Decay of mutualistic potential in aphid endosymbionts through silencing ofbiosynthetic loci: Buchnera of Diuraphis

Buchnera, the primary bacterial endosymbiont of aphids, is known to provisi on essential amino acids lacking in the hosts: diet of plant sap. The recen t discovery of silenced copies of genes for tryptophan biosynthesis (trpEG) in certain Buchnera lineages suggests a decay in symbiotic functions in so me aphid species. However, neither the distribution of pseudogenes among li neages nor the impact of this gene silencing on amino-acid availability in hosts has been assessed. In Buchnera of the aphid Diuraphis noxia, tandem r epeats of these pseudogenes have persisted in diverse lineages, and trpEG p seudogenes have originated at least twice within this aphid genus. Measures of amino-acid concentrations in Diuraphis species have shown that the pres ence of the pseudogene is associated with a decreased availability of trypt ophan, indicating that gene silencing decreases nutrient provisioning by sy mbionts. In Buchnera of Diuraphis, rates of nonsynonymous substitutions are elevated in functional trpE copies, supporting the hypothesis that pseudog ene origin and persistence reflect a reduced selection for symbiont biosynt hetic contributions. The parallel evolution of trpEG pseudogenes in Buchner a of Diuraphis and certain other aphid hosts suggests that either selection at the host level is not effective or that fitness in these aphids is not limited by tryptophan availability.