Data from: The coexistence of generalist and specialist clonal lineages in natural populations of the Irish Famine pathogen Phytophthora infestans explains local adaptation to potato and tomato

Phytophthora infestans, causing late blight on Solanaceae, is a serious threat to potato and tomato crops worldwide. P. infestans populations sampled on either potato or tomato differ in genotypes and pathogenicity, suggesting niche exclusion in the field. We hypothesized that such niche separation can reflect differential host exploitation by different P. infestans genotypes. We thus compared genotypes and phenotypes in 21 isolates sampled on potato (n = 11) or tomato (n = 10). Typing at 12 microsatellite loci assigned potato isolates to the 13_A2, 6_A1 and 1_A1 lineages, and tomato isolates to the 23_A1, 2_A1 and unclassified multi-locus genotypes. Cross-inoculations on potato and tomato leaflets showed that all isolates were pathogenic on both hosts. However: tomato isolates performed much better on tomato than did potato isolates, which performed better on potato than did tomato isolates, thus revealing a clear pattern of local adaptation. Potato isolates were significantly fitter on potato than on tomato, and are best described as potato-specialists; tomato isolates appear to be generalists, with similar pathogenicity on both hosts. Niche separation in the field may thus result mainly from the large fitness gap on tomato between generalists and unadapted potato-specialists, while the small, but significant fitness difference on potato between both types of isolates may prevent population invasion by generalists. Extreme specialization to potato seems very costly relative to performance loss on the alternative host. This study therefore shows that local adaptation and niche separation, commonly expected to involve and generate specialists, can occur with generalists.

致病疫霉（Phytophthora infestans）可引发茄科植物晚疫病，是全球范围内威胁马铃薯与番茄作物生产的重要病原物。从马铃薯或番茄寄主上采集的致病疫霉种群在基因型与致病性上存在显著差异，这暗示其在田间存在生态位排斥现象。我们据此提出假说：该生态位分离现象可反映不同致病疫霉基因型对寄主的差异化利用策略。为此，我们对21株分别采自马铃薯（n=11）与番茄（n=10）的菌株（isolate）开展基因型与表型分析。通过对12个微卫星位点（microsatellite locus）进行分型，我们将马铃薯菌株划归至13_A2、6_A1与1_A1谱系（lineage），而番茄菌株则被归入23_A1、2_A1谱系及未分类的多位点基因型（multi-locus genotype）。针对马铃薯与番茄叶片的交叉接种实验显示，所有供试菌株均可侵染两种寄主。但实验结果呈现出清晰的本地适应（local adaptation）模式：番茄菌株在番茄寄主上的致病力显著优于马铃薯菌株，而马铃薯菌株在马铃薯寄主上的致病力则强于番茄菌株。马铃薯菌株在马铃薯寄主上的适合度（fitness）显著高于其在番茄上的适合度，因此可被归类为马铃薯专化型；而番茄菌株则表现为广谱型，在两种寄主上的致病力无显著差异。由此推测，田间的生态位分离主要源于广谱型菌株与不适应的马铃薯专化型菌株在番茄寄主上存在较大的适合度差距；而两类菌株在马铃薯寄主上虽存在较小但显著的适合度差异，这一差异可阻止广谱型菌株入侵马铃薯菌株的种群。相较于在非专化寄主上的致病力损失，极端专化于马铃薯的菌株需付出极高的适合度代价。因此本研究表明，通常被认为仅会诱导专化型形成的本地适应与生态位分离现象，也可在广谱型菌株的参与下发生。