Comparative Study Of Genetic Variation And Combining Ability On S₄ And S₆ Lines In Maize

Maize is the earliest crop to use heterosis in production.In the breeding process of hybrids,combining ability and genetic variation are the important basis for parent selection.To study the genetic variation and combining ability effect of lines in different generations,and clarify their change laws between different generations,which would be great significance for improving the efficiency of maize breeding.In this study,50 S₄ lines and42 S₆ lines were derived from the elite hybrid DK008F2 population,which used as materials to study genetic variation by SSR markers.Through incomplete diallel design with two elite inbred lines as testers,the combining ability and heterosis of combinations in different generations and their correlations were analyzed to provide a theoretical basis for the breeding of maize inbred lines and hybrids.The main results were as follows:1.The results of SSR-PCR amplification by using 40 pairs of SSR primers,68 and 65genotypes,72 and 77 alleles were detected in 50 S₄ lines and 42 S₆ lines respectively.As the genetic similarity coefficient was 0.89,S₄ lines and S₆ lines and testers were classified into 7 and 5 distinct groups respectively by using UPGMA cluster method.The clustering results were basically consistent with the pedigree sources.2.The genetic variation analysis showed that,the SSR genetic distances and difference allele of S₄ lines derived from the same S₂ lines or S₆ lines derived from the same S₄ lines were less than that of those from different S₂ lines or S₄ lines.The coefficient of variation of plant height and ear height in most S₆ lines were lower than that of S₄ lines.The difference allele of"1"accounted for 8%in S₄ lines,while that of"1"or"0"accounted for 61.9%in S₆ lines.Indicated that the genetic variation among the lines gradually became smaller with the increase of generations,and began to homozygous from the S₄ lines,most of S₆ lines may tend to be homozygous.Therefore,in order to maintain genetic diversity of the population,we should selected more superior individuals to selfing at S₂ generation,while the number of that might be reduced gradually after S₄ generation.3.The variance analysis of the combining ability showed that,there were significant differences of GCA of 12 and 11 traits,and SCA of 8 and 6 traits including yield in S₄ and S₆ lines respectively;The GCA or SCA of plant height,ear length,yield or other traits were significantly interacted with location.The experiment results in two locations showed that,S₄ lines from D15 and S₆ lines from D1520 and D1528 have higher positive yield GCA;Excepted that the yield SCA of combinations with D1540-derived S₆ lines and K305 were less than that of D1207,the mean yield SCA and positive effect ratio of combinations with other D15-derived S₄ and S₆ lines and 2 testers were highter than that of D12.Therefore,through continuous determination of the combining ability in the early and middle generations by using the same tester in breeding practice,the synchronous directional breeding of inbred lines and hybrids can be achieved,thereby improving the breeding efficiency.4.According to experimet results of heterosis in two locations,the yield heterosis of S₄ combinations derived from D15 were higher than that of D12;except D1540,the average yield heterosis and positive heterosis rate of the combinations crossed other D15-derived S6 lines with the same testers were higher than that of the D12-derived.Compared with Chengdan 30,the yields of S₆ combinations K305×D152824,K305×D152823 and K365×D152031 were increased 21.26%,21.74%and 24%respectively,which had taken part in new varieties screening test of provincial levels.Which means that heterosis is relatively consistent among generations.Therefore,it is effective to select superior lines and combinations in early segregating generations,which is of important significance for accelerating the breeding process of hybrids.5.Correlation analysis shows that,SSR genetic distance between K305 and S₆ lines had significantly negative correlation with yield SCA and yield heterosis of the combination respectively,while that of those between other lines and 2 testers were not significant.For the combinations of different testers crossed with S₄ and S₆ lines,the correlation coefficient between yield heterosis and yield GCA,yield SCA,and yield TCA reached significant levels.Therefore,the SSR genetic distance between parents is not sufficient to predict the yield heterosis and yield SCA,however,selection of the high GCA lines in the segregating generations,is expected to breed the strong heterosis combinations with high SCA.