Analysis Of Effects Contributed By Different Donor Parents And Numbers Of Backcross For Maize Inbred Line Improvement

In this study,we used R08 as the recurrent parent and 18 excellent inbreeding lines as the donor parents to improve R08 line through backcross and self-cross.A total of 18 BC₁F₃-derived lines and 18 BC₂F₂-derived lines with relatively diverse genetic background were obtained and were further used to test by incomplete diallel cross,in which R08 improved lines served as the tested group and testing lines were selected form those lines which have crossed with R08 to develop commercialized hybrids.Effects contributed by different donor parents and numbers of backcross on R08 improvement were analyzed by evaluation of disease resistance,combining ability and SSR molecular markers.The main results were summarized as follows:1.The results of improved lines germination rate and disease resistance(resistant to northern leaf blight) showed that significant influence contributed by different donor parents and numbers of backcross on R08 were analyzed by these two traits,the germination rate of 28 out of 36 improved lines was significantly or very significantly higher than that of R08 and 29 lines of them were resistant or high-resistant to northern leaf blight.The results also indicated that the generation of BC₁F₃ generation(backcross once,self-cross twice) was better than BC₂F₂ generation(backcross twice,self-cross once) in terms of the improvement of germination rate and disease resistance(resistant to northern leaf blight) compared with the lines which had the same donor parents but derived from different numbers of backcross.On the contrary,there was a significant difference in aspects of germination rate,disease resistance(resistant to northern leaf blight) in the lines selected from different donor and same times of backcross.Donor parent of 2,7,11,12,13,14,15 and 18 played a larger role in improving R08's field emergence,Donor parent of 2,4,9,10,11,12,14 and 18 played a larger role in improving R08's disease resistance (resistant to northern leaf blight),Therefore,they may be excellent donor parent,but using of these donors at the time should pay attention to grasp the number of backcross.2.The results of improved lines GCA showed that there was some difference in improved lines GCA.From the whole,general combining ability(GCA) of most yield traits in the majority of improved lines traits is no decrease or increase compared with R08. The GCA of main yield traits had no significantly difference among improved progeny derived by one or two cycles of backcross.In contrast,there was a significant difference in GCA of the majority of yield traits in the lines selected from different donor and same times of backcross.Donor parent of 4,5 and 10 are excellent donor parent,which backcross lines yield/plant GCA was not only significantly or very significantly higher than that of R08,but also in most of traits GCA were positive.3.The results of 185 crosses SCA and heterosis showed that the SCA of BC₁F₃ and BC₂F₂ -derived lines crossed combinations had no significant difference,the number of the BC₁F₃ and BC₂F₂ -derived lines which the SCA was significantly or very significantly higher than categories CK was respectively 9 and 7,significantly or very significantly lower than categories CK was respectively 8 and 12.Analysis of the 49 hybrids which were categories superiority over CK 8%,the number of the BC₁F₃ and BC₂F₂ -derived lines crossed combinations showed little difference were respectively 23 and 26.Analysis of the 20 hybrids which were superiority over CK 20%,ES40,975-12 and 18-599 had more possibility to obtain high field hybrids crossed with improved lines,and BC₁F₃ and BC₂F₂ -derived lines appeared respectively 9 and 11 as the male parent.It further verified that the GCA of most yield traits in majority of the lines selected from the improved offspring was roughly the same between BC₁F₃-derived lines and BC₂F₂-derived lines,and maybe appearing lines more excellent than R08.4.The results of SSR mark analysis showed that 40 microsatellite(SSR) loci distributed uniformly on 10 chromosomes of maize.Detected in the 136 alleles,there were 93 alleles appearing in the backcross generations of individuals but lost in R08.The frequency of these alleles appearing in future generations was from 0.065 to 0.889 and the average frequency was 0.284.Furthermore,these alleles appearing in the BC₁F₃ and BC₂F₂ -derived lines showed nearly equal average frequencies were respectively 0.280 and 0.289.Genetic similar factor between the BC₁F₃-derived lines and R08 was from 0.773 to 0.920,with an average of 0.842,and It between the BC₂F₂-derived lines and R08 was from 0.738 to 0.889,with an average of 0.835.So,the genetic variation in the lines which had same donor parents and different numbers of backcross had no significant difference;on the contrary,there was a significant difference in various aspects in the lines selected from different donor and same times of backcross.5.Comprehensive analysis shows that there were some differences of improving effects contributed by different donor parents and numbers of backcross on R08.The results also indicated that the generation of BC₁F₃(backcross once,self-cross twice) was better than BC₂F₂(backcross twice,self-cross once) in terms of the improvement of germination rate and disease resistance(resistant to northern leaf blight) compared with the lines which had same donor parents and different numbers of backcross,while the GCA of most yield traits in majority of the lines selected from the improved offspring was roughly the same,The results of SSR mark analysis showed that the genetic variation in the lines had no significant difference;there was a larger difference in aspects of germination rate, disease resistance(resistant to northern leaf blight),GCA of the majority of yield traits and the results of SSR mark analysis in the lines selected from different donor and same times of backcross;donor parent of 4 and 10 played a larger role in improving R08's seedling emergence,resistant to NLB and GCA of main yield traits.Therefore,these two lines are excellent donor parent.As a result,w4-1 and w10-1 are backcross-improved lines.In conclusion,during the process of backcross for improving maize inbred lines,we could backcross once and put emphases on identification of the target traits as well as GCA evaluation to improve the breeding efficiency on the basis of the suitable donor parent.