Decoupling of genome size and sequence divergence in a symbiotic bacterium

Abstract

In contrast to genome size variation in most bacterial taxa, the small genome size of Buchnera sp. was shown to be highly conserved across genetically diverse isolates (630 to 643 kb). This exceptional size conservation may reflect the inability of this obligate mutualist to acquire foreign DNA and reduced selection for genetic novelty within a static intracellular environment.

FIG. 1

Conservation of genomes sizes in Buchnera. Buchnera chromosomes digested with rare-cutting restriction enzymes were run on pulsed field gels to estimate genome sizes. Lanes are labeled by the abbreviation for the host name (Ap, A. pisum; Rp, R. padi; Sg, S. graminum; Mr, M. rhois). Note that I-CeuI cuts once and linearizes the Buchnera chromosome. With this enzyme, genomes sizes of Buchnera from S. graminum, R. padi, and A. pisum are indistinguishable. The finding of a slightly smaller genome size of Buchnera from M. rhois is supported by several independent enzyme digests.

FIG. 2

Relationships and genome sizes of Buchnera taxa. Host abbreviations are as given in Fig. 1. Divergence times are based on the aphid fossil record (8, 17).