Distinct Patterns of Selection in Selenium-Dependent Genes between Land and Aquatic Vertebrates

Selenium (Se), a sparse element on earth, is an essential micronutrient in the vertebrate diet and its intake depends on its content in soils and waters worldwide. Selenium is required due to its function in selenoproteins, which contain selenocysteine (Sec), the 21st amino acid in the genetic code, as one of their constituent residues. Selenocysteine is analogous to the amino acid cysteine (Cys), which uses the abounding element sulfur instead. Despite the irregular distribution of Se worldwide, its distinct biochemical properties have made the substitution of Sec for Cys rare in vertebrate proteins. Still, vertebrates inhabited environments with different amounts of Se and may have distinctly adapted to it. To address this question, we compared the evolutionary forces acting on the coding sequences of selenoprotein genes and genes that regulate Se between vertebrate clades and between the Se-dependent genes and their paralogs with Cys. We find that the strength of natural selection in genes that use or regulate Se is distinct between land vertebrates and teleost fishes and more variable than in the Cys paralogs, particularly in genes involved in the preferential supply of Se to some organs and the tissue-specific expression of selenoproteins. This is compatible with vertebrates adapting to Se scarcity in land and its abundance in waters. In agreement, teleost fishes duplicated and subfunctionalized or neofunctionalized selenoprotein genes and maintained their capacity for Se transport in the body, which declined (under neutrality) for millions of years in terrestrial vertebrates. Dietary Se has thus distinctly shaped vertebrate evolution.