PHYLOGENETIC DIVERSITY OF PHYTOPATHOGENIC MYCOPLASMALIKE ORGANISMS

By using specific Primers, the 16S rRNA genes of Japanese mycoplasmali ke organisms (MLOs) were amplified by polymerase chain reactions from MLO-enriched fractions of plants infected with each of six different M LOs. Each of the polymerase chain reaction fragments (length, 1,370 nu cleotides) was directly sequenced in both strands by using 17 oligonuc leotide primers. A phylogenetic tree constructed by using the sequence data showed that these Japanese MLOs are phylogenetically diverse mic roorganisms that fall into three groups, group I (onion yellows, tomat o yellows, mulberry dwarf, and paulownia witches' broom MLOs), group I I (tsuwabuki witches' broom MLO), and group III (rice yellow dwarf MLO ). A high level of sequence homology (99%) between the Oenothera hooke ri MLO and the severe strain of the western aster yellows MLO on the o ne hand and group I MLOs on the other indicates that the O. hookeri ML O and the severe strain of the western aster yellows MLO belong to gro up I and suggests that these MLOs, isolated from two geographically se parated locations, descended from a very similar ancestor. Although gr oup I contains phylogenetically identical MLOs, the organisms are tran smitted by diverse insect vectors. The three MLO groups are more close ly related to Acholeplasma laidlawii than to Mycoplasma gallisepticum. Thus, although MLOs are phylogenetically diverse, they are evolutiona rily distant from other mollicutes. These data, together with other in formation (including phylogenetic relationships, vector specificity, p lant-pathogenic properties, and habitat in plant phloem sieve tubes), suggest that MLOs could be classified into at least three phylogenetic groups (groups I through III).