1,720,991 research outputs found
The Genetic Structure of a Maize Population: The Role of Dominance
Full text linkThe combined effects of dominance and inbreeding on covariances between relatives are still poorly understood in maize (Zea mays L.) populations. Our objectives were to address the following questions: (i) What is the importance of dominance in a maize synthetic? (ii) How does inbreeding affect the genetic variance among individuals in a maize synthetic. (iii) How do the covariance parameters compare between populations? (iv) How does breeding design impact estimators? We estimated covariance components for inbred relatives in the maize synthetic BSCB1(R)C13. Previous estimates of covariance parameters have been used to explain the ineffectiveness of inbred progeny selection in the stiff-stalk population BS13. We found that the dominance variance was larger than the additive variance for grain yield, whereas the additive variance was larger than the dominance variance for all other traits. Negative estimates of the covariance between additive and homozygous dominance deviations were found for all traits with the exception of traits associated with reproductive maturity, suggesting a negative relationship between inbred and outbred performance. The correlation between genotypic values and breeding values was lower for grain yield than for any other trait. Our results were similar to previous results found in the stiff-stalk maize population BS13, suggesting similarity in structure among populations.This article is published as Wardyn, Brandon M., Jode W. Edwards, and Kendall R. Lamkey. "The genetic structure of a maize population: The role of dominance." Crop science 47, no. 2 (2007): 467-474. doi: 10.2135/cropsci2006.05.0294. Posted with permission.</p
Dissecting the interaction of nitrogen, density, and genetics in maize
Full text linkStudies on economic optimum nitrogen rate (EONR) for corn have largely ignored interactions with plant density and hybrid. Typically, the EONR is determined by fitting a quadratic-plus-plateau model to the observed data. This study provides a novel model of a logistic function of nitrogen where the shape of the function is modified by nitrogen interactions with density. We set up a comprehensive factorial experiment (5 plant densities x 5 nitrogen rate x 4 hybrids x 2 replications x 2 environmental) to generate data to build the new framework. The coupled density by nitrogen surface response models captured 92% of the variability in the observed grain yield with a normalized root mean square error of 7%. Results indicated that EONR response to density was quadratic with variation in response among hybrids. This study showed that increases in plant density increased nitrogen use efficiency (NUE: (Yield at opt N – Yield at zero N)/EONR) from 11 to 37 kg dry matter per kg N applied, and this result was consistent for all hybrids. Plant density increased time to anthesis and anthesis to silking interval by delaying silk emergence across all hybrids. In conclusion, this is the first study that models the interaction of the logistic response of nitrogen that incorporates the effects of density and nitrogen together in a single model.</p
Heterosis, inbreeding depression and genetic divergence in maize
Full text linkHeterosis and inbreeding depression are known to be opposite phenomena that depend on allele frequencies and directional dominance. Heterosis refers to the superiority of the hybrid over its parents by an increase in the mean of crossbred individuals, while inbreeding depression refers to the reduction in the phenotypic mean of a population. Heterosis has been described as a function of the squared of the genetic divergence in allele frequency of the parents (Δ2), and dominance (d), while inbreeding depression depends on d and allele frequencies. We derived a model of heterosis based on genetic divergence in allele frequencies between the parents (Δ), dominance, and inbreeding depression. Similarly, to better understand Δ and inbreeding depression we estimate shared identical by descent (IBD) segments between inbred lines of maize. The main objective was to understand the underlying basis of heterosis and to estimate genetic diversity and progenitor’s genetic contribution based on the amount of shared IBD segments. To describe heterosis, six synthetic maize populations and eight inbred lines were used. Three crosses between synthetic populations, three between synthetic populations and the B129 inbred line, and six between inbred lines were evaluated in nine environments under a modified split-plot design with three replications. For easy deductions, heterosis model was defined under a single-locus two-alleles case and tested using a “goodness-of-fit” test. To estimate IBD segments, a set of 44 ex-PVP lines along with eight key ancestors of maize in the U.S. Corn Belt were selected. Shared IBD segments were identified by using a probabilistic approach based on a Hidden Markov Model (HMM) framework. Genetic diversity between individuals was estimated as 1 minus the kinship coefficient. Genome-wide kinship coefficients were calculated from the posterior probability of the IBD status at each locus. Our results showed that a single-locus two allele model of heterosis was adequate to describe the variation in the mean of each generation and to predict heterosis. Heterosis estimates were significantly higher for crosses involving inbred parent populations, with a limiting value when the parents reach complete inbreeding. Both model and empirical evidence of heterosis shows that population divergence in allele frequency between parents (Δ) is the key driver of heterosis, but that this divergence is achieved when the level of heterozygosis within each population decreases (i.e. increased FST). Therefore, there was a negative relationship between midparent heterosis and inbreeding depression, the latter expected to be high when panmictic populations are used and low when there is no more heterozygosity in the parents. Hence, having a deeper understanding of inbreeding could lead to better predictions of heterosis. For the 44 ex-PVP lines, long IBD segments (>14.5 Mb) were predominant between ex-PVP and key ancestor lines, suggesting a recent inbreeding, originated in less than 15 generations. There was a high genetic diversity between heterotic groups (stiff stalk and non-stiff stalk), with a reduced diversity among lines in the stiff stalk group. Consequently, we found that a small group of ancestors have contributed large proportions of the genome to important PVP lines in the U.S. Corn Belt. Finally, our results provide a high-resolution data analysis that helps in the identification of IBD regions in the genome that could be used in the quantification of the degree of divergence between parents (Δ), constituting a way to identify the best combination of parents to maximize heterosis.</p
Agronomic and phenotypic responses to 75 years of recurrent selection for yield in the Iowa Stiff Stalk synthetic maize population
Full text linkThe plant density at which 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 traits including grain yield, moisture, test weight, stalk and root lodging, flag leaf angle, anthesis-silking interval(ASI), plant height, tassel branch number, and total number of leaves 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 population has indirectly improved adaptation to high plant density. Material from an unselected base population, Iowa Stiff Stalk Synthetic (BSSS), was compared to the most advanced cycles of selection from two 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. Increasing density in advanced populations led to increased yield unlike the yield decrease seen in less advanced populations at high density, indicating an adaptation to high plant density. Increasing density in advanced populations did not increase grain moisture, test weight, or stalk lodging supporting our hypothesis of increased adaptation to high plant density in more advanced populations. Root lodging has remained unchanged.
