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The phylogenetic relationships between the mycoplasmas and bacteria have been established from a comparative analysis of their 16S rRNA oligonucleotide catalogs. The genera Mycoplasma, Spiroplasma, and Acholeplasma arose by degenerative evolution, as a deep branch of the subline of clostridial ancestry that led to Bacillus and Lactobacillus. Thermoplasma has no specific relationship to the other mycoplasmas; it belongs with the archaebacteria.
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Woese et al. (1980) studied this question.
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