Confirmation of independent introductions of an exotic plant pathogen of Cornus species, Discula destructiva, on the east and west coasts of North America

Cornus florida (flowering dogwood) and C. nuttallii (Pacific dogwood) are North American native tree species that belong to the big-bracted group of dogwoods. Cornus species are highly valued for their ornamental characteristics, and have fruits that contain high fat content for animals. Also, they are an important understory tree in natural forests. Dogwood anthracnose, caused by Discula destructiva, was observed in the late 1970s on the east and west coasts of the United States and by 1991 had quickly spread throughout most of the native ranges of C. florida and C. nuttalli. We investigated the genetic diversity and population structure of 93 D. destructiva isolates using 47 microsatellite loci developed from the sequenced genome of the type strain of D. destructiva. Clone-corrected data indicated low genetic diversity and the presence of four genetic clusters that corresponded to two major geographic areas, the eastern United States and the Pacific Northwest, and to the two collection time periods when the isolates were collected (pre- and post-1993). Linkage disequilibrium was present in five out of six subpopulations, suggesting that the fungus only reproduced asexually. Evidence of population bottlenecks was indicated across four identified genetic clusters, and was probably the result of the limited number of founding individuals on both coasts. These results support the hypothesis that D. destructiva is an exotic pathogen with independent introductions on the east and west coasts of North America. We also tested the cross-amplification of these microsatellite primers to other Discula species. Genomic DNA from 17 isolates of four other Discula species and two isolates of Juglanconis species (formerly Melanconis species) were amplified by 17 of 47 primer pairs. These primers may be useful for investigating the genetic diversity and population structure of these Discula species.

花梾木（Cornus florida，flowering dogwood）与努塔尔梾木（Cornus nuttallii，Pacific dogwood）是北美本土乔木树种，隶属于大苞片梾木类群。梾木属（Cornus）物种兼具极高观赏价值，其果实富含脂肪，可作为动物的食物来源，同时也是天然林内重要的林下乔木。由破坏性盘长孢（Discula destructiva）引起的山茱萸炭疽病，于20世纪70年代末在美国东西海岸首次被发现，至1991年已快速蔓延至花梾木与努塔尔梾木的绝大多数原生分布范围。本研究利用从破坏性盘长孢模式菌株测序基因组中开发的47个微卫星位点（microsatellite loci），对93株破坏性盘长孢分离株的遗传多样性与种群结构展开分析。经克隆校正（clone-corrected）后的数据集显示，该真菌遗传多样性较低，且存在4个遗传聚类群，这些聚类群分别对应美国东部与太平洋西北岸两大地理区域，以及分离株的两个采样时期（1993年前后）。6个亚种群中有5个检测到连锁不平衡（linkage disequilibrium）现象，表明该真菌仅以无性方式繁殖。在4个已鉴定的遗传聚类群中均存在种群瓶颈（population bottlenecks）信号，这大概率源于东西海岸两地初始定植个体数量有限。上述结果佐证了“破坏性盘长孢是一种外来病原物，且分别独立传入北美东西海岸”这一假说。本研究还测试了上述微卫星引物对其他盘长孢属（Discula）物种的跨扩增（cross-amplification）效果。对4个盘长孢属物种的17株分离株以及2株胡桃生孢菌属（Juglanconis，原名为黑梭孢属Melanconis）分离株的基因组DNA进行扩增后发现，47对引物中有17对可成功扩增。上述引物可用于后续针对这些盘长孢属物种的遗传多样性与种群结构研究。