Polyketides in insects: ecological role of these widespread chemicals and evolutionary aspects of their biogenesis

Polyketides are known to be used by insects for pheromone communication and defence against enemies. Although in microorganisms (fungi, bacteria) and plants polyketide biogenesis is known to be catalysed by polyketide synthases (PKS), no insect PKS involved in biosynthesis of pheromones or defensive compounds have yet been found. Polyketides detected in insects may also be biosynthesized by endosymbionts. From a chemical perspective, polyketide biogenesis involves the formation of a polyketide chain using carboxylic acids as precursors. Fatty acid biosynthesis also requires carboxylic acids as precursors, but utilizes fatty acid synthases (FAS) to catalyse this process. In the present review, studies of the biosynthesis of insect polyketides applying labelled carboxylic acids as precursors are outlined to exemplify chemical approaches used to elucidate insect polyketide formation. However, since compounds biosynthesised by FAS may use the same precursors, it still remains unclear whether the structures that are formed from e.g. acetate chains (acetogenins) or propanoate chains (propanogenins) are PKS or FAS products. A critical comparison of PKS and FAS architectures and activities supports the hypothesis of a common evolutionary origin of these enzyme complexes and highlights why PKS can catalyse the biosynthesis of much more complex products than can FAS. Finally, we summarise knowledge which might assist researchers in designing approaches for the detection of insect PKS genes.