Iowa Stiff Stalk Synthetic Maize Research Articles

Genome-wide association analysis of plant architecture traits using doubled haploid lines derived from different cycles of the Iowa Stiff Stalk Synthetic maize population.

Selection in the Iowa Stiff Stalk Synthetic (BSSS) maize population for high yield, grain moisture, and root and stalk lodging has indirectly modified plant architecture traits that are important for adaptation to high plant density. In this study, we developed doubled haploid (DH) lines from the BSSS maize population in the earliest cycle of recurrent selection (BSSS), cycle 17 of reciprocal recurrent selection, [BSSS(R)17] and the cross between the two cycles [BSSS/BSSS(R)C17]. We aimed to determine the phenotypic variation and changes in agronomic traits that have occurred through the recurrent selection program in this population and to identify genes or regions in the genome associated with the plant architecture changes observed in the different cycles of selection. We conducted a per se evaluation of DH lines focusing on high heritability traits important for adaptation to high planting density and grain yield. Trends for reducing flowering time, anthesis-silking interval, ear height, and the number of primary tassel branches in BSSS(R)17 DH lines compared to BSSS and BSSS/BSSS(R)C17 DH lines were observed. Additionally, the BSSS(R)C17 DH lines showed more upright flag leaf angles. Using the entire panel of DH lines increased the number of SNP markers identified within candidate genes associated with plant architecture traits. The genomic regions identified for plant architecture traits in this study may help to elucidate the genetic basis of these traits and facilitate future work about marker-assisted selection or map-based cloning in maize breeding programs.

Selection and Adaptation to High Plant Density in the Iowa Stiff Stalk Synthetic Maize (Zea mays L.) Population: II. Plant Morphology

ABSTRACTThe plant density at which maize (Zea mays L.) hybrids achieve maximum grain yield has increased throughout the hybrid era while grain yield on a per plant basis has increased little. Changes in plant characteristics including flag leaf angle, anthesis‐silking interval, plant height, tassel branch number, and total number of leaves have been characterized in comparisons of commercial hybrids representing different eras of hybrid maize production but have not been studied extensively in a temperate maize recurrent selection program. The objective of this experiment was to determine if direct selection for grain yield and agronomic performance in the Iowa Stiff Stalk Synthetic (BSSS) population has indirectly improved adaptation to high plant density. Material from an unselected base population, BSSS per se, was compared to four selected populations per se at four plant densities ranging from 38,300, to 95,700 plants ha−1. Treatments were replicated twice in each of two locations in 2 yr and arranged in a split plot design with plant density as whole plot and breeding population as subplot treatment. Selected populations had reduced anthesis‐silking interval and fewer tassel branches. Increasing plant density increased anthesis‐silking interval in the base population whereas increasing plant density did not increase anthesis‐silking interval in the most advanced populations. These results demonstrated that selected populations were better adapted to high plant density than the unselected base population.

Changes in Plant Morphology in Response to Recurrent Selection in the Iowa Stiff Stalk Synthetic Maize Population

ABSTRACTThe maize (Zea mays L.) plant phenotype has changed a great deal through the era of hybrid maize production. Some of the observed changes such as upright leaf angle, silking‐anthesis interval, and tassel branch number have well understood contributions to improved grain yield in modern hybrids. However, less is known formally about indirect selection responses for these traits in the context of recurrent selection programs. The objective of this study was to determine if recurrent selection for agronomic performance in Iowa Stiff Stalk Synthetic (BSSS) population has changed important plant traits. Thirty synthetic populations representing a total of 29 cycles of recurrent selection in three programs in BSSS were evaluated in four Iowa locations in 2008 and 2009. The most consistent changes observed across selection programs were for traits that increase light penetration into the canopy, including flag leaf angle, flag leaf size, and tassel branch number. Light‐interception traits had more consistent responses in the populations per se evaluated here than agronomic traits selected for on a testcross basis in the recurrent selection programs. Reductions in plant stature and anthesis‐silking interval were also observed, but changes were not as consistent as light‐interception traits. Selection responses for morphological traits in populations per se suggested these traits may have much simpler inheritance than typically assumed for grain yield.

