Subterranean Fishes of the World

4267

2019

AIM 2019

17

The fish Astyanax mexicanus comes in two forms: a normal river-dwelling form, and a blind cavedwelling morph. Although adult cavefish are completely eyeless, eyes first develop in embryos, in an “almost” normal manner. However, the embryonic cavefish eyes suffer several molecular and morphogenetic defects: the lens is small and undergoes apoptosis around 24 hours postfertilization, and the retina shows a coloboma phenotype with an apparent lack of optic fissure closure and a missing ventral quadrant of the retina. We aimed at understanding the mechanisms underlying this morphogenetic defect. We have thus generated identical Pachón cavefish and surface fish lines carrying a GFP reporter cassette inserted between regulatory elements of the Zic1 gene, through a CRISPR-Cas9 enhancer-trap knock-in strategy. Through 3D+time live imaging, we have then compared the morphogenesis of the optic vesicles in these reporter lines of the two morphs, from the eyefield at the end of gastrulation to the completion of eye formation at 36 hours post fertilization. In addition, through the analysis of regionalization and cell specification marker gene expression patterns (FoxG1, Tbx5a, FoxD1, Vax2, Pax2a, Bhlhe40), we have examined and compared the axial patterning of the developing retina of the two morphs. In the embryonic cavefish retina, we found defects in terms of sizes and cell movements responsible for the invagination of the optic vesicle to form the optic cup, resulting in improper patterning and coloboma. A scenario for the incorrect cavefish eye morphogenesis will be proposed, and potential molecular and cellular mechanisms will be discussed.

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