of these genes is conserved across the vertebrates. Our results show that changes in DNA methylation-based gene repression can serve as an important molecular mechanism generating phenotypic diversity during development and evolution.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Abstract Vestigial structures are key indicators of evolutionary descent but the mechanisms underlying their development are poorly understood. This study examines vestigial eye formation in the teleost Astyanax mexicanus , which consists of a sighted surface-dwelling morph and different populations of blind cave morphs. Cavefish embryos initially develop optic primordia but vestigial eyes are formed during larval development. Multiple genetic factors are involved in cavefish eye loss but none of the mutated genes have been identified. Here we identify cystathionine ß-synthase ( cbsa ), which encodes the key enzyme of the transsulfuration pathway, as a mutated gene responsible for eye degeneration in multiple cavefish populations. The inactivation of cbsa affects eye development by inducing accumulation of the transsulfuration intermediate homocysteine and defects in optic vasculature, including aneurysms and eye hemorrhages, leading to oxygen deficiency. Our findings suggest that localized modifications in the circulatory system and hypoxia had important roles in the evolution of vestigial eyes in blind cavefish.