Selection and Adaptation to High Plant Density in the Iowa Stiff Stalk Synthetic Maize (Zea mays L.) Population

The plant density at which maize (Zea mays L.) hybrids achieve maximum grain yield has increased throughout the hybrid era, whereas grain yield on a per plant basis has increased little. Changes in grain yield, grain moisture, test weight, and stalk and root lodging have been well characterized in comparisons of commercial hybrids representing different eras of hybrid maize production but have yet to be examined in a recurrent selection program. The objective of this experiment was to determine if direct selection for grain yield and agronomic performance in the Iowa Stiff Stalk Synthetic (BSSS) population has indirectly improved adaptation to high plant density. Material from an unselected base population, BSSS, was compared to the most advanced cycles of selection from three different recurrent selection programs at seven Iowa locations in 2008 and 2009. Populations were compared at densities of 38,300, 57,400, 77,500, and 95,700 plants ha−1. Treatments were replicated twice at each location and arranged in a split plot design. Selection produced notable improvement in adaptation to high plant density for grain yield and stalk lodging. Selection did not have large impacts on density response for root lodging, test weight, or grain moisture. Because of the improvement in density response for grain yield and stalk lodging without major negative changes in other traits, our data suggest that selection has improved adaptation to high plant density in these populations.

Repeatability and heritability of divergent recombination frequencies in the Iowa Stiff Stalk Synthetic (Zea mays L.)

Variability in recombination frequency was reported in the Iowa Stiff Stalk Synthetic. The objectives of the present research were to verify the differences in recombination frequency among individuals in the Iowa Stiff Stalk Synthetic maize population and to determine if the recombination frequency differences persisted among the S1 progeny. Testcrosses to measure male recombination frequency on three chromosomes (4, su1-c2; 5, a2-bt1-pr1; 9, sh1-bz1-wx1) were repeated for eight S0 individuals. Recombination frequencies were repeatably divergent among those individuals which were selected based on high or low recombination frequencies on specific chromosomes. Individuals which had been selected for long and short total map distances across the three chromosome regions produced repeatably divergent recombination frequencies only at the su1-c2 region. The recombination frequencies of the S1 lines, derived from the S0 individuals which had the most divergent recombination frequencies on a single chromosome, were significantly different. The broadsense heritability estimates derived from the regression of six S1 lines on six S0 individuals ranged from 0.69 to 0.20 for the five chromosome regions. We conclude that genetic differences for recombination frequency exist in this population and that modification by selection should be possible.

Population Means and Genetic Variances in Selected and Unselected Iowa Stiff Stalk Synthetic Maize Populations

Recurrent selection is a cyclical breeding procedure that focuses on improving the mean performance of a population by increasing the frequency of favorable alleles while maintaining adequate genetic variability for continued selection response. Iowa Stiff Stalk Synthetic (BSSS) is a maize (Zea mays L.) population that has undergone continuous recurrent selection for more than 50 yr as the base population for two independent selection programs (intra‐ and inter‐population). This study was designed to estimate the mean performance and genetic variances in BSSS after seven cycles of half‐sib (HS) progeny selection, six cycles of S2‐progeny selection, and 11 cycles of reciprocal recurrent selection (RRS). A Design II (factorial) mating design was constructed to give direct estimates of additive and dominance variance in the individual populations. Fourteen sets of four male × four female matings for each of the four populations were evaluated in a randomized incomplete block experiment grown in six environments. Half‐sib progeny and RRS methods have produced the most effective mean performance responses for grain yield in the populations per se, 0.076 and 0.104 Mg ha−1 cycle−1, respectively. The S2‐progeny selection procedure in BSSS, with a response of 0.052 Mg ha−1 cycle−1, has not performed up to theoretical expectations. Genetic variance component estimates for grain yield supported the suggestion that dominance variance is important in BSSS germplasm. Although additive variance decreased slightly, high heritability estimates suggest that further improvement in population means should be achieved by each selection method. Mean performance and estimated genetic variability for other important agronomic traits (grain moisture, root and stalk lodging, and ear height) generally showed favorable response to selection.

