Reduction of bacterial genome size and expansion resulting from obligate intracellular lifestyle and adaptation to soil habitat

Prokaryotic organisms are exposed in the course of evolution to various imp acts, resulting often in drastic changes of their genome size. Depending on circumstances, the same lineage may diverge into species having substantia lly reduced genomes, or such whose genomes have undergone considerable enla rgement. Genome reduction is a consequence of obligate intracellular lifest yle rendering numerous genes expendable. Another consequence of intracellul ar lifestyle is reduction of effective population size and limited possibil ity of gene acquirement via lateral transfer. This causes a state of relaxe d selection resulting in accumulation of mildly deleterious mutations that can not be corrected by recombination with the wild type copy. Thus, gene l oss is usually irreversible. Additionally, constant environment of the euka ryotic cell renders that some bacterial genes involved in DNA repair are ex pandable. The loss of these genes is a probable cause of mutational bias re sulting in a high A+T content. While causes of genome reduction are rather indisputable, those resulting i n genome expansion seem to be less obvious. Presumably, the genome enlargem ent is an indirect consequence of adaptation to changing environmental cond itions and requires the acquisition and integration of numerous genes. It s eems that the need for a great number of capabilities is common among soil bacteria irrespective of their phylogenetic relationship. However, this wou ld not be possible if soil bacteria lacked indigenous abilities to exchange and accumulate genetic information. The latter are considerably facilitate d when housekeeping genes are physically separated from adaptive loci which are useful only in certain circumstances.