A GENOME-WIDE APPROACH TO PLANT-HOST PATHOGENICITY IN XYLELLA FASTIDIOSA: MULTIGENIC METHODS FOR IDENTIFYING STRAINS, FOR STUDYING THE ROLE OF INTER-STRAIN RECOMBINATION, AND FOR IDENTIFYING PATHOGENICITY CANDIDATE GENES

• Author(s): Nunney, Leonard; Luck, Robert; Stouthamer, R;
• Abstract: We have developed a multilocus sequence typing (MLST) system for identifying the known pathovars of Xylella fastidiosa (including subsp. fastidiosa that causes Pierces disease). This identification system is based on allelic variation at seven housekeeping genes (holC, nuoL, leuA, gltT, cysG, petC and lacF) and a public MLST database has been established at www.mlst.net (see Scally et al. 2005). This easily accessible system will allow for the rapid recognition of novel variants as they arise, since it can be continuously updated by researchers who have sequenced new isolates for the seven genes. We tested the effectiveness of the MLST system using 25 isolates of Xylella fastidiosa (Xf) from five different host plants: grapevine (Pierces disease, PD), oleander (oleander leaf scorch, OLS), oak (oak leaf scorch, OAK), almond (almond leaf scorch, ALS) and peach (phony peach scorch, PP). An eBURST analysis identified six clonal complexes (CCs), using the grouping criterion that each member of a CC must be identical to at least one other member at five or more of the seven loci. These clonal complexes corresponded to phylogenetic clades that we had previously identified (Schuenzel et al. 2005), including subspecific clades of fastidiosa and sandyi (CC1 & 2), while CC3-5 defined host-specific sub-clades of the subsp. multiplex. CC6 identified a multiplex-like group characterized by a high frequency of inter-subspecific recombination. To begin to understand the potential role of recombination in the creation of new pathovars, we also used the MLST data (plus three additional loci) to estimate the relative contribution of recombination and mutation to the observed variability. Recombination between different alleles was estimated to give rise to 76% of the nucleotide changes and 31% of the allelic changes observed. However, sequence data also suggests that inter-subspecific recombination has started relatively recently. This new phenomenon may lead to an increased rate of pathovar formation.
• Publication Date: Dec 2005
• Journal: 2005 Pierce's Disease Research Symposium