High levels of diversity and population structure in the potato late blight pathogen at the Mexico center of origin

Globally destructive crop pathogens often emerge by migrating out of their native ranges. These pathogens are often diverse at their center of origin, and may exhibit adaptive variation in the invaded range via multiple introductions from different source populations. However, source populations are generally unidentified or poorly studied compared to invasive populations. Phytophthora infestans, the causal agent of late blight, is one of the most costly pathogens of potato and tomato worldwide. Mexico is the center of origin and diversity of P. infestans and migration events out of Mexico have enormously impacted disease dynamics in North America and Europe. The debate over the origin of the pathogen, and population studies of P. infestans in Mexico, have focused on the Toluca Valley, whereas neighboring regions have been little studied. We examined the population structure of P. infestans across central Mexico, including samples from Michoacán, Tlaxcala, and Toluca. We found high levels of diversity consistent with sexual reproduction in Michoacán and Tlaxcala, and population subdivision that was strongly associated with geographical region. We determined that population structure in Central Mexico has contributed to diversity in introduced populations based on relatedness of U.S. clonal lineages to Mexican isolates from different regions. Our results suggest that P. infestans exists as a metapopulation in Central Mexico, and this population structure could be contributing to the repeated re-emergence of P. infestans in the U.S. and elsewhere.

全球范围内具有破坏性的作物病原菌往往通过脱离原生分布范围的迁移而暴发成灾。这类病原菌在其起源中心往往具有丰富的遗传多样性，且在入侵区域可通过来自不同源种群的多次引种事件产生适应性变异。然而，相较于入侵种群，源种群通常尚未被明确鉴定，或是研究程度极低。致病疫霉（Phytophthora infestans）是引发晚疫病的病原菌，也是全球范围内为害马铃薯与番茄最为严重的病原菌之一。墨西哥是致病疫霉的起源与多样性中心，其向外的迁移事件极大地影响了北美与欧洲的病害流行动态。围绕该病原菌起源的争议，以及针对墨西哥境内致病疫霉的种群研究，均以托卢卡谷地为核心对象，而邻近区域则鲜有研究。本研究对墨西哥中部地区的致病疫霉种群结构展开了调查，采样覆盖米却肯州、特拉斯卡拉州与托卢卡谷地。研究结果显示，米却肯州与特拉斯卡拉州的种群具有高水平的遗传多样性，符合有性生殖的特征，且种群分化与地理区域呈现显著关联。通过比对美国克隆谱系与不同区域墨西哥分离株的亲缘关系，我们明确墨西哥中部的种群结构是导致入侵种群多样性的重要因素。本研究结果表明，致病疫霉在墨西哥中部以集合种群（metapopulation）形式存在，这种种群结构可能是促成美国及其他地区致病疫霉反复再暴发的重要原因。