EFFECT OF DIET ON THE FREE AMINO-ACID POOLS OF SYMBIOTIC AND APOSYMBIOTIC PEA APHIDS, ACYRTHOSIPHON-PISUM

Free amino acid pools were analysed in Acyrthosiphon pisum reared on Vicia faba L. and on three artificial diets with different amino acid profiles: diets A and B copying the very unbalanced profiles of phloem saps of alfalfa and broad bean respectively, diet C deriving from aphid carcass analysis. Total free amino acid levels ranged from 46.9 +/- 2.3 nmol . mg(-1) fresh weight for aphids maintained on host plant to 86.0 +/- 3.6 nmol . mg(-1) for those reared on diet B. Whatever the food source was, the free amino acid pools of aphids displayed roughly similar patterns, except for lysine and arginine. The role played by the intracellular symbionts in this homeostasis was investigated with aposymbiotic aphids reared on the same diets. The treated aphids had significantly higher free amino acid content (119.2 +/- 1.7 to 140.0 +/- 7.7 nmol . mg(-1) fresh weight) than control aphids. In contrast to the symbiotic situation, the well balanced free amino acid pools were not maintained in aposymbiotic aphids: four amino acids were found in higher concentrations in aposymbiotic aphids (asparagine, aspartic acid, glutamine and proline), while isoleucine, leucine, tyrosine, phenylalanine, threonine and glutamic acid were in lower concentrations. These results are consistent with the hypothesis that symbiotic bacteria contribute to the nutrition of aphids by the synthesis of essential amino acids. The observed negative correlation of free amino acid levels with aphid performances suggested a possible use of this parameter as an indirect criteria to measure the quality of a natural food for aphid development, and to test the nutritional fitness of an aphid population to its host plant.