Performance and Genetic Variance among S1 Lines and Testcrosses of Iowa Stiff Stalk Synthetic Maize

Iowa Stiff Stalk Synthetic (BSSSC0) maize (Zea mays L.) population has been used in two long‐term recurrent selection programs: reciprocal recurrent selection (R) and half‐sib followed by S2‐progeny (S) recurrent selection. Advanced populations after nine cycles of reciprocals recurrent selection [BSSS(R)C9] and seven cycles of half‐sib and three cycels of S2 progeny selection [BS13(S)C3] were evaluated. Objectives of this study were to compare performance and amount of genetic variation present among S1 lines from BSSSC0, BSSS(R)C9, BS13(S)C3, and BSSS(R)C9 ✕ BS13(S)C3(C3 ✕ C9) and, also, to compare performance and genetic variation among testcrosses of the S1 lines by using two testers of different heterotic background, B73 and Mo17. The S1 lines from BSSS(S)C9 and BS13(S)C3 showed significant increases in grain yield compared with S1 lines from BSSSC0. The C3 ✕ C9 S1 lines showed high‐parent heterosis for grain yield. Genetic variance component estimates of S1 lines were significant in all four populations, and C3 ✕ C9 reflected midparent estimates between the estimates for C3 and C9. Mean grain yield for Mo17 testcrosses was significantly greater than for B73 testcrosses within each of the four populations. Testcrosses involving C3 and a related inbred, B73, showed a nonsignificant grain yield variance estimate. The average yield for the testcrosses of S1 lines from C3 ✕ C9 for both testers was intermediate between the testcrosses of S1 lines from C3 and C9; thus, the high‐parent heterosis observed for C3 ✕ C9 S1 lines was not observed for the testcross means.

Inheritance of photosynthesis in a diallel among eight maize inbred lines from Iowa Stiff Stalk Synthetic

Photosynthesis is a trait that should be improved in a selection program for yield potential of maize (Zea mays L.). We measured leaf CO2-exchange rate (CER), an estimate of photosynthetic efficiency, of a complete diallel (parents, F1 crosses, and their reciprocals) among eight inbred lines (4 with low and 4 with high CER) from the Iowa Stiff Stalk Synthetic maize population. Measurements were made during vegetative (CER 1) and grain filling (CER 2) stages of growth, and the experiment was conducted two years at one location. We measured large differences among crosses and significant heterosis for high CER at CER 1 (0.0 to 25.1%) and CER 2 (0.0 to 53.8%). Several crosses exhibited overdominant phenotypes for high CER at both growth stages, and one cross showed significant overdominance for low CER at CER 1. General combining ability effects (gca) were the largest components of among-cross variation at both CER 1 and CER 2. Specific combining ability (sca) also was significant at both growth stages, but gca effects were 9.4 and 4.8 times larger than sca effects at CER 1 and CER 2, respectively. Furthermore, high CER lines showed positive gca effects, and low CER lines showed negative gca effects at each stage. Maternal and reciprocal effects were not significant; thus, CER in these crosses was controlled largely by additive effects of nuclear genes. A high positive genotypic correlation (r=0.74) between CER 1 and CER 2 suggested that selection at either growth stage would improve CER throughout the growing season.

Variability and Selection Advance for Photosynthesis in Iowa Stiff Stalk Synthetic Maize Population1

Photosynthesis is a primary trait influencing yield potential, and it should be improved in a yield‐potential selection program. Information concerning the genotypic variability and inheritance of photosynthetic efficiency in maize (Zea mays L.) must be obtained before a yield‐potential selection program is initiated.We used CO2‐exchange rate (CER) of sections from excised leaves to estimate photosynthetic efficiency of 64 random inbred fines of maize derived from Iowa Stiff Stalk Synthetic (BSSS). Our study was conducted for years at one location. Measurements of CER taken during vegetative growth (CER 1) and during grain filling (CER 2) were used to estimate genotypic variances and heritabilities and to calculate selection advances for these two stages of plant development. We also evaluated the possibility of using specific leaf weight and leaf thickness as index traits for CER selection.Results showed that estimates of additive variance were 1.7 and 4.9 times larger than estimates of genotype ✕ environment variance at CER 1 and CER 2, respectively. Narrow‐sense heritabilities were 0.58 for CER l and 0.80 for CER 2, and realized heritability values were 0.72 (CER 1) and 0.66 (CER 2). A high genotypic correlation between CER at CER 1 and CER 2 indicated selection at either developmental stage would produce nearly identical results. Actual selection advances (15% selection intensity) represented increases of 14 and 22% above population means for CER 1 and CER 2, respectively. Our data suggested that measurement of two replications of four plants/plot grown in two environments would provide most efficient selection advance for inbreds. We concluded that selection for improved CER in BSSS would be possible and that selection must be for CER per se.