AbstractThe genetic relatedness of the phytoplasmas associated with papaya yellow crinkle (PYC)and papaya mosaic (PM) diseases is not known. To study this, the techniques of southern hybridisation, RFLP analysis of PCR products and sequence analysis were used. From southern hybridisation experiments it was determined that the chromosomal sites for the endonucleases EcoRI and Hindiil was the same for all papaya phytoplasmas studied regardless of whether they were associated with PYC or PM. In contrast to this, the RFLP analysis of the 16S - 23S rRNA region of the genome which can be amplified using PCR with the primer pair Pl and m23S revealed that there are several slight differences within the papaya phytoplasma population. These differences cannot be directly attributed to one disease or the other because the variations can be found in the fragment pattern of the P1/m23S PCR product of both the PYC and PM phytoplasmas.
The RFLP variations observed arose mainly within the PYC population collected from the study site at Katherine (NT). The AluI and RsaI digested PCR product of these samples showed three possible RFLP patterns and in addition to this two Katherine samples were not digested by EstEll unlike all the other samples used in this study. This diversity was studied further by sequence analysis of the intergenic spacer between the 16S and 23S rRNA genes. This region was chosen for sequence analysis because it is relatively variable in comparison to the adjacent genes, making it more likely to contain some of the sequence differences indicated by the RFLP analysis. The spacer sequence for two papaya samples from Katherine was determined and in one case found to be the same as that of the reference phytoplasma V4. This V4 is the sweet potato little leaf(SPLL) phytoplasma which has been transmitted to periwinkle via the parasitic vine dodder (Cascuta sp.). The V4 reference phytoplasma produces slightly different RFLP patterns to that of SPLL The other PYC spacer which was sequenced was found to have two insertions (positions 5 and 53) and two mismatches (positions 54 and 178) when compared to the sequence of V4. The insertions in this sequence explains one of the polymorphisms observed in the PYC samples collected from Katherine.
|Date of Award||Oct 1996|
|Supervisor||Karen Gibb (Supervisor)|