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Figure 2 | Biology Direct

Figure 2

From: Massive comparative genomic analysis reveals convergent evolution of specialized bacteria

Figure 2

Proposed scenario for genome evolution and specialization. Model of evolution involving 3 steps en route to specialization to an intracellular lifestyle. The different steps correspond to the different levels of dependency to eukaryotic cells, the minimum is for the free-living bacteria and the maximum is for obligate intracellular bacteria. Each Step corresponds to a bacterial community sharing common habitat and relationship with eukaryotic cells. Bacterial specialization to an intracellular lifestyle is characterized by gene loss including transcriptional regulators and rRNA operons. Free-living promiscuous bacteria have large genomes because of a high level of gene importation. They also have a large number of rRNA operons. Obligate intracellular bacteria have reduced genomes with few rRNA operons and transcription regulators. When bacterial lineages make the transition from free-living to permanent associations with hosts, they undergo a major loss of genes. Restriction to an intracellular environment limits the opportunity to acquire foreign genes from other bacteria therefore the balance between acquired and lost genes in specialized bacteria is in favour of genome reduction. Irreversible massive gene decay implies that specialization to an intracellular lifestyle is a one-way road. Differential gene loss is noted in mutualistic and parasitic bacterial groups. Data in circles represent the mean (± s.d.) of genome size in megabases (GS), GC content (GC), rRNA operon (Op), and number of genes assigned to transcriptional regulation (TR) in each bacterial community. Numbers on the arrows represent the average number of lost genes ± the standard error in order to compute confidence intervals for the estimated loss ratio (proportion of genes lost with respect to the whole number of genes present at least in one bacterium).

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