Heterosis refers to the better performance of cross progeny compared with inbred parents, and its utilization contributes greatly to agricultural production. Several hypotheses have been proposed to explain heterosis mainly including dominance, over-dominance (or pseudo-overdominance) and epistasis. However, systematic dissection and verification of these hypotheses are rarely documented. Here, comparison of heterosis level across different traits showed that the strong heterosis of composite traits (such as yield) could be attributed to the multiplicative effects of moderate heterosis of component traits, whether at the genome or locus level. Yield heterosis was regulated by a complex trait-QTL network that was characterized by obvious centre-periphery structure, hub QTL, complex up/downstream and positive/negative feedback relationships. More importantly, we showed that better-parent heterosis on yield could be produced in a cross of two near-isogenic lines by the pyramiding and complementation of two major heterotic QTL showing partial-dominance on yield components. The causal gene (BnaA9.CYP78A9) of QC14 was identified, and its heterotic effect results from the heterozygous status of a CACTA-like transposable element in its upstream regulatory region, which led to partial dominance at expression and auxin levels, thus resulting in non-additive expression of downstream responsive genes involved in cell cycle and proliferation, eventually leading to the heterosis of cell number. Taken together, the results at the phenotypic, genetic and molecular levels were highly consistent, which demonstrated that the pyramiding effect of heterotic QTL and the multiplicative effect of individual component traits could well explain substantial parts of yield heterosis in oilseed rape. These results provide in-depth insights into the genetic architecture and molecular mechanism of yield heterosis.

中文翻译：

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对油菜的系统解剖为了解产量杂种优势的遗传结构和分子机制提供了见解

杂种优势是指杂交后代与近交亲本相比具有更好的性能，其利用对农业生产有很大贡献。人们提出了几种假说来解释杂种优势，主要包括优势、过度优势（或假超优势）和上位性。然而，对这些假设的系统剖析和验证很少有记录。在这里，不同性状杂种优势水平的比较表明，无论是在基因组还是基因座水平，复合性状（例如产量）的强杂种优势可以归因于组成性状的中等杂种优势的乘法效应。产量杂种优势受复杂的性状QTL网络调控，该网络具有明显的中心-外围结构、枢纽QTL、复杂的上下游和正负反馈关系。更重要的是，我们表明，通过对产量组成部分显示部分优势的两个主要杂种优势QTL的金字塔化和互补，可以在两个近等基因系的杂交中产生更好的亲本产量杂种优势。因果基因（鉴定出QC14的杂种优势效应是由于其上游调控区CACTA样转座元件的杂合状态，导致表达和生长素水平的部分优势，从而导致非加性表达下游响应基因参与细胞周期和增殖，最终导致细胞数量的杂种优势。综合来看，表型、遗传和分子水平上的结果高度一致，表明杂种优势QTL的金字塔效应和单个组成性状的倍增效应可以很好地解释油菜产量杂种优势的大部分。这些结果提供了对产量杂种优势的遗传结构和分子机制的深入见解。