Fine Mapping Of QHd1, A Minor QTL Having Pleiotropic Effects For Heading Date And Yield Traits Indica Rice

In rice(Oryza sativa L.), heading date is a crucial determinant for ecological adaption and grain yield is an immediate indicator of the productivity. They are key factors determining the commercial potential of a rice variety. Optimizing heading date for maximizing grain yield requires a better understanding of the major and minor genes controlling both traits. In this study, QTL mapping was first conducted using three primary populations and the influence of heading date variation on the QTL detection was analysed. Then, based on these results, secondary populations with a homogenouse genetic background were developed, and used to fine-map a minor QTL with pleiotropism for heading date and yield traits. The main results are as follows:1. The influence of heading date variation on the detection of QTLs for heading date and yield traits in riceThree recombinant inbred line(RIL) populations featured by distinct heading date var iations were used to detect QTLs for heading date and yield traits. Among them, ZM-RIL and XM-RIL were derived from the cross between medium restorer line Milyang 46 and early maintainer lines Zhenshan 97 B and Xieqingzao B, respectively and showed great variations on heading date, while the TI populatin were constructed using the cross between medium restored lines Teqing and IRBB lines and had a high-uniform heading date. They were each tested for three years and measured for heading date and yield traits. QTL analysis was performed by QTL Network 2.0 and a total of 13 and 51 main-effect QTLs, together with 11 and 20 digenic interactions, were detected for heading date and yield traits, respectively. Additive effect was shown to be the major genetic component responsible for the traits under study and epistaseis also played an important role, while few of the QTLs and epistases showed signif icant genotype-by-environment interaction. Compared with main-effect QTLs detected in ZM and XM, those found in TI had it own distinctive features. For heading date, they had smaller additive effects but explained a relatively large proportion of phenotypic variance(R2). For panicle number and grain number, they had larger R2 and additive effects, respectively. For grain weight, they were approximately twofold in both the number and the overall R2. These results indicate that the use of segregating population with low variation on heading date not only increases the power of detecting minor QTLs for heading date itself, but also greatly improves the detection of QTLs for yield traits. While a considerable proportion of the QTLs coincided in positions with those that have been cloned, some of the others were shown to have a consistent effect across years and populations, including q HD11 for heading date, q TGW10 for grain weight, q TGW6.1/q GY6 for grain weight and grain yield, and q NGP2/q TGW2.3 for grain number and grain weight.2. Fine mapping of the minor QTL q Hd1, analyzing its pleiotropism for heading date and yield tarits, and preliminarily exploring the underlying molecular mechanismsAccording to the above results of QTL primary mapping, secondary populations with major QTLregions fixed were constructed to detect minor QTLs for heading date. In a previous study, residual heterozygotes were selected from rice populations with advanced generations derived from the indica rice cross Zhenshan 97B(ZS97B)///ZS97B//ZS97B/Milyang 46, and used to constructed BC2F8:9 populations, by which q Hd1, a minor QTL for heading date, was delimited into a 761 Kb interval on the long arm terminal of chromosome 1. In the present study, rice populations were constructing using sequential residual heterozygotes that showed smaller but sequential heterozygous segments in the target interval with a higher homozygouse genetic background, by which q Hd1 was fine-mapped.Then, its pleiotropism for heading date and yield traits, response to the fluctuation of natural photo-temperature conditions and the underlying molecular mechanisms were analyzed. First, by using BC2F10 populations and its derived BC2F10:11 near isogenic line(NIL) sets featured by sequential segregating regions in the target interval, q Hd1 was fine-mapped into a 95.0 Kb region f lanked by RM12102 and RM12108, and its pleiotropism for yield traits was determined. At the subsequent experiments of validation, including those conducted using BC2F12 and BC2F13 populations in Zhejiang and Hainan, respectively, and using NIL sets at both sites for years, it was observed that the genotypic effects of q Hd1 on heading date and yield traits varied greatly, mainly regarding to the magnitude, which suggested the existence of q Hd1 × environment interaction. Then, in the experiment of interval-sowing with five different sowing dates, this interacton was confirmed. With the postponement of sowing date, the genotypic effects of q Hd1 on heading date and grain yield per plant showed an increasing tendency; on the other hand, significant variations were detected for different yield component traits in different sowing-date treatments with the tendency of panicle number per plant—grain/spikelete number per panicle—1000-grain weight. When considering the photo-temperature condition during this experiment, that is escalating temperature with generally long days, it was inferred that the phenotypic effects of q Hd1 was sensitive to the variation of temperature. The candidate region contains ten annotated genes including two genes with functional information related to the control of heading date. By sequencing and expression analyses, it could be speculated that the target gene underlying q Hd1 was Os MADS51. A 9.7 Kb structural variation was identif ied at the 1st intron of Os MADS51 and might influence the expression of the promotion pathway of rice flowering, Os MADS51-Ehd1-RFT1/Hd3 a. These results lay a foundation for the cloning of q Hd1.