Lack of Host Specialization on Winter Annual Grasses in the Fungal Seed Bank Pathogen Pyrenophora semeniperda

Generalist plant pathogens may have wide host ranges, but many exhibit varying degrees of host specialization, with multiple pathogen races that have narrower host ranges. These races are often genetically distinct, with each race causing highest disease incidence on its host of origin. We examined host specialization in the seed pathogen Pyrenophora semeniperda by reciprocally inoculating pathogen strains from Bromus tectorum and from four other winter annual grass weeds (Bromus diandrus, Bromus rubens, Bromus arvensis and Taeniatherum caput-medusae) onto dormant seeds of B. tectorum and each alternate host. We found that host species varied in resistance and pathogen strains varied in aggressiveness, but there was no evidence for host specialization. Most variation in aggressiveness was among strains within populations and was expressed similarly on both hosts, resulting in a positive correlation between strain-level disease incidence on B. tectorum and on the alternate host. In spite of this lack of host specialization, we detected weak but significant population genetic structure as a function of host species using two neutral marker systems that yielded similar results. This genetic structure is most likely due to founder effects, as the pathogen is known to be dispersed with host seeds. All host species were highly susceptible to their own pathogen races. Tolerance to infection (i.e., the ability to germinate even when infected and thereby avoid seed mortality) increased as a function of seed germination rate, which in turn increased as dormancy was lost. Pyrenophora semeniperda apparently does not require host specialization to fully exploit these winter annual grass species, which share many life history features that make them ideal hosts for this pathogen.

广宿主植物病原体（Generalist plant pathogens）通常具备宽泛的宿主范围，但多数类群表现出不同程度的宿主专化性（host specialization），其下存在多个宿主范围更窄的病原小种（pathogen races）。这些小种往往具有显著遗传分化，每个小种在其原生宿主上引发的病害发病率最高。本研究以种子病原（seed pathogen）Pyrenophora semeniperda为研究对象，通过互接种试验，将从Bromus tectorum以及另外4种冬生一年生禾本科杂草（包括Bromus diandrus、Bromus rubens、Bromus arvensis和Taeniatherum caput-medusae）上分离得到的病原菌株，分别接种至Bromus tectorum与其余各供试宿主的休眠种子上，以此探究该病原菌的宿主专化性。研究结果显示，不同宿主物种的抗病性（resistance）存在显著差异，不同病原菌株的致病力（aggressiveness）亦各不相同，但未发现宿主专化性的相关证据。致病力的绝大多数变异源于种群内部菌株间的差异，且在两类宿主上的表现模式一致，最终导致菌株在Bromus tectorum与交替宿主上的病害发病率呈正相关。尽管未检测到宿主专化性，本研究通过两套结果一致的中性分子标记系统（neutral marker systems）分析发现，以宿主物种为分化依据可区分出微弱但显著的种群遗传结构（population genetic structure）。该遗传结构最可能由奠基者效应（founder effects）导致——已有研究表明该病原菌可随宿主种子进行传播。所有供试宿主物种对自身来源的病原小种均表现出高度易感。宿主对侵染的耐受性（即即便遭受侵染仍可萌发，从而规避种子死亡的能力）随种子发芽率（germination rate）升高而增强，而发芽率又随休眠破除而提升。Pyrenophora semeniperda似乎无需依赖宿主专化性，即可充分利用这类冬生一年生禾草物种——这类宿主共享诸多关键生活史特征，使其成为该病原菌的理想宿主。