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 expo sure in biosynthesis of the aphid-specific polyketides, sorbic acid an d quinone pigments. Injection of adult potato aphids, Macrosiphum euph orbiae (Thomas), with the antibiotic rifampicin did not alter the leve l of sorbic or myristic acid in triglycerides of resultant progeny; pi gmentation was also unaffected. However, antibiotic injection did prod uce marked physiological effects; progeny from injected aphids were sm aller, slower to mature, and not fecund. Light microscopy confirmed th at only 8% of rifampicin-treated aphids contained mycetocytes; thus, s ymbiont involvement in the production of this unusual UV-quenching sho rt chain fatty acid is not supported. Following multigenerational expo sure to long wavelength UV light, no substantial changes in sorbic aci d content were detected in the potato aphid or the oleander aphid, Aph is nerii Fonscolombe. Pigments from UV-exposed oleander aphids had a p eak absorbance at 390 nm, 70 nm lower than unexposed aphids. This sugg ests a photo-protective role for the pigments of the sunlight-inhabiti ng A. nerii; by contrast, no changes were observed in pigments of M. e uphorbiae which usually feeds in the shade. Injection of adult potato aphids with sodium [1-C-14]-acetate rapidly labeled both sorbic acid a nd pigments, particularly among the latter a yellow pigment which co-c hromatographed with the dominant C15 yellow pigment of the oleander ap hid. These data support the hypothesis that aphid C30 pigments are bui lt up by coupling of ''monomeric type'' C15 pigments. Although aphid a nd not symbiont enzymes appear to synthesize these acetogenins, a poss ible biosynthetic link between sorbic acid and aphid pigments requires further clarification. (C) 1994 Wiley-Liss, Inc.