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Chapter 45 : Evolution of Mycoplasma pneumoniae and Mycoplasmal Infections
Category: Fungi and Fungal Pathogenesis; Bacterial Pathogenesis
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In the mid-1970s, two models of mycoplasma evolution were proposed. The first model considered that mycoplasmas were polyphyletic and had arisen by degenerate evolution and diversification of different bacterial lineages, with different mycoplasmas originating from different branches of the bacterial phylogenetic tree. The second model was that mycoplasmas arose very early in the evolution of living forms on Earth and ancestral mycoplasmas were precursors of bacteria. Earlier phylogenetic work indicated that genome size reductions must have occurred on at least two and possibly four separate occasions in the course of Mollicutes evolution. Mycoplasma with smaller genome sizes (<1,00 kb) are on three of the most rapidly evolving mycoplasma phylogenetic branches: the Mycoplasma pneumoniae, Ureaplasma, and M. hominis group branches. Complete genome analysis of mycoplasmas fails to detect orthologues of most genes in the two systems for homologous recombination. Phylogenomic studies suggest that many genes involved in a variety of DNA repair pathways have been lost in Mycoplasma. Defects in mismatch repair might increase pathogenicity because of an increase in the mutation rate, which allows a faster evolutionary response to immune systems and other host defenses. Sequence repeats leading to recombination events constitute evolutionary reservoirs that can be recruited by adaptive strategies involving sequence varation. Antigenic epitopes that are shared by different mycoplasmas and host cells have been proposed as possible factors involved in the evasion of host defense mechanisms and in the induction of auto-antibodies observed during infection.
A suggested scenario of Mycoplasma evolutionary history. Adapted from Maniloff, 2002.
Mycoplasma evolutionary adaptation to cell parasitism.