|Yokomi, Raymond - Ray|
Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/20/2008
Publication Date: 8/15/2008
Citation: Mello, A.F., Yokomi, R.K., Mulcher, U., Chen, J., Wayadande, A., Fletcher, J. 2008. Genetic Diversity of Spiroplasma citri strains from Different Regions, Hosts, and Isolation Dates. Phytopathology. 98:960-968. Interpretive Summary: Citrus stubborn disease (CSD) is caused by Spiroplasma citri, a phloem-limited prokaryote which is both graft transmissible and leafhopper vectored. Incidence of CSD appears to be increasing in some citrus groves in central California. A molecular analysis of representative S. citri strains collected recently was compared with strains maintained in a culture collection from the last 20 years. Using a method called randomly amplified polymorphic DNA (RAPD)-polymerase chain reaction (PCR) and 20 different primer pairs, no evidence was found for the emergence or development of a new strain. Therefore, one conclusion was that the increasing CSD incidence was not attributed to a new strain. However, consistent genetic differences were discernable in S. citri isolated from carrots and daikon. This evidence suggested that certain host plants other than citrus may more readily lead to development of new strains.
Technical Abstract: Spiroplasma citri, a phloem-limited, leafhopper-transmitted pathogen, causes citrus stubborn disease (CSD). Losses due to CSD in California orchards has grown over the past decade. To investigate the possibility of introduction or emergence of a new spiroplasma strain, a study of genetic diversity among S. citri strains from various locations was conducted using RAPD-PCR of 35 strains cultured from 1980-1993, and of another 35 strains cultured from 2005-2006. Based on analysis with 20 primer pairs, considerable diversity was observed among strains, however, no unique genetic signatures were associated with recently collected strains compared with those collected 15-28 years ago. Further, no geographically associated pattern was distinguishable. S. citri from carrot and daikon radish was found to contain some unique DNA fragments, suggesting some host plant influence. Multiple strains from single trees also showed genetic diversity. Sequencing of five RAPD bands that differed among S. citri strains showed that diversity-related gene sequences include virus fragments, a membrane lipoprotein, a DNA modification enzyme, and a mobilization element. No differences in colony morphology were observed among the strains. The lack of correlation between PCR patterns and isolation date or collection site is inconsistent with the hypothesis that recent infections are due to the introduction or emergence of novel pathogen strains.