Parallel evolution of cox genes in H₂S-tolerant fish as key adaptation to a toxic environment

Populations that repeatedly adapt to the same environmental stressor offer a unique opportunity to study adaptation, especially if there are a priori predictions about the genetic basis underlying phenotypic evolution. Hydrogen sulphide (H₂S) blocks the cytochrome-c oxidase complex (COX), predicting the evolution of decreased H₂S susceptibility of the COX in three populations in the Poecilia mexicana complex that have colonized H ₂S-containing springs. Here, we demonstrate that decreased H ₂S susceptibility of COX evolved in parallel in two sulphide lineages, as evidenced by shared amino acid substitutions in cox1 and cox3 genes. One of the shared substitutions likely triggers conformational changes in COX1 blocking the access of H₂S. In a third sulphide population, we detect no decreased H₂S susceptibility of COX, suggesting that H ₂S resistance is achieved through another mechanism. Our study thus demonstrates that even closely related lineages follow both parallel and disparate molecular evolutionary paths to adaptation in response to the same selection pressure.