Heterosis Of Resynthesized Brassica Napus And Its Utilization In Hybrid Breeding

It is generally believed that higher genetic distance between their respective parents results in higher heterosis. Although Brassica napus is one of the most successful crops in heterosis utilization worldwide, its genetic basis is narrow when compared with its diploid parent Brassica rapa and Brassica olecerea. The gene pool of elite rapeseed breeding material has been further eroded by an emphasis on specific traits like high yield, double low quality etc, and the genetic variability in this important oilseed crop is restricted with regard to many valuable characters for breeding purposes. The resynthesized Brassica napus crossed by its diploid parents is a useful strategy for broadening the genetic basis of breeding material, and large genetic distance to rapeseed cultivars make it possible for hybrid rapeseed breeding. So far, few thorough studies have been made on the agronomic and genetic performance of hybrids between resynthesized and natural rapeseed(NRF1).In this study, many technologies with quantative genetics, cytogenetics, physiology in hormonal level and molecular biology were first used to analysize seed yield traits of NRF1i in detail. An 8×8 complete diallel experiment using four B. napus cultivars (BN) and four resynthesized rapeseed lines (RS) as parents was designed to study heterosis and genetic inheritance for yield components. SSR and SRAP techniques were employed to assess genetic diversity in 24 materials, including 16 resynthesized rapeseed and 8 cultivars. Relationship between genetic distance and F1 performance was analyzed. Endogenous hormones were tested in three F1 hybrids and their parents to determine heterosis and their relationship to yield components. Self-incompatibility, pollen fertility and meiosis of NRF1 were analysed to detemine the main factor resulting in low seed per pod in NRF1. The main results are as follows:1. Heterosis and genetic inheritance of NRF1 combinations1.1 All the 56 hybrids had positive mid-parent heterosis (MPH) in seed yield per plant with a mean of 32.77%, except Rl×R7. As for yield components, pods per plant had highest heterosis with a mean of 24.49%, followed with seeds per pod, with a mean of 15.71%, heterosis for 1000 seed weight is low, with a mean of 1.22%. Significant heterosis for seed oil content was also observed, but it was less than that of yield per plant. Among 56 hybrids, the mid-parent heterosis for seed oil content ranged from -8.547% to 15.07% with an average of 4.43%. About 78.6% combinations (44/56) had positive heterosis for seed oil content.1.2 The 56 hybrids were divided into three types: RS×RS, BN×BN and NRF1. Compared with the both parents, NRF1 showed better agronomic characters than mid-parent values in all traits. The NRF1 were also taller in plant height, longer in primary branches and main inflorescence, more in branches and increased in pod number per plant when compared with cultivar check (ZS10).1.3 There were seven NRF) which showed positive check heterosis in seed yield per plant, accounted for 21.88%, among which 05R1×XY15 had the second highest seed yield per plant in all the 56 hybrids, indicating that NRF1 had great potential in increasing seed yield. The NRF1 had much more pods per plant (436.96) than BNxBN ( 370.89) types, while had lower seeds per pod. Because 1000 seed weight is normal in NRF1, seeds per pod is the limiting factor to increasing plant yield.1.4 Diallel analysis showed that both additive and dominant effects were significant for yield components, additive gene effects were larger than non-additive gene effects in yield per plant and seed per pod, while non-additive gene effects were larger than additive gene effects in pods per plant and 1000-seed weight. Dominant genes had positive effects on yield per plant, pods per plant and 1000-seed weight and negative effects on seeds per pod (r=-0.86, -0.62, -0.41 and 0.47, respectively). The two highest yield hybrids ZS10×XY15 and 05R1×XY15 showed best general combing ability (GCA) and special combing ability (SCA), it is requisite to select parents with high GCA and combinations with high SCA to obtain high seed yield F1s.1.5 There was no significant correlations between genetic distance and F1 yield per plant, it is difficult to predict F1 heterosis by molecular markers.2 Genetic distance and genetic diversity in resynthesized Brassica napus2.1 With SSR and SRAP molecular markers, total 168 polymorphic bands were amplified in the 24 materials, among which 73 bands were obtained from 18 pair SSRs, with a mean of 4 bands per SSR marker, 95 bands were obtained from 19 pair SRAPs, with a mean of 5 bands per SRAP marker, the polymorphism in SRAP is better than in SSR.2.2 With UPGMA (Unweighted Pair Group Mathematics Average) clustering and PCA (Principal Component Analysis), the resynthesized Brassica napus and rapeseed cultivars were significantly divided into two groups, indicating that they had different genetic basis and the former had more genetic diversity.3 Endogenous hormones in hybrids and their relationship to seed yield3.1 Endogenous hormones changed dramatically in different stages and different stages, the trends of variation were similar in three F1 hybrids for GA and iPA, which reached their highest levels at flowering stage with a mean of 876.26 ng/g FW and 1084.04ng/g FW, respectively.3.2 Except for 1000 seed weight, the other yield traits showed positive relationship to iPA in all stages, and high heterosis of iPA among three F1 combinations were detected, indicating there might be important roles of iPA to yield traits. The high level of iPA in hybrids make it possible to produce more meristem and then more pods per plant, and got higher seed yield in hybrids finally.3.3 The materials showed significant differences for four endogenous hormones in buds at flowering stage, indicating genetic variation of hormones in this stage; GA and iPA contents reached their highest level in buds at flowering stage, implying that this stage was the main stage of synthesis and metabolism of hormones.4 Possible mechanisms resulted in the low seed per pod in the NRF1 hybrids4.1 The results of bud self-pollination and flower self-pollination indicated that there was obvious self-incompatibility in NRF1, which was significantly correlated to seed per pod. The NRF1 from R2 as resynthesized rapeseed parent showed best pods per bud, seed per bud and seed per pod, indicating inferior self-incompatibility in NRF1 when R2 as resynthesized rapeseed parent. The results were in accordance with the former study in the second chapter: four NR hybrids 05Rl×S72, 05RlxHY15, 05Rl×XY15 and 05R4×XY15 showed positive check heterosis in seed per pod, 05R1 and R2 here was the same material. The four combinations were the best ones of all 32 NRF1 in seed yield per plant. 4.2 Meiosis of NRF1 hybrids was somewhat abnormal and might be responsible for the lower pollen fertility. However, no significant abnormality was found in first and second anaphases and neither in tetrad stage. No positive correlation was found between flower organ abnormality and pollen fertility. That meant the above two factors might be responsible for the lower seed per pod, but not the main factors.