Nat. Ecol. Evol. https://doi.org/10.1038/s41559-018-0569-4 (2018).

Eyes aren’t necessary to live in the dark, and many dark-adapted creatures have lost them entirely. Such is the case for morphs of Mexican tetras that have adapted to live in caves. For the first few days post-fertilization, cave tetras will develop eyes just like their sunlight-soaked river counterparts. Their eyes soon begin to degenerate though, and are completely absent in adults.

But unlike other blind species, there are no apparent genetic mutations driving the loss. Instead, the cause looks to be epigenetic. RNA sequencing revealed increased expression of dnmt3bb.1in cavefish compared to river; this enzyme methylates DNA, which can modify gene expression without changing the DNA itself. Meanwhile, expression of genes involved with eye development was reduced. Chemically inhibiting the methylation process partially “rescued” the developing eyes that received the injection.