By sequencing the genomes of symbiotic microbes, scientists are currently uncovering the biological and mechanistic basis for these mutualistic associations. One of the primary genomic characteristics of obligate bacterial symbionts is a massive reduction in genome size compared to their free-living counterparts ?a phenomenon called "genome streamlining." Additional genome sequences of bacterial symbionts are needed, however, to more fully understand the biological basis for these associations.
Research assistants Patrick Degnan (now a doctoral student in the Department of Ecology and Evolutionary Biology at the University of Arizona) and Adam Lazarus worked with Wernegreen to sequence the entire genome ?all 791,654 nucleotides ?of Blochmannia pennsylvanicus, the endosymbiont that is specifically associated with the black carpenter ant (Camponotus pennsylvanicus). In order to trace genetic changes that occurred in the context of this ant-bacterial mutualism, they then compared the B. pennsylvanicus genome to the sequence from a related carpenter ant mutualist, B. floridanus.
Although the two Blochmannia species diverged between 16 and 20 million years ago, Wernegreen's group made a striking observation: All 635 genes shared between the two genomes were completely conserved in terms of order and strand orientation.
This is a remarkable observation, given that bacteria are particularly noted for their rapidly evolving genomes characterized by extensive recombination, gene transfer, inversion, and translocation. In comparison, the genomes of E. coli and Salmonella, which diverged between 100-150 million years ago, have undergone extensive changes in their genomic architecture. Interestingly, the observations of Wernegreen regarding B. pennsylvanicus were consistent with those previously described for the 150-200 million-year history of Buchn
Source:Cold Spring Harbor Laboratory