Decoupling of genome size and sequence divergence in a symbiotic bacterium

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