Genetic Analysis Of Allohexaploid In Brassica With Emphasis On The Resistance Against Sclerotinia Sclerotiorum

Brassica species has a rich gene pool, which can provide materials and genetic diversity resources for interspecific and intergeneric hybridization to cultivate new species. Allohexaploid resynthesized in Brassica is not only a model material to research structure variations and evolutionary relationship among A, B, C genome in Brassica, but also a bridge to transfer elitic traits for rapeseed breeding. The objectives of present study are to develop allohexploid derived from tetraploids and diploids and to transfter the resistance against Sclerotina sclerotiorum from B. oleracea via hexaploids and pentaploid between it and B. napus. The results are as follows:(1) Synthesis of Brassica allohexaploidAmong three types of crosses to develop allohexaploid, with use of9varieties of B.juncea (AABB),9varieties of B. oleracea (CC),12varieties of B. carinata (BBCC),9varieties of B. rapa (AA),1B. napus (AACC) and2varieties of B. nigra (BB) as parents, the crossability between B. carinata and B. rapa was highest, followed by B. juncea and B. oleracea, while the crossability between B. nigra and B. napus was lowest. Three chromosome doubling approaches were used in our research including dropping growth point, immersing roots, tissue culture in the medium with colchicines. All approaches were treated successfully, but the efficiency was different. In2010, only28plants of4lines of S1generation survived finally because of the bad condition and some other reasons.(2) Comparative research of two types of hexaploidsComparison of pollen fertility, seed setting rates of self-pollination and open pollination between two types of hexaploids showed that no significant difference existed in pollen fertility among7lines. While, there were significant difference among seed setting rates of self-pollination and open pollination (p<0.01). Besides, there was significant correlation between self-setting rate and pollen fertility (r=0.704, p <0.05) and significant correlation between setting rates of self and open pollination (r=0.96, p<0.01).In order to study the crossability between two types of hexaploids, crosses and reciprocal crosses between two types of hexaploids were performed. Result showed that there was significant difference between different female (p<0.05). AABB.CC as female was easier to obtain seeds while two types of hexaploid have the same compatibility with B. napus. In order to verify the crossability, pollen-stigma recognition was conducted. No matter the hybridization of two types of hexaploids or hexaploids and B. napus, germination of pollen grains and growth of pollen tube was normal, and it was speculated that no barriers exist before fertilization.(3) Preliminary study of heterosis between two types of hexaploidsHeterosis test through randomized block design of hexaploids and hybrids between hexaploids were performed in2012. Through the field observation during seedling, buds and flowering periods, it was obvious that the hybrids had stronger yield advantages (for example, more branches, higher and stronger plant) than two parents. However, analisis of seeds per silique showed a certain degree mid-parent heterosis which suggested that reproduction was not too strong. Analysis of seed test indicated that, index has greater influence on yield as plant height and number of primer branches of hybrids exhibited stronger over-parent heterosis, and better than CK ZhongYou821.(4) Identification of resistance against S. sclerotirumBecause the B. oleracea used for allohexaploid resynthesized had been identified as exellent resistance resource against S.sclerotiorum, we attempted to establish a new way to transfer resistance gene from B. oleracea to hexapliod, and then transfer to B. napus. Total of5times identification of resistance against S. sclerotiorum carried out during two years showed that all of the hexaploids and the hybrids between hexaploids and B. napus exhibited high level of resistance against S.sclerotiorum as compared with the control ZhongYou821. At the same time, a significant and positive correlation for disease symptom score were detected between in vivo and in vitro stem inoculation (r=0.93, p＜0.01), while high correlation of resistance against S.sclerotiorum existed between stem and leaf inoculation (r=0.48, p＜0.05).