Relating genotype and phenotype for tryptophan synthesis in an aphid-bacterial symbiosis

The symbiotic bacteria Buchnera provide their aphid hosts with tryptophan and other essential amino acids. Tryptophan production by Buchnera varied among 12 parthenogenetic clones of the pea aphid Acyrthosiphon pisum (Harris), as determined from both the incorporation of radioactivity from C-14-anthranilate into tryptophan and the protein-tryptophan growth rate of larval aphids on tryptophan-free diet. The values of tryptophan production obtained for the two methods were correlated significantly with each other but not with the level of amplification of the Buchnera genes trpEG , which code for anthranilate synthase, a key enzyme in tryptophan biosynthetic pathway. This study provides the first direct demonstration of interclonal variation in production of any nutrient in an aphid-Buchnera symbiosis and indicates that a key aspect of Buchnera phenotype (tryptophan production) does not vary in a simple fashion with Buchnera genotype.