The aposymbiotic aphid: An analysis of chlortetracycline-treated pea aphid, Acyrthosiphon pisum

Abstract

Acyrthosiphon pisum, whose symbiotic bacteria have been selectively disrupted by chlortetracycline, are known as aposymbionts. These aphids had significantly higher free amino acid content (108 nmol mg −1 fresh weight) than aphids containing their bacteria (65 nmol mg −1 fresh weight), but their protein content was depressed (0.034 and 0.041 mg mg −1 fresh weight for aposymbiotic and control aphids, respectively). Chlortetracycline treatment also influenced the amino acid composition of both the aphids and their honeydew. The carcasses of the aposymbionts had disproportionately high levels of the nitrogen-rich amino acids, asparagine and glutamine, and low levels of phenylalanine, isoleucine and threonine; and their honeydew was dominated by asparagine and glutamine, while that of untreated aphids contained high concentrations of arginine and histidine. It is suggested that: (a) protein synthesis in aposymbiotic aphids is limited by low availability of essential amino acids, which untreated aphids derive from their symbiotic bacteria; and (b) ammonia, a major nitrogenous waste product of the aphid, is utilized by the bacteria in untreated aphids, but it is detoxified, primarily by incorporation into glutamine in the aposymbiotic aphid. These data are consistent with the view that the bacteria contribute to the nutrition of aphids by the synthesis of essential amino acids.