背景:胞质溶胶中的油体或脂滴(LD)是种子的亚细胞储存区室和向发芽种子提供能量的脂质代谢位点。主要的LD相关蛋白是脂氧合酶,磷脂酶,油质蛋白,TAG-脂肪酶,甾醇质,钙质和SEIPINs;参与促进发芽和增强过氧化作用,导致异味。然而,自然选择如何平衡富含脂质的种子中的矛盾过程仍然回避。本研究旨在预测主要油料种子直系同源进化枝之间的选择特征以及选择效应与基因表达的相关性。
结果:分析了主要含油作物的LD相关基因,以预测系统发育紧密直系系物集群中的自然选择特征,以了解适应性进化。正向选择是推动直系同源物以特定谱系方式进化和多样化的主要力量。在94个基因中发现了显着的正选择效应,特别是在油质蛋白和TAG脂肪酶中,在44个基因中使用过量的非同义替换进行纯化,而35个基因对选择效应是中性的。在十字花科中未发现对油棕LOX基因的显着选择影响。在低花生的T谱系油质蛋白和LOX基因中检测到大量影响选择特征的有害突变。T谱系油质蛋白基因主要涉及花药,绒毡层和花药壁形态发生。在蓖麻和芝麻中,>85%的PLD基因处于选择状态,而芥菜和向日葵的选择压力较低。硬脂质,在脂滴组织中起重要作用的caleosin和SEIPINs主要在种子中表达,并且处于相当大的正选择压力下。在旁系同源物和同源物之间表达差异明显,其中一个基因与另一个基因相比具有功能优势。与异味相关的LOX基因Glyma.13g347500在发芽期间未表达,而是其旁系Glyma.13g347600在甘氨酸max中显示表达。PLD-α基因在除种子外的所有组织中均有表达,δ基因在种子和分生组织中表达,而β和γ基因在叶片中表达。
结论:参与种子萌发和脂质代谢的基因处于强阳性选择状态,尽管物种差异是可辨别的。本研究鉴定了提高种子油含量和发芽的合适候选基因,其中定向选择可以变得更加富有成果。
BACKGROUND: Oil bodies or lipid droplets (LDs) in the cytosol are the subcellular storage compartments of seeds and the sites of lipid metabolism providing energy to the germinating seeds. Major LD-associated proteins are lipoxygenases, phospholipaseD, oleosins, TAG-lipases, steroleosins, caleosins and SEIPINs; involved in facilitating germination and enhancing peroxidation resulting in off-flavours. However, how natural selection is balancing contradictory processes in lipid-rich seeds remains evasive. The present study was aimed at the prediction of selection signatures among orthologous clades in major oilseeds and the correlation of selection effect with gene expression.
RESULTS: The LD-associated genes from the major oil-bearing crops were analyzed to predict natural selection signatures in phylogenetically close-knit ortholog clusters to understand adaptive evolution. Positive selection was the major force driving the evolution and diversification of orthologs in a lineage-specific manner. Significant positive selection effects were found in 94 genes particularly in oleosin and TAG-lipases, purifying with excess of non-synonymous substitution in 44 genes while 35 genes were neutral to selection effects. No significant selection impact was noticed in Brassicaceae as against LOX genes of oil palm. A heavy load of deleterious mutations affecting selection signatures was detected in T-lineage oleosins and LOX genes of Arachis hypogaea. The T-lineage oleosin genes were involved in mainly anther, tapetum and anther wall morphogenesis. In Ricinus communis and Sesamum indicum > 85% of PLD genes were under selection whereas selection pressures were low in Brassica juncea and Helianthus annuus. Steroleosin, caleosin and SEIPINs with large roles in lipid droplet organization expressed mostly in seeds and were under considerable positive selection pressures. Expression divergence was evident among paralogs and homeologs with one gene attaining functional superiority compared to the other. The LOX gene Glyma.13g347500 associated with off-flavor was not expressed during germination, rather its paralog Glyma.13g347600 showed expression in Glycine max. PLD-α genes were expressed on all the tissues except the seed,δ genes in seed and meristem while β and γ genes expressed in the leaf.
CONCLUSIONS: The genes involved in seed germination and lipid metabolism were under strong positive selection, although species differences were discernable. The present study identifies suitable candidate genes enhancing seed oil content and germination wherein directional selection can become more fruitful.