Genetic relationship between 1000-grain weight and its main components
Grain length and grain width are the main components of 1000-grain weight, which are closely related to each other. Studies by Ramya (2010) and Gegas (2010) showed that the 1000-grain weight positive correlated with the grain length and grain width, and the correlation between 1000-grain weight and grain width was higher than that between 1000-grain weight and grain length. In this study, the correlation coefficient between 1000-grain weight and grain width, 1000-grain weight and grain length, as well as grain length and grain width were 0.75, 0.46, and 0.29, respectively. The correlation between 1000-grain weight and grain width was much larger than that between 1000-grain weight and grain length. Similar results are detected in this study. The phenomenon that QTLs control associated traits tends to be distributed in the same or similar chromosome intervals, which was known as that one gene had multiple effects or tight linkage (Liang et al. 2010). In this study, fifteen Loci with multiple effects were detected, involving 37 QTLs that accounted for 90.2% of total numbers of QTLs detected. of which one locus simultaneously regulated 1000-grain weight, grain length and grain width, 11 loci simultaneously regulated 1000-grain weight and grain width, and 2 loci simultaneously regulated 1000-grain weight and grain length, one locus controlled both grain length and grain width. The positive additive effect values of different traits ' QTLs at the same loci were all derived from the same parent, which revealed the positive correlation at molecular level among 1000-grain weight, grain length and grain width.
Comparison of the present study with previous studies
By comparing the results of this study and previous studies, it was found that only nine alleles concerned with 1000-grain weight, grain length or grain width were in the same marker range as previously reported, while the others could not be identified for lack of a common molecular marker. Besides, the poor comparability of QTL mapping among different results may result from population size, genetic background, molecular marker types and so on.
Kumari et al. (2018) detected a locus for 1000-grain weight, grain length and grain width in the region Xgwm429-Xgwm148 on 2B chromosome. Luo et al. (2016), Quarrie et al. (2005), and Groos et al. (2005) have also detected QTLs for 1000-grain weight near the mark Xgwm148. Ramya et al. (2010) detected a QTL for 1000-grain weight near the marker Xbarc55 on 2B chromosome, which was stably expressed in multiple environments. By comparison in the high-density genetic maps of micro-satellite markers (Somers et al. 2004), it was found that the distance between Xbarc55 and Xgwm148 was only 8 cM. QGWT-2B.1 and QGW-2B.1 detected in this study located in the chromosome region Wsnp_Ex_c1962_369626-Xgwm148, and had the same molecular marker Xgwm148 as above studies. It was further confirmed that there was a locus for 1000-grain weight and related traits on chromosome 2B, which was closely linked to the molecular marker Xgwm148.
Chen et al. (2014) detected three main-effect QTLs for 1000-grain weight on chromosome 4B, of which, QGW4B-5 were found to be located in the same marker interval with QGWT-4B.1 and QGW-4B.1 detected in this study. There were three same tight-linked markers BS00021984, Tdurum and IAV971. Li (2017) also detected a QTL for grain length (QKl-WA-4B.1a) in this chromosome region, and this QTL had the same linked marker wsnp_Ex_c3119_5763762 as QGWT-4B.1 and QGW-4B.1. At a distance of about 10cM from this locus, a locus simultaneously controlling 1000-grain weight and grain width, namely QGWT-4B.2 and QGW-4B.2, was also detected in this study. Chen et al. (2014) also detected a main-effect QTL (QGW4B-17) for 1000-grain weight Within a similar chromosomal region, therefore, these two Adjacent Chromosome region on chromosome 4B may also be important for the expression of grain-related traits, and more in-depth research is needed.
Ramya et al. (2010) detected a loci simultaneously controlled 1000-grain weight, grain length and grain width in the region Xbarc74-Xgwm499 of 5B chromosome. Li et al. (2015d), Krishnappa et al. (2017), Li et al. (2012), and Quarrie et al. (2005) detected one, and Tang et al. (2011) Detected Two QTLs for 1000-grain weight, linked with Molecular Marker Xgwm499. in this study, QGWT-5B and QGL-5B.2 were located nearby of Xgwm499 on chromosome 5B, and QGWT-5B is a major QTL stably expressed in 4 environments.
Golabadi et al. (2010) identified a main-effect QTL for 1000-grain weight on the 7B chromosome stably expressed under drought stress and normal water condition, which was closely linked with Xcfd22-7B, the genetic distance between Xcfd22-7B and Xwmc517 was 13cM by comparison in the high-density genetic maps of micro-satellite markers (Somers et al. 2004). In this study, the main-effect QTLs for GWT, GW and GL were also detected within a marker interval of about 10cM genetic distance from the Xwmc517. Whether these QTLs were the same as above loci should be further verified.
QTL with stable expression in multiple environments
The ultimate goal of QTL mapping is to detect stable and reliable quantitative trait loci and to closely link molecular markers for molecular marker assisted selection (MAS). 1000-grain weight, grain length and grain width are all typical quantitative traits, which are greatly affected by environment (Garcia-Suarez et al. 2010). QTLs expressed only in specific environment tend to be less effective, less stable and less reliable, while QTLs detected in multiple environments tend to have higher effect values and reliable. Two of the 51 QTLs for 1000-grain weight and related traits detected in this study were detected in four environments, six were detected in three environments, and 14 were detected in two environments. Total of 22 QTLs were detected in two or more environments, accounting for 43% of the number of QTLs detected in this study. It is necessary to deeply analyze and precisely locate the stable QTLs for Gene Cloning and marker-assisted selection.