Genetic Diversity Study of Big Bracted Dogwood (Cornus ssp.) Cultivars and Wild Collected Accessions

Big-bracted dogwoods are popular ornamental trees known for their beautiful spring blooms with showy bracts and four-season appeal. The two most widely grown species are Cornus florida and Cornus kousa, native to Eastern North America and East Asia, respectively, although C. nuttallii from the Pacific Northwest is also grown on occasion and has been used in breeding. Despite their horticultural prominence, there is little information available regarding genetic diversity, population structure, relatedness, and subspecies origins of dogwood cultivars. 313 cultivars, wild-collected plants, and Rutgers University breeding selections, focusing on C. florida, C. kousa, and interspecific hybrids, were genotyped using double digest restriction-site associated DNA sequencing (ddRadseq) generating thousands of single nucleotide polymorphism (SNP) and insertion deletion (Indel) markers. The research validated pedigrees of interspecific F1 hybrids and found many of the recent Rutgers breeding selections contain small regions of pacific dogwood (C. nuttallii) DNA introgressed into C. kousa backgrounds. For C. florida, pink-bracted plants formed a distinct clade from those with white-bracts with the Mexican C. florida ssp. urbiniana forming an outgroup. For C. kousa, Chinese-collected plants (ssp. chinensis) were a distinct subspecies with clear separation from Japanese and Korean accessions (ssp. kousa) and cultivars were designated as ssp. chinensis, ssp. kousa, or ssp. hybrid. Using this information, a Kompetitive allele specific PCR (KASP) assay genotyping panel was designed to determine C. kousa trees' subspecies makeup. Further, results revealed many cases of genetically identical cultivars being sold under different names, especially for pink-bracted cultivars of both species. Additionally, reported parent-progeny relationships were evaluated and either validated or discredited. This diversity study elucidates origins, diversity, and relationships of a large population of big-bracted dogwoods. The results can inform plant breeders, arboreta, and the ornamental plant industry, as most modern cultivars and popular historic cultivars are represented.

大苞片梾木（Big-bracted dogwoods）是广受欢迎的观赏树种，以其艳丽的春季苞片绽放和四季观赏价值而闻名。目前最广泛栽培的两个物种为多花梾木（Cornus florida）和四照花（Cornus kousa），分别原产于北美东部和东亚；此外太平洋西北岸原产的太平洋梾木（C. nuttallii）也偶有栽培，并曾被用于育种工作。尽管该类群在园艺领域极具影响力，但目前关于梾木栽培品种的遗传多样性、种群结构、亲缘关系以及亚种起源的相关研究信息仍较为匮乏。本研究针对多花梾木、四照花及其种间杂种，对313份栽培品种、野生采集植株以及罗格斯大学（Rutgers University）的育种选育材料进行了双酶切限制性位点关联DNA测序（double digest restriction-site associated DNA sequencing, ddRADseq），共获得数千个单核苷酸多态性（single nucleotide polymorphism, SNP）和插入缺失（insertion deletion, Indel）分子标记。研究结果验证了种间F1杂种的系谱关系，并发现许多近期的罗格斯大学育种选育材料中存在少量太平洋梾木（C. nuttallii）的DNA片段渐渗入四照花遗传背景的现象。针对多花梾木，苞片粉色的植株与苞片白色的植株构成两个独立的演化支，而墨西哥原产的多花梾木乌比纳亚种（C. florida ssp. urbiniana）则作为外类群。针对四照花，中国采集的中华亚种（ssp. chinensis）植株与日本、朝鲜采集的原亚种（ssp. kousa）植株存在显著遗传分化，所有供试材料被划分为中华亚种、原亚种或杂交亚种三类。基于上述研究结果，本研究设计了一套竞争性等位基因特异性PCR（Kompetitive allele specific PCR, KASP）基因分型检测组合，用于鉴定四照花植株的亚种组成。此外，研究还发现大量同名异株的情况，即遗传背景完全一致的栽培品种被冠以不同商品名进行销售，这一现象在两个物种的粉色苞片栽培品种中尤为突出。同时，本研究对已报道的亲本-子代亲缘关系进行了评估，部分关系得到验证，部分则被推翻。本项多样性研究阐明了大苞片梾木庞大种群的起源、遗传多样性及亲缘关系，由于研究涵盖了绝大多数现代栽培品种和主流历史栽培品种，其结果可为植物育种者、树木园以及观赏植物产业提供重要的理论参考与实践指导。