Construction Of Chromosome Segment Substitution Lines Carrying Japonica Segments In Indica Background And Its Effect On Heterosis For Yield-Relalted Traits

Rice yield improvement guarantees the food security. So the development of high-yield varieties is one of the most important objectives in rice breeding. Indica and Japonica are two subspecies of Asian cultivated rice. Tremendous differentiation between them resulted in great heterosis of inter-specific crosses. It is an optimal strategy to take advantage of sub-specific heterosis for further elevation in rice production. In this study, a set of chromosome segment substitution lines (CSSLs) were constructed from an elite Indica hybrid parental line Zhenshan 97B (ZS97B) and the sequenced Japonica cv. Nipponbare (NP) to study the genetic effects of Japonica segments on yield and yield-related traits in the genetic background of Indica. In order to characterize the effects of Japonica segments on heterosis, the population of CSSLs/Minghui63 (CSSLs/MH63) was used. The major results are as follows:1. A series of 148 CSSLs were generated by subsequent backcrossing and marker-assisted selection. In this CSSL population, recovery of the ZS97B genome ranged from 92.0% to 99.9% with an average of 97.8%. 78 of 148 CSSLs contained a single segment and 51 carried two. Each line carried an average of 1.6 segments. The target-segment of NP span an average of 23.1cM and the total length was 3396.8 cM that was 2.3 times of rice genome. The CSSLs totally represented the whole genome of NP except for RM314 and RM527 on chromosome 6, the telomere region on chromosome 8 and RM6396 on chromosome 12.2. Phenotypic evaluation of CSSLs was carried out in three environments. The traits characterized are heading date (HD), plant height (PH), panicles per plant (PPP), number of primary branches (PB), number of secondary branches (SB), panicle length (PL), panicle weight (PW), grains per panicle (GN), spikelets per panicle (SB), seed setting ratio (SSR), kilo-grain weight (KGW) and yield per plant (YD). Transgressive segregations observed in all these traits implied that the introgressed NP segments had an effect on agronomic traits. Analyzing through a specific method, a total of 131 chromosome regions controlling different traits were detected in three environments. Among them, 71 regions show positive effects while the other 60 were negative when harboring NP alleles.3. The genetic effects of NP segments composed of additive, dominant and over-dominant. Genetic effects differentiated among different introgression segments. Taken all the traits into consideration, the percentage of loci showing additive, dominant or over-dominant was 43, 13 and 44 percent, respectively. These results suggested that additive and over-dominant played an important role in trait performance.4. CSSLs/MH63 was developed by selecting CSSLs as female and MH63 as the male parents, to study the effects of NP segments on heterosis. Twelve traits were evaluated in two environments. Taken all these traits into consideration, 22.6 percent of hybrids significantly deviated from SY63, which differentiated more than one standard deviation. So was 10.1 percent for more than two standard deviations. In 2007, 7.9 percent of hybrids deviated from ZS97B/MH63 (Shanyou63, SY63) more than two standard deviations on YD when carrying NP segments, so it was 5.7, 14.3 and 5.0 percent for KGW, GN and PPP, respectively. Similar trends were observed in 2006. A total of 86 over-check heterosis loci (HL_CK) were identified in two environments. 49 (57.0 percent) out of 86 loci showed positive effects, implying NP segments improved the performance of hybrids. The other 37 (43.0 percent) showed negative effects.5. Mid-parent heterosis of each CSSLs/MH63 cross was calculated and used as input data for identifying mid-parent heterosis loci (HL_MP). 118 HL_MP were detected in two environments. 67 HL_MP (56.8%) showed positive effect such that heterozygotes had higher heterosis values than the means of the two homozygotes. The other 51 (43.2%) showed negative effects such that heterozygotes had lower heterosis values than the mean of the two homozygotes.6. Comparing 86 HL_CK with regions affecting CSSLs performance (QTL), only 16.3 percent (14 loci) of HL_CK were repeatedly detected in QTL, the other 83.7 percent was specified. This indicated that most HL_CK were different from QTL. No significant correlations between the performance of CSSLs/MH63 and CSSLs were observed on PPP, PB, SB, KGW, YD and so on. These results implied that the CSSLs couldn't be used to predict the performance of CSSLs/MH63, especially for YD. The hybrids must be developed to study the heterosis effects of introgressed segments.Comparing HL_MP with HL_CK, 77.1 percent of loci were specifically observed in HL_MP or in HL_CK. This suggested that the interaction between different alleles could enhance the effects of heterosis loci. The epistasis between heterosis loci and genetic background was not excluded.7. 23 out of 118 HL_MP were co-localized with those identified in ZS97/MH63 population, and the others may be newly discovered. By comparing the heterosis effects of loci between CSSLs/MH63 and ZS97B/MH63, 23 HL_MP can be divided into two groups. First, heterosis loci in CSSLs/MH63 and ZS97B/MH63 showed the same direction of effects. Second, heterosis loci in CSSLs/MH63 and ZS97B/MH63 showed the reverse direction of effects.8. The twenty-three loci showing positive effects were simultaneously detected both in HL_CK and HL_MP. Among them, 15 loci caused positive effects on PPP, PB, PL, PW, GN, SN, SSR and YD, respectively. These results may play an important role in heterosis improvement and in studying the molecular basis of heterosis.