Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Arthropods produce a great variety of natural compounds, many of which have unexplored biosynthesis. Among the armored harvestmen (Arachnida: Opiliones) of the suborder Laniatores, the defensive gland exudates contain vinyl ketones and other constituents of supposed polyketide origin. We have studied the biosynthesis of 1-hepten-3-one in the Neotropical harvestman Iporangaia pustulosa by feeding individuals with C-13-labeled precursors, demonstrating its mixed acetate/propionate origin. C-13 NMR spectroscopy showed an unusual labeling pattern suggesting different propionate sources for starting and extender units. Our analysis also indicates the presence of methylmalonyl-CoA mutase, converting acetate into propionyl-CoA via succinyl-CoA, together with other C-3 unit routes. This is the first biosynthetic study of alkyl vinyl ketones in arthropods. Our results shed light on the origin and diversification of chemical compounds in a major arthropod group.3Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)PetrobrasFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [GM 02/00381-0, 08/06604-7]