Research On The Genetic Basis Related To Heterosis In Brassica

Heterosis or hybrid vigor is an important genetic phenomenon in organic sphere. The utilization of heterosis breeding is one of the efficient ways to improve the yield, qualities, stress tolerance and other characters of crops. Heterosis is also widely used in plant breeding of Brassica genus. With various species and multiple utilizations such as vegetable, oil resources, medicinal plants, ornamental plants, feed resources and so on, plants in Brassica genus possess important economic values and are planted world wide. But due to the limited understanding of the genetic basis underlying heterosis, the utilizations of heterosis in plant breeding are greatly restricted. In this study, four inbred lines from three subspecies of Brassica genus (Brassica campestris L., syn. B. rapa L.), i.e. 'Bcajh97-01' (ssp. chinensis Makino cv. Aijiaohuang), 'Bchyc98-02' (ssp. pekinensis Olsson cv. Huangyacai), 'Bcbmj98-03' (ssp. rapifera Sinsk cv. Baimanjing), 'Bcshq98-04' (ssp. chinensis L. Hanelt cv. Shanghai-qing) and their hybrid offspring were used to study the genetic basis of heterosis. QTL based heterosis analysis, gene expression differences analysis, photosynthetic heterosis and the mechanism of low P tolerance were orderly studied. The main results are listed below.(1) The biomass and its component trait, net photosynthesis rate and chlorophyll content of four parents and their nine hybrids were investigated. Results showed that the biomass component traits of most hybrids showed obvious heterosis in all the investigated stages, i.e. 20, 45, 70 days after planting. The biomass showed the highest heterosis of 181.2% over better parent. All photosynthetic rates of hybrids, with 7.2% - 29.5% better parent heterosis, were significantly higher than their parents. The photosynthetic rates were also significantly positively correlated with plant height, leaf number, leaf width, biomass and dry weight. No obvious heterosis for chlorophyll content was found for all hybrids.(2) A genentic linkage map based on a F2 population derived from the cross of 'Bcajh97-01' and 'Bcbmj98-03' was constructed. This map composed of 141 AFLP makers, 17 linkage groups, and totally spanned 2063 cM. Genetic distance between adjacent markers ranged from 0.4 to 37.3 cM and averaged at 16.6 cM. This map was subsequently used in the QTL mapping of biomass, over ground weight, plant height, plant breath 1 & 2, leaf length and width of the biggest leaf, leaf number, taproot diameter 1 & 2 and taproot weight. A total of 23 main effect QTLs were detected using composite interval mapping. A single QTL can explain 4.4% - 47.8% phenotypic variance, and QTLs for each trait can account for 16.96% to 61.59% of the total phenotypic variance. Sixty five percent (15/23) of these QTLs showed obvious overdominance.(3) Two locus interactions for the 11 traits were analyzed using EPISTACY software. Four hundred and forty four two locus interactions were detected at the default criteria of P < 0.005, with an average of 40 epistatic loci per trait. Seventy-six and 43 of them remained significant at 0.001 and 0.0005 level. It also could be observed that some epistases were shared by the highly correlated traits, i.e. pair of traits or composite/component trait.(4) The gene expression profiles of 'Bcajh97-01', 'Bcbmj98-03' and their hybrid were compared using cDNA-AFLP technology. Seventeen fragments that expressed uniquely in hybrid were successfully cloned and analyzed. Fifteen of them belonged to 14 UniGene: 4 related to chloroplast, 2 related to mitochondrion, and others related to endomembrane system, cytosol and so on. The processes participant of these genes were nucleotide metabolism, carbohydrate metabolism, oligopeptide transport, protein biosynthesis, intracellular protein transport, protein complex assembly, protein folding, proteolysis and so on. And the functions involved were kinase, hydrolase and O-methyltransferase activity, chloroplast Biogenesis, nucleic acid binding, and transporter activity and so on.(5) 'Bcajh97-01', 'Bcbmj98-03' and their reciprocal hybrids were used to analyze photosynthetic characteristics by simultaneous measurement of leaf gas exchange and chlorophyll fluorescence parameters. The ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO), a key enzyme in the Calvin cycle was also investigated accordingly. Results showed that the net CO₂ assimilation rates (Pn) of hybrids were significantly higher than that of their parents in the experiment, leading to a strong heterosis in photosynthesis. Hybrids also increased Stomatal conductance (Gs) but had no influence on intercellular CO₂ concentration (Ci) and Stomatal limitation (l). The concentrations of total chlorophyll in the hybrids were between their parents with only a weak mid-parent heterosis. Hybrids significantly increased the quantum efficiency of PSII photochemistry (Φ_PSII) and the electron transport rate (ETR), which were accompanied by a similar increase in photochemical quenching (qP). Additionally, hybrids also had evident heterosis in the maximum carboxylation rate of RuBisCO (Vcmax), maximum electron transport rate (Jmax), and the RuBisCO protein content.(6) The performances of 'Bcajh97-01' 'Bchyc98-02' and their hybrid under P-deficient conditions were studied. Biomass, root growth, photosynthesis, chlorophyll fluorescence, respiration and element absorptions were investigated. Results showed that low P treatment significantly reduced the shoot weight and net photosynthetic rate of both parents, but little had been done to the hybrid. Hybrid increased its longest fibrous root length for about 61.4% in low p treatment than in control, but no significant different were found in root length for both parents. Low P treatment had less influences on hybrid in P acquire efficiency (PAE), P utilization efficiency (PUE) and P content per plant than on its parents. The acid Phosphatase activity in leaf and root were both increased. Low P treatment also significantly decreased the respiration rate and simultaneity increased the ratio of alternative pathway respiration. It was interesting that the cytochrome pathway respiration of hybrid under low P condition was significantly higher than its parents. Low P treatment also had some influences on the absorption of other element, such as N, K, Ca, and Mg.