Advanced populations had reduced ASI. Plant density did not affect flag leaf angle which became more vertical in advanced populations. Increasing plant density in advanced populations increased plant height while not effecting ASI or tassel branch number; supporting our hypothesis of increased adaptation to high plant density.</p
Density response of maize canopy architecture in adapted and unadapted synthetic populations
Full text linkABSTRACT
Since the 1950's, the average maize grain yield, on a per unit area basis, has risen exponentially and without a pause. Associated with this increase have been changes in shoot morphology which permit more light penetration into the canopy. Changes in plant traits including plant height, leaf number, individual leaf area, vertical leaf angle, tassel size and weight, and leaf area density distribution along the main stem have been reported in the literature; however, the response of canopy components to changes in plant density has not been examined in closed populations and at today's densities. The objective of this study was to: (i) analyze canopy traits (leaf angle / leaf area) to determine how canopy architecture has changed; (ii) determine if canopy architecture interacted with density either directly or indirectly. Materials from unselected base populations, Iowa Stiff Stalk Synthetic (BSSS) and Iowa Corn Borer Synthetic no.1, were compared to the most advanced cycles of selection at four locations near Ames, Carroll, Crawfordsville, and Keystone, IA, in 2011.
Populations were compared at six densities ranging from 3.0 to 9.5 plants m-1. Each breeding population by density combination was replicated once at each location and arranged in a split plot design. Increased densities resulted in reduced numbers of total nodes, lower ear height, shorter plant stature, smaller tassels, more upright leaf angles with smaller leaf areas at the top sector of the canopy and more horizontal leaf angles with larger leaf areas lower in the canopy. More importantly, the shape of the canopy was affected by plant height, ear height, node of attachment of the ear, and density.</p
Effects of recurrent selection for yield on plant growth across planting densities in maize (Zea mays L.)
Full text linkBreeding for higher grain yield in maize (Zea mays L.), utilizing increased selection densities, has produced varieties that are adapted to grow at higher population densities. The effects of increased planting density on grain yield and final phenotypes are well known, but the effects of density on plant growth across the growing season have been less widely characterized. The objectives of this study were: 1) examine the effects of high planting density on growth rates, growth timing, and biomass partitioning of the ear, stalk, and tassel; 2) characterize the difference in density effects in maize populations before and after recurrent selection for grain yield; 3) characterize heterosis and hybrid performance in plant growth and biomass partitioning; 4) characterize the effects of recurrent selection on heterosis levels in plant growth. Four populations, Iowa Stiff Stalk Synthetic (BSSS), Iowa Synthetic Corn Borer #1 (BSCB1), and the populations derived from the 17th cycle of reciprocal recurrent selection, BSSS(R)C17 and BSCB1(R)C17, along with the BSSS/BSCB1 and BSSS(R)C17/BSCB1(R)C17 population crosses were utilized in this study. The populations were growth at 3.23, 6.46, 9.69, and 12.92 plants m-2 at six locations near Ames, IA over 4 years. Increased density lowered maximum growth rates for all plant organs, but reduction in ear length, plant height, and ear and stalk biomass growth rates occurred at higher densities in the selected populations and population crosses compared to the unselected populations. Increased planting density affected plant organ growth timing differently in BSSS and BSCB1. High density delayed stalk biomass accumulation in BSCB1 but not BSSS, and delayed ear length growth and ear and tassel biomass accumulation in BSSS but not BSCB1. Growth delays the cycle 0 populations were not present in the cycle 17 populations, or in the cycle 0 population cross. BSCB1 and the BSSS/BSCB1 population cross partitioned lower levels of biomass to the ear and had smaller harvest indices at high density, while biomass partitioning and harvest index were not affected by density in the selected population and population cross. Heterosis was present in final phenotypes, growth midpoints, growing period length, and maximum growth rates. Heterosis levels for final phenotypes and maximum growth rates increased with selection. Increases in heterosis levels were due to depressed per se population performance and slight increases in selected population cross phenotypes. BSCB1 was often the dominant parent in population crosses in regards to growth midpoint timing, maximum growth rates, harvest indices, and density response.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Predicted Genetic Gain and Inbreeding Depression with General Inbreeding Levels in Selection Candidates and Offspring
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