[Evolution of gene orders in mycoplasmas (Bacteria, Firmicutes, Mollicutes)]

Quantitative methods of estimation of similarity between gene orders have been used to compare the genomes of 14 strains of mycoplasmas and 2 strains of phytoplasmas, i.e., all genomes of bacteria of the class Mollicutes deciphered to date. Reconstructions of the mycoplasma phylogeny based on comparisons of (a) gene orders in a chromosome and (b) nucleotide or amino acid sequences have proved to be almost identical, which confirms that quantitative measures of gene order similarity can be used for meaningful phylogenetic reconstructions. Genomic rearrangements have been almost equally frequent in the evolutions of three main groups of mycoplasmas. A gene order changes by 1% approximately every 7 Myr or less (the calculation is based on the assumption that a 1% change in the nucleotide sequence of the 16S rRNA gene requires, on average, 50 Myr). In contrast to another analyzed group of obligately parasitic bacteria (rickettsiae), no distinct tendency towards a decrease in the rate of genomic rearrangements has been found in the evolution of mycoplasmas.