A Study On The Genetic Origin Of Apomixis Boehmeria Tricuspis

Genetic study of apomixis has drawn great attention because of its importance in crop breeding. Two apomixis type and a sexual type from B.tricuspis were studied. In order to analyse the genetic origin of apomixis type, some cytogenics and molecular biologic methods were used.The chromosome number of the two apomixis type are 2n=42(triploid) and 2n =28(diploid), while the sexual type is 2n=28. Their karyotype are all the 4A type. The symmetry of their karyotype are: sexual type > diploid apomixis type > triploid apomixis type. This suggests their positions in evolution(low→high): sexual type→diploid apomixis type→triploid apomixis type.There is normal meiosis action to most PMCs in sexual type, while abnormal meiosis action to a few PMCs. Some may result in 2n male gametes. To triploid apomixis type, a lot of trivalents were observed in PMCs, suggest that the triploid apomixis type is auto-triploid.Analysis based on RAPD data showed these 3 types had very high similar coefficiency: 0.957 between sexual type and diploid apomixis type, 0.881 between 2 apomixis types, and 0.864 between sexual type and triploid apomixis type. They were classified into a group and the triploid apomixis type was at the highest position.So it can be concluded that diploid apomixis type is originated from the sexual type by mutation, while the triploid apomixis type is autoploidy. The triploid type might origin in two ways: 2n gametophyte of diploid apomixis types fused with the male gametophyte (2n+n) from sexual type;2n gametophyte of sexual type fused with the n gametophyte of itself (2n+n) .All the studies showed that the 3 types of B.tricuspis almost had the same genetic background and could be ideal materials for apomixis study.