How an aphid ( Acyrthosiphon pisum) symbiosis responds to variation in dietary nitrogen

Abstract

This study concerns the responses of the pea aphid Acyrthosiphon pisum to chemically-defined diets containing different concentrations of nitrogen (45–270 mM amino acids) and how these responses are influenced by the antibiotic chlortetracycline, which selectively disrupts the aphids' symbiotic bacteria. The chlortetracycline-treated aphids ingested less diet than untreated controls, and the difference between the two groups of insects could be ascribed largely to the greater size of the untreated aphids. Diet uptake increased with decreasing dietary amino acid concentration, in the range 202-89 mM for untreated aphids and 202-135 mM for chlortetracycline-treated aphids. This compensatory feeding response contributed to the close similarity of aphid growth rates on these diets. The honeydew of untreated and chlortetracycline-treated aphids contained 6–10 and 10–30 mol% of the dietary amino acid concentration, respectively, and the values (in percentage terms) did not vary consistently with dietary nitrogen. The concentration of non-essential amino acids, but not essential amino acids, was significantly higher in the honeydew of chlortetracycline-treated aphids than in untreated aphids' honeydew. The selective utilization of non-essential amino acids by untreated aphids is consistent with the hypothesis that the symbiotic bacteria utilize non-essential amino acids as a nitrogen source for essential amino acid synthesis, a process known as nitrogen upgrading.