Gene Survival in Emergent Genomes
M. Pilar Francino
from: Horizontal Gene Transfer in Microorganisms (Edited by: M. Pilar Francino). Caister Academic Press, U.K. (2012)
As genome sequences have accumulated for closely related bacteria, our view of bacterial genomes has radically changed. Genome comparisons have demonstrated that little 16S sequence divergence can be accompanied by large differences in total gene repertoire, and even populations of a single 16S rRNA species are made up of vast numbers of genomic varieties. Much of the observed variation among closely related genomes is attributable to gains and losses of genes that are acquired horizontally, in addition to genomic rearrangements, including gene duplications. The genomic flexibility that results from these mechanisms certainly contributes to the ability of bacteria to survive and adapt in varying environmental challenges. However, the duplicability and transferability of individual genes imply that natural selection should operate, not only at the organismal level, but also at the level of the gene. Therefore, it can be argued that genes are semiautonomous entities capable of responding to natural selection at different levels: selfishly, because they can "reproduce" by duplication and "migrate" by Horizontal Gene Transfer (HGT), and cooperatively, because their long-term survival depends on their ability to be replicated as parts of integrated, emerging genomes. The extent of gene autonomy, or the tightness of the association between gene and genome, can greatly vary, mostly depending on the relative strengths of the selective processes acting at each of these two different, but directly related, levels of organization read more ...