Schuenzel Erin L, Scally Mark, Stouthamer Richard, Nunney Leonard
Department of Biology, University of California, Riverside, CA 92521.
Appl Environ Microbiol. 2005 Jul;71(7):3832-9. doi: 10.1128/AEM.71.7.3832-3839.2005.
Xylella fastidiosa is a pathogen that causes leaf scorch and related diseases in over 100 plant species, including Pierce's disease in grapevines (PD), phony peach disease (PP), plum leaf scald (PLS), and leaf scorch in almond (ALS), oak (OAK), and oleander (OLS). We used a high-resolution DNA sequence approach to investigate the evolutionary relationships, geographic variation, and divergence times among the X. fastidiosa isolates causing these diseases in North America. Using a large data set of 10 coding loci and 26 isolates, the phylogeny of X. fastidiosa defined three major clades. Two of these clades correspond to the recently identified X. fastidiosa subspecies piercei (PD and some ALS isolates) and X. fastidiosa subsp. multiplex (OAK, PP, PLS, and some ALS isolates). The third clade grouped all of the OLS isolates into a genetically distinct group, named X. fastidiosa subsp. sandyi. These well-differentiated clades indicate that, historically, X. fastidiosa has been a clonal organism. Based on their synonymous-site divergence ( approximately 3%), these three clades probably originated more than 15,000 years ago, long before the introduction of the nonnative plants that characterize most infections. The sister clades of X. fastidiosa subsp. sandyi and X. fastidiosa subsp. piercei have synonymous-site evolutionary rates 2.9 times faster than X. fastidiosa subsp. multiplex, possibly due to generation time differences. Within X. fastidiosa subsp. multiplex, a low level ( approximately 0.1%) of genetic differentiation indicates the recent divergence of ALS isolates from the PP, PLS, and OAK isolates due to host plant adaptation and/or allopatry. The low level of variation within the X. fastidiosa subsp. piercei and X. fastidiosa subsp. sandyi clades, despite their antiquity, suggests strong selection, possibly driven by host plant adaptation.
桑氏木质部小菌（Xylella fastidiosa）是一种病原体，可在100多种植物中引发叶焦病及相关病害，包括葡萄上的皮尔氏病（PD）、假桃病（PP）、李叶焦病（PLS），以及杏仁（ALS）、橡树（OAK）和夹竹桃（OLS）上的叶焦病。我们采用高分辨率DNA序列方法，研究了在北美引发这些病害的桑氏木质部小菌分离株之间的进化关系、地理变异和分化时间。利用包含10个编码位点和26个分离株的大型数据集，桑氏木质部小菌的系统发育确定了三个主要分支。其中两个分支分别对应最近鉴定出的桑氏木质部小菌皮尔斯亚种（PD和一些ALS分离株）和桑氏木质部小菌复合亚种（OAK、PP、PLS和一些ALS分离株）。第三个分支将所有OLS分离株归为一个遗传上不同的组，命名为桑氏木质部小菌桑地亚种。这些分化良好的分支表明，从历史上看，桑氏木质部小菌是一种克隆生物。基于它们的同义位点分歧（约3%），这三个分支可能起源于15000多年前，远早于大多数感染所特有的非本地植物的引入。桑氏木质部小菌桑地亚种和桑氏木质部小菌皮尔斯亚种的姐妹分支的同义位点进化速率比桑氏木质部小菌复合亚种快2.9倍，这可能是由于世代时间差异所致。在桑氏木质部小菌复合亚种内，低水平（约0.1%）的遗传分化表明，由于宿主植物适应和/或异域分布，ALS分离株与PP、PLS和OAK分离株最近才发生分化。尽管桑氏木质部小菌皮尔斯亚种和桑氏木质部小菌桑地亚种分支历史悠久，但其中的变异水平较低，这表明可能存在由宿主植物适应驱动的强烈选择。
