Table_3_Construction of High-Density Genetic Maps and Detection of QTLs Associated With Huanglongbing Tolerance in Citrus.docx

Huanglongbing (HLB), or citrus greening, is the most devastating disease in citrus worldwide. Commercial citrus varieties including sweet orange (Citrus sinensis) are highly susceptible to HLB, and trifoliate orange (Poncirus trifoliata, a close Citrus relative) is widely considered resistant or highly tolerant to HLB. In this study, an intergeneric F1 population of sweet orange and trifoliate orange was genotyped by Genotyping-by-Sequencing, and high-density SNP-based genetic maps were constructed separately for trifoliate orange and sweet orange. The two genetic maps exhibited high synteny and high coverage of the citrus genome. Progenies of the F1 population and their parents were planted in a replicated field trial, exposed to intense HLB pressure for 3 years, and then evaluated for susceptibility to HLB over 2 years. The F1 population exhibited a wide range in severity of HLB foliar symptom and canopy damage. Genome-wide QTL analysis based on the phenotypic data of foliar symptom and canopy damage in 2 years identified three clusters of repeatable QTLs in trifoliate orange linkage groups LG-t6, LG-t8 and LG-t9. Co-localization of QTLs for two traits was observed within all three regions. Additionally, one cluster of QTLs in sweet orange (linkage group LG-s7) was also detected. The majority of the identified QTLs each explained 18–30% of the phenotypic variation, indicating their major role in determining HLB responses. These results show, for the first time, a quantitative genetic nature yet the presence of major loci for the HLB tolerance in trifoliate orange. The results suggest that sweet orange also contains useful genetic factor(s) for improving HLB tolerance in commercial citrus varieties. Findings from this study should be very valuable and timely to researchers worldwide as they are hastily searching for genetic solutions to the devastating HLB crisis through breeding, genetic engineering, or genome editing.

黄龙病（HLB），亦称柑橘黄化病，是全球柑橘类作物中最具毁灭性的病害。包括甜橙（Citrus sinensis）在内的商业柑橘品种对HLB高度易感，而三叶橙（Poncirus trifoliata，一种与柑橘近缘的植物）则普遍被认为对HLB具有抗性或高度耐受性。在本研究中，通过对甜橙与三叶橙的杂交F1群体进行基因分型测序，分别构建了三叶橙和甜橙的高密度SNP基遗传图谱。这两个遗传图谱展示了柑橘基因组的高度同源性和高覆盖度。F1群体的后代及其亲本在重复的田间试验中种植，连续3年遭受强烈的HLB压力，并在接下来的2年内对其对HLB的敏感性进行了评估。F1群体在HLB叶片症状和树冠损伤的严重程度上表现出广泛的差异。基于两年内叶片症状和树冠损伤表型数据的全基因组QTL分析，在三叶橙的连锁群LG-t6、LG-t8和LG-t9中识别出三个可重复的QTL群。在所有三个区域中均观察到两个性状QTLs的共定位。此外，在甜橙（连锁群LG-s7）中也检测到一个QTL群。所识别的大多数QTLs各自解释了18-30%的表型变异，表明它们在决定HLB反应中的主要作用。这些结果首次表明，三叶橙对HLB耐受性具有数量遗传性质，且存在主要位点。结果表明，甜橙也含有可用于提高商业柑橘品种HLB耐受性的有用遗传因子。本研究的结果对于全球研究人员来说应当极具价值和时效性，因为他们正急于通过育种、基因工程或基因组编辑等方法寻找解决破坏性的HLB危机的遗传解决方案。