A test of the hypotheses that nitrogen is upgraded and recycled in an aphid ( Acyrthosiphon pisum) symbiosis

Abstract

The growth and developmental rates (i.e. “performance”) of larval pea aphids, Acyrthosiphon pisum, varied significantly with the concentration of amino acids (in the range 45–270 mM) and essential amino acid content (between 20 and 50 mol% of the total amino acids) in chemically-defined diets. The antibiotic chlortetracycline, which (at 50 μg ml −1) selectively disrupts the bacterial symbionts of aphids, depressed aphid performance on all diets, especially those with amino acid composition similar to phloem sap. Furthermore, the growth of chlortetracycline-treated aphids was reduced on diets with low essential amino acid content to a greater extent than untreated aphids, if the diet resembled phloem sap but not on a diet suitable for the long-term culture of aphids. This suggests that the upgrading of dietary non-essential amino acids to essential amino acids by the symbiotic bacteria might be significant to aphids feeding on diets with unbalanced amino acid content, such as phloem sap. However, the reduction in performance of chlortetracycline-treated aphids on diets of low amino acid concentration with balanced amino acid was not proportionately greater than in untreated aphids. This result indicates that, under the experimental conditions adopted, nitrogen recycling is not significant in the aphid symbiosis.