Biosynthesis of sorbic acid in aphids: An investigation into symbiont involvement and potential relationship with aphid pigments

Studies were undertaken to determine the role of symbionts and UV exposure in biosynthesis of the aphid‐specific polyketides, sorbic acid and quinone pigments. Injection of adult potato aphids, Macrosiphum euphorbiae (Thomas), with the antibiotic rifampicin did not alter the level of sorbic or myristic acid in triglycerides of resultant progeny; pigmentation was also unaffected. However, antibiotic injection did produce marked physiological effects; progeny from injected aphids were smaller, slower to mature, and not fecund. Light microscopy confirmed that only 8% of rifampicin‐treated aphids contained mycetocytes; thus, symbiont involvement in the production of this unusual UV‐quenching short chain fatty acid is not supported. Following multigenerational exposure to long wavelength UV light, no substantial changes in sorbic acid content were detected in the potato aphid or the oleander aphid, Aphis nerii Fonscolombe. Pigments from UV‐exposed oleander aphids had a peak absorbance at 390 nm, 70 nm lower than unexposed aphids. This suggests a photo‐protective role for the pigments of the sunlight‐inhabiting A. nerii; by contrast, no changes were observed in pigments of M. euphorbiae which usually feeds in the shade. Injection of adult potato aphids with sodium [1‐¹⁴C]‐acetate rapidly labeled both sorbic acid and pigments, particularly among the latter a yellow pigment which co‐chromatographed with the dominant C15 yellow pigment of the oleander aphid. These data support the hypothesis that aphid C30 pigments are built up by coupling of “monomeric type” C15 pigments. Although aphid and not symbiont enzymes appear to synthesize these acetogenins, a possible biosynthetic link between sorbic acid and aphid pigments requires further clarification. © 1994 Wiley‐Liss, Inc.