| The maize inbred line R08 was an elite line with good combining ability, multiple resistance to diseases, big ear and extensive adaption in southwest of China. More than twenty varieties were released from R08 for maize production. However, the elite line would be susceptible to maize Northern corn leaf blight (NCLB) at some seasons with low germination rate. To improve the two disadvantages, eighteen excellent inbreeding lines were used as the donor parents to make a cross with R08, the recurrent parent, to develop backcross inbred lines. In 2008, a total of 18 BC1F3-derived lines and 18 BC2F2-derived lines with relatively diverse genetic background were obtained. These improved lines were further used as the tested group to test by incomplete diallel cross with the testing line selected from those lines which have crossed with R08 to develop commercialized hybrids. The evaluation of genetic diversity of SSRs, disease resistance to NCBL, germination rate, and combining ability of main agronomic phenotypes were determined by different donor parents and numbers of backcross on R08 improvement. In the autumn of 2009, the assessment of disease resistance to NCBL and germination rate was further investigated according to high generation of BC1F5 and BC2F4 obtained by proceeding self-cross. The main results were summarized as follows:1. A total of 136 discernible bands were obtained according to forty core SSRs, which equally distributed in maize chromosomes.93 were present in the BC-derived lines and absent in the recurrent parent, R08. These fragments occurred at frequencies varying from 0.065 to 0.889, with average of 0.284, and the bands average frequencies occurred in BC2F2 (0.289) were almost the same as occurred in BC1F3 (0.280). The BC-derived lines from different source populations with the recurrent parent, R08 had the averaged GS estimate varying from 0.773 to 0.920 with average of 0.842 for BC1F3 and 0.738 to 0.899 with average of 0.835 for BC2F2. These results indicated that indiscrimination variation was found in generation from different numbers of backcrosses and more genetic diversity was observed among lines derived from different donor parents.On the other hand, the ability of distinguishing donors and receptor for 4 SSR markers linked with Ht2 and Ht3 was varied. There was no specific amplifications were found in resistance or susceptible improved backcross lines for all of the primers. The practicability of these markers assisted selection was required to be further investigated.2. The anlysis of disease resistance to NCBL showed that significant effect was contributed by different donor parents, and phenotypes were varied to a certain extent between low and high generations from backcross different numbers. Twenty-nine out of 36 BC1F3 and BC2F2 lines were resistant or high-resistant to NCBL, and 16 out of 26 BC1F5 and BC2F4 lines were mid-resistance to NCBL. The comparave analysis of different backcrosses derived from same donor indicated that the improvement effect of resistance to NCBL for backcross once self-cross twice (BC1F3) generation was better than backcross twice self-cross once (BC2F2) generation; and no significant difference was found in high generation from BC1F5 and BC2F4 lines. In addition, significant variance was found among lines from different donors and same backcrosses in term of resistance to NCBL. The performance of lines from low-and high-generation backcross revealed that Luyuan92, Chuan 321,313, Shuang741,835, and Qi 319 were better donors in term of improvemen resistance to NCBL for R08, but the numbers of backcross should be proper.3. The anlysis of field generation rate showed that significant variance was contributed by both different donors and different backcross numbers. Twenty-eight out of 36 BC1F3 and BC2F2 lines were significantly or very significantly higher than R08, and 10 out of 26 BC1F5 and BC2F4 lines were significantly or very significantly higher than R08. The comparave analysis of different backcrosses derived from same donor indicated that the improvement effect of generation rate for backcross once self-cross twice (BC1F3) generation was better than backcross twice self-cross once (BC2F2) generation; but reversed in high generation of BC1F5 and BC2F4 lines. In addition, significant variance was also observed among lines from different donors and same backcrosses generation. The performance of lines from low-and high-generation backcross revealed that Shen137, Qi 319, Shuang741, Huangyesi,313, CAL73, Huotanghuang and K14 might be better donor parents for improvemen generation rate of R08.4. The general combining ability (GCA) analysis showed that the performances of improved lines from different donors and different backcross numbers were varied in GCA. As a whole, the GCA of most quantitative agricultural traits of improved lines was no decrease or increase compared with recurrent parent, R08. The GCA of main yield traits had no significantly difference between BC1F3 and BC2F2 from same donor and different backcross. In contrast, there was a significant variance in GCA of the majority yield traits in the lines selected from different donor and same backcross generation. The performance of improved lines from donor parents of Chuan 321, Chang 7-2 and and K14 were excellent, which yield GCA per plant of backcross lines was not only significantly or very significantly higher than that of R08, but also most of were positive.5. The analysis of specific combining ability (SCA) and heterosis showed that the number of the BC1F3 and BC2F2 derived lines which SCA was significantly or very significantly higher than those categories CK was 9 and 7, respectively, and significantly or very significantly lower than those categories CK was respectively 8 and 12. The results indicated that there was no significant difference in SCA of combinations from BC1F3 and BC2F2 with test lines. 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 23 and 26, respectively. The analysis of consolidate superiority over CK showed that high yield combination would be most occurred in the crosses between improved lines and test lines ES40,975-12 and 18-599. In term of donors for superior combinations, there were 9 and 11 for BC1F3 and BC2F2 lines, respectively. It further validated that there was no distinctive GCA for most quantitative yield traits in improved backcross lines, and more excellent individuals than R08 would be found in BC1F3 and BC2F2 lines.6. Comprehensive analysis showed that the effects of backcross improvement by different donors and backcross numbers on R08 were varied, while the GCA of most yield traits for backcross once (BC1F3) and twice (BC2F2) generation was roughly the same. What more, the results of SSR analysis showed that there was no significant genetic diversity between the two generation lines. In terms of disease resistance to NCBL and germination rate for low generation (BC1F3, BC2F2) and high generation (BC1F5, BC2F4), heterozygossity effect might be in action and had a greater impact on the generation rate especially. The majority diversity was contributed by the lines derived from different donors through analysis of SSRs, resistance to NCBL, germination rate, and GCA of most yield traits. Two excellent donor parents, Chuan321 and Chang7-2 were distinguished in combination analysis of resistance to NCBL, germination rate, and GCA, which offspring would be elite backcross improved lines with favorable characters. 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 during the low and high-generation as well as GCA evaluation to improve the breeding efficiency on the basis of the suitable donor parent.
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