Horizontally acquired cysteine synthase genes undergo functional divergence in lepidopteran herbivores.

Horizontal gene transfer (HGT) plays an important role in evolutionary processes as organisms adapt to their environments, and now cases of gene duplication after HGT in eukaryotes are emerging at an increasing rate. However, the fate and roles of the duplicated genes over time in eukaryotes remain unclear. Here we conducted a comprehensive analysis of the evolution of cysteine synthase (CYS) in lepidopteran insects. Our results indicate that HGT-derived CYS genes are widespread and have undergone duplication following horizontal transfer in many lepidopteran insects. Moreover, lepidopteran CYS proteins not only have β-cyanoalanine synthase activity but also possess cysteine synthase activity that is involved in sulfur amino acid biosynthesis. Duplicated CYS genes show marked divergence in gene expression patterns and enzymatic properties, suggesting that they probably have undergone subfunctionalization and/or neofunctionalization in Lepidoptera. The gene transfer of CYS genes and subsequent duplication appears to have facilitated the adaptation of lepidopteran insects to different diets and promoted their ecological diversification. Overall, this study provides valuable insights into the ecological and evolutionary contributions of CYS in lepidopteran insects.