Co-opted Genes of Algal Origin Protect C. elegans against Cyanogenic Toxins

SUMMARY Amygdalin is a cyanogenic glycoside widely used by many plants in herbivore defense. Poisonous to most animals, amygdalin-derived cyanide is detoxified by potent enzymes commonly found in bacteria and plants but not most animals. Here we show that the nematode C. elegans can detoxify amygdalin by a genetic pathway comprising cysl-1, egl-9, hif-1 and cysl-2 . Essential for amygdalin resistance, cysl-1 encodes a protein similar to cysteine synthetic enzymes in bacteria and plants, but functionally co-opted in C. elegans . We identify exclusively HIF-activating egl-9 mutations in a cysl-1 suppressor screen and show that cysl-1 confers amygdalin resistance by regulating HIF-1-dependent cysl-2 transcription to protect against amygdalin toxicity. Phylogenetic analysis suggests cysl-1 and cysl-2 were likely acquired from green algae through horizontal gene transfer (HGT) and functionally co-opted in protection against amygdalin. Our studies reveal that HGT-mediated evolutionary changes can facilitate host survival and adaptation to adverse environment stresses and biogenic toxins.