Microbial genotype and insect fitness in an aphid-bacterial symbiosis

1. The impact of the genotype of symbiotic micro-organisms on ecologically important traits of their animal hosts was explored using the symbiosis between aphids and the bacterium Buchnera. This symbiosis has a nutritional basis, with aphids dependent on Buchnera-derived essential amino acids. The genetic variation studied was copy number of the Buchnera genes trpEG, coding for anthranilate synthase in the biosynthetic pathway of the essential amino acid tryptophan. TrpEG amplification has been predicted to enhance tryptophan biosynthesis rates by Buchnera. 2. TrpEG amplification of Buchnera varied from 2 to 16-fold relative to other trp genes among aphids from natural populations of the Pea Aphid Acyrthosiphon pisum (Harris). 3. The fitness of aphids on Vicia faba plants of low phloem tryptophan content under field conditions and on tryptophan-free chemically defined diets under laboratory conditions did not vary significantly with trpEG copy number of Buchnera. It is concluded that naturally occurring variation in trpEG copy number does not have large and readily detectable effects on aphid fitness. 4. This first study on the impact of genetic diversity of Buchnera on ecologically important traits of the insect host illustrates the importance of empirical checks on the validity of plausible inferences made from genomic or other molecular data.