Selection strategy for developing maize inbred lines with drought and disease resistance in Uganda.
Binaissa, U. Levene Castiano
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Breeding for host plant resistance along with performance traits requires the use of an effective breeding method and selection strategy. This study of a maize population identified efficient selection indices and a selection strategy for maize lines that combines selecting for traits of primary interest such as anthesis-silking interval (as a proxy for drought tolerance), and resistance to both Turcicum leaf blight (TLB) and maize streak virus (MSV) with other characteristics that directly and indirectly influence grain yield. Such a selection strategy and related indices should contribute to reducing the time required for breeding, and saving resources for further development and testing. Testcrosses were generated in season 2011B at Kasese, with single cross (SC) testers from heterotic groups A and B used as males, planted in multiple rows, and pollen-bulked. F3 lines were used as females, and planted ear-to-row in single-row plots. These crosses were then tested in three locations (Serere, Namulonge and Bulindi), in order to evaluate the performance of families in their early-segregating generations. Results for anthesis-silking-interval (ASI) showed a high frequency of F2 plants falling within the range of -3 to 0 days (67.4%), while only 29.2% of F3 families fell in that range. In the inbreds, partial dominance conditioned for low disease pressure of TLB was indicated by 90% of F2 progeny and 81% of F3 progeny scoring resistant. In the F3 families, 63.4% showed resistance to MSV, suggesting both some effect of segregation and a high response of the genotypes to environment. And selection among F2 and F3 progeny of a biparental cross has contributed for improvement of the performance of lines. Significant GxE terms occurred frequently, but the GxE variance components were usually lower in magnitude than the pooled error. Variance components for lines for yield (YLD), number of kernel rows (KR), ear per plant (EPP), Turcicum leaf blight (TLB), maize streak virus (MSV), silking date (SD) and anthesis silking interval (ASI) were larger than LxE interactions. Suggesting that early generation test-crossing could be a good strategy for developing hybrid maize with the desirable traits for yield, Turcicum leaf blight, maize streak virus, and drought tolerance. Results showed high broad sense heritability (BSH) values of 71% for AD and 60% for SD. The relatively high heritability makes the across environment selection for earliness easier on xv the basis of phenotype. Meanwhile, levels of heritability across locations were moderate for ASI (55%), MSV (46%) and TLB (59%), indicating the influence of environment on disease incidence and severity. Nine lines have been identified as the best, based on a consistent ranking correlated with maximum vigour in the hybrids in all four of the indices proposed, using AD, SD, ASI, yield and resistance to TLB and MSV as key components in the selection indices. These nine lines are WL-429-33; WL-429-40, WL-429-49; WL-429-37; WL-429-57; WL-429-34; WL-429-4, WL-429-50 and WL-429-119. The strategy used for the present study was to utilize all the indices developed, applying them from inbred development up through the testcrosses, tracking the best and most consistent genotypes. Recommendations emerging from this study include the use of effective blocking, more testing sites and careful attention to experimental precision to minimise random variation and error variance. Also, since the across-environment mean for total yield and cobs-per-plant indicated low heritability, which was probably due to little genetic variance among the parents used to derive the population, we recommends using diverse crosses to develop breeding populations, and then selecting for yield-per-plant in later generations. In the present study we noted that selecting by the selection index in either the F3 generation or with the F3-testcrosses was the best strategy for identifying the desirable individuals, and may improve the breeder’s consistency in selecting the most promising lines with the best potential for multi-trait combination hybrid (as for MSV and TLB resistance, drought tolerance and high yield). And also noted that the highest gains for selection ( ≥5%) are obtained when the selection is done at the testcross level, especially for traits with a high level of heritability in early stages (such as for MSV, TLB or ASI). In the determination of heterotic group, results led us to conclude that there is no clear evidence supporting these lines belonging to heterotic group B, since most of them had SCA estimates of >1 SE in only one environment (14 lines in Serere) and 1 SE is not a very strong criterion to distinguish them, since 16% of lines (about 10 lines) are expected to have SCA values exceeding 1 SE by chance. However, there was a strong Line x Tester x Environment interaction, indicating the sensitivity to environmental influences of the performance of a specific line with a specific tester