Analysis Of Genetic Diversity And Heterosis Prediction In Rapeseed (Brassica Napus L.)

Heterosis as a common biological phenomenon in nature is an efficient approach to increase crop yield. The genetic diversity study is the basis of germplasm evaluation and heterosis application. Thus, understanding the genetic diversity of parental lines and partitioning parents to different genetic groups can assist the selection for combinations with strong heterosis, elevating the efficiency of heterosis breeding. Different methods have been attempted for heterosis prediction over a long time period, however, none of them is sufficient. The development of molecular markers and its application in biological researches provide technical means for the prediction of heterosis in crops.This study used 49 Brassica napus lines and two accessions from allian species, Orychophragmus violaceous (Zhugecai) and Sinapis arvensis (Xinjiang wild rape). SSR and SRAP markers were used to investigate the genetic diversity and genetic groups of these materials, revealing the genetic difference on molecular level. The materials for heterosis study consisted of four restorer lines (R1, R2, R3, R6), three maintainer lines (1055B, 6098B, 8908B) of Shaan 2A CMS and 12 F₁ hybrids from 4 restorer lines and 3 respective sterility lines of the maintainers. The correlation between genetic distance and heterosis of various agronomic traits was analyzed based on the differences of SSR and SRAP markers. The main results are as following:1. The genetic groups classified by SRAP markers were more similar to pedigree information than those by SSR markers, and SRAP markers were more effective for the genetic analysis of closely related lines. Genetic distances (GDs) based on SSR markers were significantly and positively correlated to the genetic distances based on SRAP markers (r=0.857**). The genetic distance estimated based on SRAP markers was higher than that based on SSR. markers under the same number of polymorphic fragments.2. The middle parent heterosis (MPH) of important agronomic traits ranged from 1.8% to 39.4%. The MPH of yield per block was the highest (39.4%), and that of 1000-seed weight was the lowest (1.8%). High parent heterosis (HPH) of these traits ranged from -5.3% to 24.9%. Among them, those of 1000-seed weight and number of primary branches were negative. But again, the yield per block had the highest HPH (24.9%). Among all the possible combinations, 1055A×R6 had the highest HPH on yield per block, which reached 42.7%, indicating it is a hybrid combination with high yield potential.3. The special combining ability (SCA) of yield per plant was significantly and positively correlated to that of number of silliques per plant (r=0.730**). The SCA of yield per plant is negatively correlated to that of 1000-seed weight. Among the yield component traits, number of siliques contributed the most to yield (72.45%), the rest was contributed by number of seeds per silique, whereas 1000-seed weight had negative effect on seed yield.4. Results from analysis of variance indicated that the quadratic and cubic curves based on SRAP GDs were almost congruent with yield heterosis. There is a tendency of rising first, and then descending, indicating that combinations with parental lines of proper GDs have the highest yield heterosis. GDs are not correlative with oil content heterosis on the materials used in this study. 5. Coefficient between genetic distance based on polymorphism of SRAP markers and yield heterosis is 0.654, which is significant at 0.05 levels, whereas it is 0.423 based on polymorphism of SSR markers, which is not significant. Thus, for the prediction of yield heterosis, SRAP markers are better than SSR markers.