Another important feature of B. pennsylvanicus tested by the researchers was the rate of protein evolution, as measured by amino acid changes, since the divergence of B. pennsylvanicus from their free-living ancestors. The researchers showed that the amino acid sequences of Blochmannia have diverged approximately 50 times more quickly than proteins in free-living bacteria. According to the scientists, endosymbiont proteins may be more tolerant of amino acid changes when they first become associated with their hosts, and this may account for the rapid rates of protein evolution observed.
In addition, protein sequences of the two Blochmannia species exhibited different rates of evolution; divergence rates were approximately two times faster in B. floridanus than in B. pennsylvanicus. The authors suggested that these lineage-specific differences may reflect life history differences of their respective ant hosts.
When the observations of the current study are coupled with results from previous studies, a new model for bacterial genome evolution in the context of a host-symbiont relationship emerges. As Wernegreen and her colleagues explain in their Genome Research article, long-term genome stasis is a striking characteristic of insect mutualists, and it may severely constrain the evolutionary potential of these symbiotic microbes.
However, whether rapid rates of protein evolution are important for the adaptation of insect mutualists remains unclear. While the current study documents rapid changes in amino acid sequence through evolutionary time, some studies suggest that most changes in proteins are slightly harmful to the bacterium and by extension, to its host.
"Overall, the picture emerging is one of genome deterioration, with the loss of many gene functions, and extreme genome stability," says Werne
Source:Cold Spring Harbor Laboratory