Injection of essential amino acids substitutes for bacterial supply in aposymbiotic pea aphids (Acyrthosiphon pisum)

The symbiotic bacteria Buchnera contribute to the nutrition of pea aphids, Acyrthosiphon pisum, through the provision of essential amino acids which a re lacking in the diet. However, chemically defined diets, containing nutri tionally adequate amounts of essential amino acids, fail to rescue aposymbi otic aphids, in which the bacteria have been disrupted with antibiotics. In this study the injection of a mixture of essential amino acids into the ha emocoel of aposymbiotic aphids was shown to alleviate, at least partially, the impact of symbiont loss. Specifically, the total amino acid content in the tissues of aposymbiotic aphids was reduced by approximately 40% to leve ls comparable with symbiotic insects, and there was a 1.7-fold increase in the number of embryos, suggesting that the availability of essential amino acids promotes aphid protein synthesis by rejuvenating the free amino acid pool of aposymbiotic aphids. In addition, a similar effect on the total ami no acid content was observed when phenylalanine alone, but not glutamine, l ysine or tryptophan, was injected into the haemocoel of aposymbiotic aphids , and there was also a significant increase in the number of embryos follow ing injection of phenylalanine or tryptophan alone. The impact of amino aci d injection on the embryo complement of aposymbiotic aphids was limited to an increase in the number of embryos, with no increase in basal embryo size . It is proposed that older embryos may rely on their own complement of sym biotic bacteria for essential amino acid provisioning. Taken together, the data highlight the importance of bacterial provisioning of essential amino acids, particularly the aromatic amino acids, in the intact symbiosis.