5S ribosomal RNA gene repeat sequences define at least eight groups of plant trypanosomatids (Phytomonas spp.): Phloem-restricted pathogens form a distinct section

Trypanosomatids isolated from plants have been assigned typically into the genus Phytomonas. Such designations do not reflect the biology of the diver se isolates; confusion may arise due to the transient presence in plants of monogenetic (insect) trypanosomatids deposited by phytophagous bugs. To de velop further molecular markers for the plant kinetoplastids, we have obtai ned the DNA sequence of the 5S ribosomal RNA gene from 24 isolates harveste d from phloem, latex, and fruit. Small, distinct sequence differences were found at the 3'-ends of the transcribed regions; substantial sequence and s ize differences were found in the non-transcribed regions. Alignment of the gene sequences from all the isolates suggested the presence of eight group ings. While six groups contained isolates from single plant tissues, groups C and A contained isolates from both fruit and latex. The DNA sequences of the 10 phloem-restricted pathogenic isolates from South America and the Ca rribean were highly conserved and thus comprised a single group (H). The co nserved nature of the 5S ribosomal RNA genes in these plant pathogens suppo rts the proposal that they be considered as a distinct section, the phloemi cola.