Construction of the mycoplasma evolutionary tree from 5S rRNA sequence data.

Abstract

The 5S rRNA sequences of eubacteria and mycoplasmas were analyzed and a phylogenetic tree was constructed. Sequences of 5S rRNA from Clostridium innocuum, Acholeplasma laidlawii, A. modicum, Anaeroplasma bactoclasticum, A. abactoclasticum, Ureaplasma urealyticum, Mycoplasma mycoides ssp. mycoides, M. pneumoniae and Mycoplasma gallisepticum was determined. Analysis of these and published sequences shows that mycoplasmas form a coherent phylogenetic group that, with C. innocuum, arose as a branch of the low G + C gram-positive tree, near the lactobacilli and streptococci. The initial event in mycoplasma phylogeny was formation of the Acholeplasma branch; hence, loss of cell wall probably occurred at the time of genome reduction to .apprxeq. 1000 megadaltons (MDa). A subsequent branch produced the Spiroplasma. This branch appears to have been the origin of sterol-requiring mycoplasmas. During development of the Spiroplasma branch there were several independent genome reductions, each to .apprxeq. 500 MDa, resulting in Mycoplasma and Ureaplasma spp. Mycoplasmas, particularly species with the smallest genomes, have high mutation rates, suggesting that they are in a state of rapid evolution.