Bremia lactucae is a devastating pathogen causing heavy yield losses in lettuce production worldwide. Long time it was thought to infect various members of the Asteraceae but recent phylogenetic investigations revealed that B. lactucae is restricted to cultivated Lactuca sativa and its wild progenitor Lactuca serriola. Many of the Dm resistance genes in cultivated lettuce were crossbred from L. serriola, but often a fast overcoming of the resistance in the crop by new downy mildew races was observed. Consequently, it seems important to investigate the population genetic relationships of wild prickly lettuce and cultivated lettuce, in order to determine, if the Bremia lineages infecting these hosts are distinct or intermingled. In the first case, searching for resistance in prickly lettuce is straightforward, in the second case it would rather be futile. Bremia belongs to the genera of downy mildews with pyriform haustoria, which feature highly variable ITS-repeats that can be used as high-resolution markers for population genetics studies. Here we report that Bremia lactucae on Lactuca sativa is not homogeneous but emerged from several host shifts from Lactuca serriola and that Bremia accessions from both species are not forming distinct genepools. While the pathogen population from L. serriola showed a balanced pattern, the pathogen population from L. sativa has a directional selection pattern reflecting founder effects after host jumps, resulting in a fast colonisation of susceptible lettuce cultivars. Thus, the wild population of Bremia lactucae seems to play a major role in the epidemiology of downy mildew on cultivated lettuce. This also explains, why resistance breeding using resistance genes from prickly lettuce had little success for achieving durable resistance in cultivated lettuce and calls for both more detailed research into Bremia on wild lettuce and new breeding strategies using less closely related lettuce species.

Bremia lactucae是一种破坏性病原体，导致全球生菜生产严重减产。长期以来，人们认为它会感染菊科的各种成员，但最近的系统发育研究表明，B. lactucae仅限于栽培的Lactuca sativa及其野生祖先Lactuca serriola。栽培生菜中的许多 Dm 抗性基因是从L. serriola杂交的，但经常观察到新的霜霉病小种迅速克服了作物中的抗性。因此，调查野生刺莴苣和栽培莴苣的种群遗传关系似乎很重要，以确定Bremia感染这些宿主的谱系是不同的或混合的。在第一种情况下，在刺生菜中寻找抗药性很简单，在第二种情况下则是徒劳的。Bremia属于具有梨状吸器的霜霉病属，具有高度可变的 ITS 重复序列，可用作种群遗传学研究的高分辨率标记。在这里，我们报告了Lactuca sativa上的Bremia lactucae不是同质的，而是从Lactuca serriola 的几个宿主转移中出现的，并且来自两个物种的Breemia种质没有形成不同的基因库。而来自L. serriola的病原体种群显示平衡模式，来自L. sativa的病原体种群具有定向选择模式，反映了宿主跳跃后的创始人效应，导致易感生菜品种的快速定植。因此，Bremia lactucae的野生种群似乎在栽培生菜霜霉病的流行病学中起主要作用。这也解释了为什么使用多刺莴苣的抗性基因进行抗性育种在栽培生菜中实现持久抗性方面收效甚微，并呼吁对野生生菜的Bremia进行更详细的研究，以及使用不太密切相关的生菜物种进行新的育种策略。