WHY PEA APHIDS (ACYRTHOSIPHON-PISUM) LACKING SYMBIOTIC BACTERIA HAVE ELEVATED LEVELS OF THE AMINO-ACID GLUTAMINE

Aposgmbiotic pea aphids Acyrthosiphon pisum (i.e. aphids lacking funct ional symbiotic bacteria) were generated by treatment with the antibio tic rifampicin, Eight-day-old aposymbiotic aphids produced honeydew co ntaining 16-30 mol% glutamine, while the honeydew of symbiotic aphids (containing bacteria) contain <1 mol% glutamine. The elevated glutamin e production by aposymbiotic aphids was correlated with 7.4-fold and 1 .5-fold greater concentrations of ammonia in the honeydew and tissues, respectively, and a 1.8-fold greater activity of glutamine synthetase in the aphid tissues. The interpretation that glutamine is synthesize d from the assimilation of ammonia into glutamate via glutamine synthe tase is supported by a significant increase in the glutamine synthetas e activity of symbiotic and aposymbiotic aphids reared on ammonia-supp lemented diets. Elevated rates of both ammonia synthesis in aposymbiot ic aphids and ammonia consumption by the bacteria in symbiotic aphids could contribute to the enhanced glutamine production by the aposymbio nts.