Intraspecific variation in symbiont genomes: Bottlenecks and the aphid-buchnera association

Buchnera are maternally transmitted bacterial endosymbionts that synthesize amino acids that are limiting in the diet of their aphid hosts. Previous s tudies demonstrated accelerated sequence evolution in Buchnera compared to free-living bacteria, especially for nonsynonymous substitutions. Two mecha nisms may explain this acceleration: relaxed purifying selection and increa sed fixation of slightly deleterious alleles under drift. Here, we test the divergent predictions of these hypotheses for intraspecific polymorphism u sing Buchnera associated with natural populations of the ragweed aphid, Uro leucon ambrosiae. Contrary to expectations under relaxed selection, U. ambr osiae from across the United States yielded strikingly low sequence diversi ty at three Buchnera loci (dnaN, trpBC, trpEG), revealing polymorphism thre e orders of magnitude lower than in enteric bacteria. An excess of nonsynon ymous polymorphism and of rare alleles was also observed. Local sampling of additional dnaN sequences revealed similar patterns of polymorphism and no evidence of food plant-associated genetic structure. Aphid mitochondrial s equences further suggested that host bottlenecks and large-scare dispersal may contribute to genetic homogenization of aphids and symbionts. Together, our results support reduced N-e as a primary cause of accelerated sequence evolution in Buchnera. However, our study cannot rule out the possibility that mechanisms other than bottlenecks also contribute to reduced N-e at ap hid and endosymbiont loci.