Phylogenetic diversity of phytopathogenic mycoplasmalike organisms

By using specific primers, the 16S rRNA genes of Japanese mycoplasmalike organisms (MLOs) were amplified by polymerase chain reactions from MLO-enriched fractions of plants infected with each of six different MLOs. Each of the polymerase chain reaction fragments (length, 1,370 nucleotides) was directly sequenced in both strands by using 17 oligonucleotide primers. A phylogenetic tree constructed by using the sequence data showed that these Japanese MLOs are phylogenetically diverse microorganisms that fall into three groups, group I (onion yellows, tomato yellows, mulberry dwarf, and paulownia witches' broom MLOs), group II (tsuwabuki witches' broom MLO), and group III (rice yellow dwarf MLO). A high level of sequence homology (99%) between the Oenothera hookeri MLO and the severe strain of the western aster yellows MLO on the one hand and group I MLOs on the other indicates that the O. hookeri MLO and the severe strain of the western aster yellows MLO belong to group I and suggests that these MLOs, isolated from two geographically separated locations, descended from a very similar ancestor. Although group I contains phylogenetically identical MLOs, the organisms are transmitted by diverse insect vectors. The three MLO groups are more closely related to Acholeplasma laidlawii than to Mycoplasma gallisepticum. Thus, although MLOs are phylogenetically diverse, they are evolutionarily distant from other mollicutes. These data, together with other information (including phylogenetic relationships, vector specificity, plant-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).