Table_4_Dissecting the Genetic Architecture of Melon Chilling Tolerance at the Seedling Stage by Association Mapping and Identification of the Elite Alleles.DOCX

Low temperature is an important abiotic stress that negatively affects morphological growth and fruit development in melon (Cucumis melo L.). Chilling stress at the seedling stage causes seedling injury and poor stand establishment, prolonging vegetative growth and delaying fruit harvest. In this study, association mapping was performed for chilling tolerance at the seedling stage on an expanded melon core collection containing 212 diverse accessions by 272 SSRs and 27 CAPSs. Chilling tolerance of the melon seedlings was evaluated by calculating the chilling injury index (CII) in 2016 and 2017. Genetic diversity analysis of the whole accession panel presented two main groups, which corresponded to the two subspecies of C. melo, melo, and agrestis. Both the subspecies were sensitive to chilling but with agrestis being more tolerant. Genome-wide association study (GWAS) was conducted, respectively, on the whole panel and the two subspecies, totally detecting 51 loci that contributed to 74 marker-trait associations. Of these associations, 35 were detected in the whole panel, 21 in melo, and 18 in agrestis. About half of the associations identified in the two subspecies were also observed in the whole panel, and seven associations were shared by both the subspecies. CMCT505_Chr.1 was repeatedly detected in different populations with high phenotypic contribution and could be a key locus controlling chilling tolerance in C. melo. Nine loci were selected for evaluation of the phenotypic effects related to their alleles, which identified 11 elite alleles contributing to seedling chilling tolerance. Four such alleles existed in both the subspecies and six in either of the two subspecies. Analysis of 20 parental combinations for their allelic status and phenotypic values showed that the elite alleles collectively contributed to enhancement of the chilling tolerance. Tagging the loci responsible for chilling tolerance may simultaneously favor dissecting the complex adaptability traits and elevate the efficiency to improve chilling tolerance using marker-assisted selection in melon.

低温是影响甜瓜（Cucumis melo L.）形态生长与果实发育的重要非生物胁迫因子。苗期冷胁迫会导致幼苗受冷害、成苗不良，延长营养生长周期并推迟果实采收期。本研究以包含212份遗传多样性种质的扩展甜瓜核心种质为材料，利用272个简单序列重复标记（Simple Sequence Repeats, SSRs）和27个酶切扩增多态性序列（Cleaved Amplified Polymorphic Sequences, CAPSs），开展苗期耐冷性的关联作图分析。2016年和2017年，通过计算冷害指数（Chilling Injury Index, CII）对甜瓜幼苗的耐冷性进行评价。对全部供试种质的遗传多样性分析显示，其可划分为两个类群，分别对应甜瓜的两个亚种：melo亚种与agrestis亚种。两个亚种均对冷胁迫敏感，但agrestis亚种的耐冷性更强。本研究分别针对全部种质群体及两个亚种开展全基因组关联分析（Genome-Wide Association Study, GWAS），共检测到51个位点，涉及74个标记-性状关联。其中，全部种质群体中检测到35个关联，melo亚种中检测到21个，agrestis亚种中检测到18个。两个亚种中鉴定出的标记-性状关联约有一半在全部种质群体中也可被检测到，且有7个关联为两个亚种所共有。位点CMCT505_Chr.1在不同群体中均被重复检测到，且表型贡献率较高，可能是调控甜瓜耐冷性的关键位点。本研究筛选出9个位点用于评估其等位基因的表型效应，共鉴定出11个可提升幼苗耐冷性的优异等位基因：其中4个优异等位基因在两个亚种中均存在，6个仅存在于其中一个亚种。对20组亲本组合的等位基因组成及表型值进行分析后发现，这些优异等位基因可协同提升甜瓜幼苗的耐冷性。本研究鉴定到的耐冷性相关位点，既有助于解析复杂的适应性性状机制，也可提升甜瓜标记辅助选择（Marker-Assisted Selection, MAS）改良耐冷性的育种效率。