Sun Shuai, Wu Ying, Lin Xiuyun, Wang Jie, Yu Jiamiao, Sun Yue, Miao Yiling, Li Qiuping, Sanguinet Karen A, Liu Bao
Key Laboratory of Molecular Epigenetics of the Ministry of Education (MOE), Northeast Normal University, Changchun China.
Department of Crop and Soil Sciences, Washington State University, Pullman, Washington, United States of America.
PLoS One. 2017 Mar 1;12(3):e0172919. doi: 10.1371/journal.pone.0172919. eCollection 2017.
Hybrid weakness, a phenomenon opposite to heterosis, refers to inferior growth and development in a hybrid relative to its pure-line parents. Little attention has been paid to the phenomenological or mechanistic aspect of hybrid weakness, probably due to its rare occurrence.
METHODOLOGY/PRINCIPAL FINDINGS: Here, using a set of interspecific triploid F1 hybrids between Oryza sativa, ssp. japonica (genome AA) and a tetraploid wild rice species, O. alta (genome, CCDD), we investigated the phenotypic and physiological differences between the F1 hybrids and their parents under normal and nitrogen-limiting conditions. We quantified the expression levels of 21 key genes involved in three important pathways pertinent to the assayed phenotypic and physiological traits by real-time qRT-PCR. Further, we assayed expression partitioning of parental alleles for eight genes in the F1 hybrids relative to the in silico "hybrids" (parental cDNA mixture) under both normal and N-limiting conditions by using locus-specific cDNA pyrosequencing.
CONCLUSIONS/SIGNIFICANCE: We report that the F1 hybrids showed weakness in several phenotypic traits at the final seedling-stage compared with their corresponding mid-parent values (MPVs). Nine of the 21 studied genes showed contrasted expression levels between hybrids and parents (MPVs) under normal vs. N-limiting conditions. Interestingly, under N-limiting conditions, the overtly enhanced partitioning of maternal allele expression in the hybrids for eight assayed genes echo their attenuated hybrid weakness in phenotypes, an observation further bolstered by more resemblance of hybrids to the maternal parent under N-limiting conditions compared to normal conditions in a suite of measured physiological traits. Our observations suggest that both overall expression level and differential partitioning of parental alleles of critical genes contribute to condition-specific hybrid weakness.
杂种劣势是一种与杂种优势相反的现象，指杂种相对于其纯系亲本而言生长发育较差。由于杂种劣势很少发生，因此其现象学或机制方面很少受到关注。
方法/主要发现：在这里，我们使用一组粳稻亚种（基因组AA）与四倍体野生稻物种高杆野生稻（基因组CCDD）之间的种间三倍体F1杂种，研究了F1杂种及其亲本在正常和氮限制条件下的表型和生理差异。我们通过实时定量逆转录聚合酶链反应（qRT-PCR）对参与与所测定的表型和生理性状相关的三个重要途径的21个关键基因的表达水平进行了定量。此外，我们通过使用位点特异性cDNA焦磷酸测序，在正常和氮限制条件下，测定了F1杂种中八个基因的亲本等位基因相对于计算机模拟“杂种”（亲本cDNA混合物）的表达分配。
结论/意义：我们报告说，与相应的中亲值（MPV）相比，F1杂种在最终苗期的几个表型性状上表现出劣势。在正常与氮限制条件下，21个研究基因中的9个在杂种和亲本（MPV）之间表现出相反的表达水平。有趣的是，在氮限制条件下，杂种中八个被测基因的母本等位基因表达明显增强，这与它们在表型上减弱的杂种劣势相呼应，在一系列测量的生理性状中，与正常条件相比，氮限制条件下杂种与母本的相似性更高，这一观察结果进一步得到了支持。我们的观察结果表明，关键基因的总体表达水平和亲本等位基因的差异分配都导致了特定条件下的杂种劣势。
