PROKARYOTIC OXYGEN-RESPONSE AND THE EVOLUTION OF GENOME SIZES

The hypotheses that prokaryotic DNA content is the outcome of sequential large-scale of genome duplications (Wallace and Morowitz, 1973, Chromosoma 40: 121; Zipkas and Riley, 1975, PNAS 72: 1354) have been analyzed a database constructed from the published reports of prokaryotic genomes whose size has been determined by pulse-field gradient electrophoresis. Although this database is likely to be biased and does not represent the full range of prokaryotic diversity, the discontinous distribution of DNA content that we have observed does not favor the idea that genomes have been shaped by succesive rounds of whole-genome duplications involving first an ancestral minigenome.

When genome size distribution was analyzed in relation with the oxygen-response of the different bacteria as reported in the literature (anaerobes, facultatives, microaerophilic and aerobes), we found that obligate anaerobic prokaryotes and/or microaerophilic bacteria are endowed with genomes much more smaller than the aerobic bacteria, although there is considerable variation among the latter. Larger genomes are typical of free-living prokaryotes with complex life cycles, which must have evolved once significant amounts of free-oxygen became available in the primitive environment. Our results suggest that the DNA content of aerobic bacteria is the outcome not of genome duplications, but instead is the outcome of small-scale DNA duplications, slippage, horizontal transfer, and other mechanisms.