The assimilation and allocation of nutrients by symbiotic and aposymbioticpea aphids, Acyrthosiphon pisum

The failure of a nutritionally balanced diet to ameliorate the impact of sy mbiont disruption in the pea aphid Acyrthosiphon pisum (Harris) was investi gated using two approaches. The assimilation of dietary nutrients by aphids was investigated using chemically-defined diets containing H-3-labelled in ulin and C-14-labelled sucrose or amino acids. Symbiotic aphids (i.e., aphi ds containing their bacteria) had a high sucrose demand and assimilated 72% of sucrose ingested in the diet, whereas the assimilation of sucrose by ap osymbiotic aphids (in which the bacteria had been disrupted), was significa ntly reduced to 47%. The assimilation of individual dietary amino acids by symbiotic aphids varied between 61 and 92%, and there was no impact on the feeding or assimilation rate when the aphids were fed a `phloem sap-like' d iet containing a reduced amount of essential amino acids. Consequently, the absolute amount of each essential amino acid assimilated by symbiotic aphi ds feeding on a `phloem sap-like' diet was reduced by 36-59%. Aposymbiotic aphids consistently assimilated a lower proportion of ingested amino acids, and lysine in particular was poorly assimilated from the diet. In a second experiment, the allocation of free amino acids in the haemocoel to aphid e mbryos was investigated following microinjection of C-14-labelled amino aci ds. After 2 h, radiolabel could be detected at varying levels from the embr yo complement of both symbiotic and aposymbiotic aphids, indicating rapid b ut selective uptake by the embryos. The essential amino acids phenylalanine and lysine were incorporated into the protein fraction of embryo tissues, but the rate of incorporation per unit biomass was approximately 4-fold hig her in the embryos of aposymbiotic aphids, possibly reflecting increased de mand due to the lack of amino acid provisioning from the symbiotic bacteria .