| The improvement and innovation of germplasm resources is an important topic in current maize breeding.Backcrossing can quickly introduce genes that control other good traits into the recurrent parent,which is one of the effective methods for polymerizing elite genes and increasing breeding efficiency.In this study,maize inbred lines K11 and K62 were used as recurrent parents,36 improved lines were bred using 3 excellent inbred lines respectively as donors by different backcross times,and the 114 combinations obtained by 3×38 incomplete diallel model with 3 backbone lines as testers,were used as materials to study the main agronomic traits and genetic variations by molecular markers,combining ability and yield heterosis of the improved lines,and to compare the improvement effects of different donors and backcross times.The objective is to clarify the application potential of the improved lines in breeding practice.The main results are as follows:1.Analyses of main trait variations showed that there were differences in the improvement effects of different donors and backcross times on recurrent parents K11 and K62.Donor A2,A3,B1 and B2 have significant effects on reducing plant height,ear height and leaf number,but A2 and B3 have effects of increasing tassel branches;donors A1,A2 and A3 can effectively increase ear rows and Kernels per row,and reduce the barren ear tip length,thereby increasing the yield.Although B1,B2 and B3 could significantly increase the ear length,but increase the barren ear tip length also,thus the improvement of yield was not significant.For the above traits,the differences of improvement effects are not obvious among backcrossing 1 to 5 times,hence the backcrossing may be done 1 to 2 times in the breeding practice.2.The studies by using molecular markers showed that,81 genotypes were detected with40 pairs of SSR primers in 2 recurrent parents and 36 improved lines,with an average of2.025 per pair of primers,and a total of 99 alleles were detected,with an average of 2.475.The genetic similarity coefficients between the recurrent parents and its improved lines increased with the increase of backcross times.The testers and improved lines were divided into 8 groups by unweighted pair-group method with arithmetic means(UPGMA)with genetic similarity coefficient 0.86 as threshold,and the clustering results were basicallyconsistent with the pedigree sources.The average difference loci between K11 and improved lines derived from donors A1,A2,and A3 were 6.0,9.3,and 10.2 respectively,while that of K62 and improved lines from B1,B2,and B3 were 7.3,5.3,and 1.7 respectively.After backcrossing 0 to 5 times,the average difference loci between improved lines and 2corresponding recurrent parents were 20.7 to 1.3 and 13.3 to 0 respectively.The average different loci decreased with the increase of backcrossing time,and more than 4 backcross times the different loci between the improved lines and recurrent parents are 0 to 2,which can be judged to be the same or similar.Therefore,in the breeding practice,the backcross times should be flexibly determined according to the improvement objectives.3.The results of combined analysis of variance of combining ability showed that the differences of GCAs of all 13 detected traits including yield reached a significant level,and the SCAs of 6 traits including plant height reached a significant level.The interaction of GCAs of 5 traits and SCAs of 9 traits including yield with the locations reached a very significant level,which indicates that combining ability of these traits are greatly affected by the environments.The effects of different donors and backcross times on the combining abilities of different traits for recurrent parents were quite different.In the two locations,the numbers of yield GCAs of the improved lines derived from donor A3,B1 and B3 positively higher than that of recurrent parents,were 8,12 and 5 respectively,which indicates that the improvement effects of these three donors on yield GCA are significant.with the increase of backcrossing times,the average yield GCA effect values of the improved line tends to be consistent with the recurrent parents,and the numbers of positive and significant improved lines with backcrossing 0 to 5 times are 6,7,6,5,6 and 6 respectively,which indicates that the effects of different backcross times on yield GCA is not obvious different,hence the backcrossing may be done 1 to 2 times in the breeding programs.4.Analysis of yield heterosis showed that,the numbers of yield increased combinations derived from improved lines compared with that of corresponding recurrent parent(classified CK)were 68 and 54 in the trials of Tieling and Xingyang respectively,accounting for 59.6%and 47.4% of the total combinations respectively,and the increase rates were 0.19% to27.70% and 0.08% to 22.43% respectively.Among them,donors A3 and B1 had the best improvement effects,and the numbers of positively significant improved combinations wereup to 20 in 2 locations,followed by A2 and B3;the numbers of positively significant improved combinations with backcrossing 0 and 1 times were 18 and 17 respectively,and their improvement effects are better.The numbers of yield increased combinations derived from improved lines compared with Zhengdan 958(unified CK)were 65 and 8 in 2 locations respectively,accounting for 57.0% and 7.0% of the total combinations respectively,and the increase rate were 0.83% to 16.53% and 0.57% to 10.09% respectively.Among them,donors A3,B1 and B3 had significant improvement effects,the numbers of positively significant improved combinations were 10,23 and 16 respectively in 2 locations,which had good effects on yield heterosis;the numbers of positive and significant combinations with backcrossing 0 and 1 times were 14 and 11 respectively,which is basically the same as the improvement effects on heterosis compared with classified CK.The yield of K324×B1-0increased by 17.59% and 10.09% in Tieling and Xingyang trials respectively,and that of 11 combinations including K128×B3-1 increased more than 10% compared with Zhengdan 958 at Tieling location,reaching a significant level.Those combinations are recommended to higher level trials.5.Comprehensive analysis showed that,compared with the corresponding recurrent parents,the different SSR loci of improved lines A3-1,B1-1,B1-2 and B3-1 were more than 4,and the yield increased significantly or were equivalent,and the plant height and ear height were significantly decreased,and their yield GCA effect values increased significantly,which are the better parents for high yield breeding.The negative GCA effect values of plant height and ear height of A2-3 and B2-2 decreased significantly,which can be used as potential parents for dwarf breeding.The positive GCA effect values of 100-kernel weight and kernel depth of B1-0 was significantly increased,which is an excellent material for improving kernel traits. |
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