Department of Cell and Systems Biology, University of Toronto, 25 Willcocks St., Toronto, ON, M5S 3B2, Canada.
BMC Microbiol. 2012 Jul 16;12:141. doi: 10.1186/1471-2180-12-141.
Hazelnut (Corylus avellana) decline disease in Greece and Italy is caused by the convergent evolution of two distantly related lineages of Pseudomonas syringae pv. avellanae (Pav). We sequenced the genomes of three Pav isolates to determine if their convergent virulence phenotype had a common genetic basis due to either genetic exchange between lineages or parallel evolution.
We found little evidence for horizontal transfer (recombination) of genes between Pav lineages, but two large genomic islands (GIs) have been recently acquired by one of the lineages. Evolutionary analyses of the genes encoding type III secreted effectors (T3SEs) that are translocated into host cells and are important for both suppressing and eliciting defense responses show that the two Pav lineages have dramatically different T3SE profiles, with only two shared putatively functional T3SEs. One Pav lineage has undergone unprecedented secretome remodeling, including the acquisition of eleven new T3SEs and the loss or pseudogenization of 15, including five of the six core T3SE families that are present in the other Pav lineage. Molecular dating indicates that divergence within both of the Pav lineages predates their observation in the field. This suggest that both Pav lineages have been cryptically infecting hazelnut trees or wild relatives for many years, and that the emergence of hazelnut decline in the 1970s may have been due to changes in agricultural practice.
These data show that divergent lineages of P. syringae can converge on identical disease etiology on the same host plant using different virulence mechanisms and that dramatic shifts in the arsenal of T3SEs can accompany disease emergence.
希腊和意大利的榛树衰退病是由两个远缘谱系的假单胞菌丁香假单胞菌 pv.榛树（Pav）的趋同进化引起的。我们对三个 Pav 分离株的基因组进行了测序，以确定它们趋同的毒力表型是否由于谱系之间的遗传交换或平行进化而具有共同的遗传基础。
我们几乎没有发现 Pav 谱系之间基因水平转移（重组）的证据，但最近有两个大的基因组岛（GIs）被一个谱系获得。对编码 III 型分泌效应物（T3SEs）的基因进行进化分析，这些效应物被转运到宿主细胞中，对抑制和引发防御反应都很重要，结果表明，两个 Pav 谱系的 T3SE 谱截然不同，只有两个共享的假定功能 T3SE。一个 Pav 谱系经历了前所未有的分泌系统重塑，包括获得十一个新的 T3SE 和 15 个的丢失或假基因化，其中包括存在于另一个 Pav 谱系中的六个核心 T3SE 家族中的五个。分子年代测定表明，两个 Pav 谱系的分化都早于它们在野外的观察。这表明，两个 Pav 谱系多年来一直秘密感染榛树或野生亲缘植物，而榛树衰退病在 20 世纪 70 年代的出现可能是由于农业实践的改变。
这些数据表明，不同谱系的假单胞菌可以在同一宿主植物上使用不同的毒力机制趋同于相同的疾病病因，并且 T3SE 武器库的剧烈变化可以伴随着疾病的出现。
