Structures and biosynthesis of the ascarosides. a) The simple ascarosides ascr#1, ascr#3, ascr#9, and ascr#10, which serve signaling functions in C. elegans and many other nematode species. b) Modular ascarosides incorporating building blocks from folate (ascr#8), neurotransmitter (osas#10), tryptophan (icas#3, icas#10), and short-chain fatty acid (mbas#3) metabolism from C. elegans, and npar#1 from the satellite model organism P. pacificus, which features a nucleoside-derived building block. c) A model for the biosynthesis of ascarosides. Iterative 4-step β-oxidation of long-chain ascarosides (1) produces short-chain precursors (3) of modular ascarosides (4).[19]