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

Published

Journal Article

Buchnera are maternally transmitted bacterial endosymbionts that synthesize amino acids that are limiting in the diet of their aphid hosts. Previous studies demonstrated accelerated sequence evolution in Buchnera compared to free-living bacteria, especially for nonsynonymous substitutions. Two mechanisms may explain this acceleration: relaxed purifying selection and increased fixation of slightly deleterious alleles under drift. Here, we test the divergent predictions of these hypotheses for intraspecific polymorphism using Buchnera associated with natural populations of the ragweed aphid, Uroleucon ambrosiae. Contrary to expectations under relaxed selection, U. ambrosiae from across the United States yielded strikingly low sequence diversity at three Buchnera loci (dnaN, trpBC, trpEG), revealing polymorphism three orders of magnitude lower than in enteric bacteria. An excess of nonsynonymous 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 sequences further suggested that host bottlenecks and large-scale 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 aphid and endosymbiont loci.

Full Text

Duke Authors

Cited Authors

  • Funk, DJ; Wernegreen, JJ; Moran, NA

Published Date

  • February 2001

Published In

Volume / Issue

  • 157 / 2

Start / End Page

  • 477 - 489

PubMed ID

  • 11156972

Pubmed Central ID

  • 11156972

Electronic International Standard Serial Number (EISSN)

  • 1943-2631

International Standard Serial Number (ISSN)

  • 0016-6731

Language

  • eng