Table_6_A High-Density Linkage Map of the Forage Grass Eragrostis curvula and Localization of the Diplospory Locus.docx

Eragrostis curvula (Schrad.) Nees (weeping lovegrass) is an apomictic species native to Southern Africa that is used as forage grass in semiarid regions of Argentina. Apomixis is a mechanism for clonal propagation through seeds that involves the avoidance of meiosis to generate an unreduced embryo sac (apomeiosis), parthenogenesis, and viable endosperm formation in a fertilization-dependent or -independent manner. Here, we constructed the first saturated linkage map of tetraploid E. curvula using both traditional (AFLP and SSR) and high-throughput molecular markers (GBS-SNP) and identified the locus controlling diplospory. We also identified putative regulatory regions affecting the expressivity of this trait and syntenic relationships with genomes of other grass species. We obtained a tetraploid mapping population from a cross between a full sexual genotype (OTA-S) with a facultative apomictic individual of cv. Don Walter. Phenotypic characterization of F1 hybrids by cytoembryological analysis yielded a 1:1 ratio of apomictic vs. sexual plants (34:27, X2 = 0.37), which agrees with the model of inheritance of a single dominant genetic factor. The final number of markers was 1,114 for OTA-S and 2,019 for Don Walter. These markers were distributed into 40 linkage groups per parental genotype, which is consistent with the number of E. curvula chromosomes (containing 2 to 123 markers per linkage group). The total length of the OTA-S map was 1,335 cM, with an average marker density of 1.22 cM per marker. The Don Walter map was 1,976.2 cM, with an average marker density of 0.98 cM/marker. The locus responsible for diplospory was mapped on Don Walter linkage group 3, with other 65 markers. QTL analyses of the expressivity of diplospory in the F1 hybrids revealed the presence of two main QTLs, located 3.27 and 15 cM from the diplospory locus. Both QTLs explained 28.6% of phenotypic variation. Syntenic analysis allowed us to establish the groups of homologs/homeologs for each linkage map. The genetic linkage map reported in this study, the first such map for E. curvula, is the most saturated map for the genus Eragrostis and one of the most saturated maps for a polyploid forage grass species.

垂穗画眉草（Eragrostis curvula (Schrad.) Nees，俗称垂穗知风草（weeping lovegrass））是原产于非洲南部的无融合生殖（apomixis）物种，在阿根廷半干旱地区被用作饲草。无融合生殖是一种通过种子进行无性繁殖的机制，其过程涵盖跳过减数分裂以形成未减数胚囊（apomeiosis，无融合减数分裂）、孤雌生殖（parthenogenesis），并以依赖受精或不依赖受精的方式形成可育胚乳（endosperm）。本研究构建了首张四倍体垂穗画眉草的饱和连锁图谱，同时采用传统分子标记（扩增片段长度多态性（AFLP）、简单序列重复（SSR））与高通量分子标记（简化基因组测序单核苷酸多态性（GBS-SNP）），定位了控制二倍体孢子生殖（diplospory）的位点。此外，本研究还鉴定了影响该性状表现的潜在调控区域，以及与其他禾本科物种基因组的共线性（syntenic）关系。我们从完全有性生殖基因型OTA-S与栽培品种Don Walter的兼性无融合生殖个体的杂交组合中，获得了四倍体作图群体。通过细胞胚胎学分析对F1杂种进行表型鉴定，结果显示无融合生殖植株与有性生殖植株的比例为1:1（34株:27株，χ²=0.37），符合单显性遗传因子的遗传模型。亲本OTA-S与Don Walter的最终有效标记数分别为1114个和2019个。这些标记被分配至每个亲本的40个连锁群（linkage group）中，这与垂穗画眉草的染色体数目一致，每个连锁群包含2至123个标记。OTA-S连锁图谱的总长度为1335厘摩（cM），平均标记密度为1.22 cM/标记；Don Walter连锁图谱的总长度为1976.2 cM，平均标记密度为0.98 cM/标记。控制二倍体孢子生殖的位点被定位在Don Walter的第3连锁群上，与该位点相邻的共有65个标记。对F1杂种中二倍体孢子生殖的表现度进行数量性状位点（QTL）分析，发现存在两个主要QTL，分别位于距二倍体孢子生殖位点3.27 cM和15 cM处。这两个QTL共解释了28.6%的表型变异。共线性分析帮助我们确定了两张连锁图谱的同源/部分同源染色体（homologs/homeologs）群。本研究报道的遗传连锁图谱是首张垂穗画眉草的连锁图谱，也是画眉草属（Eragrostis）中饱和度最高的连锁图谱，同时是多倍体饲草物种中饱和度最高的连锁图谱之一。