Hybrid maize breeding with doubled haploids: I. One-stage versus two-stage selection for testcross performance

Optimum allocation of resources is of fundamental importance for the efficiency of breeding programs. The objectives of our study were to (1) determine the optimum allocation for the number of lines and test locations in hybrid maize breeding with doubled haploids (DHs) regarding two optimization criteria, the selection gain ΔG _k and the probability P _k of identifying superior genotypes, (2) compare both optimization criteria including their standard deviations (SDs), and (3) investigate the influence of production costs of DHs on the optimum allocation. For different budgets, number of finally selected lines, ratios of variance components, and production costs of DHs, the optimum allocation of test resources under one- and two-stage selection for testcross performance with a given tester was determined by using Monte Carlo simulations. In one-stage selection, lines are tested in field trials in a single year. In two-stage selection, optimum allocation of resources involves evaluation of (1) a large number of lines in a small number of test locations in the first year and (2) a small number of the selected superior lines in a large number of test locations in the second year, thereby maximizing both optimization criteria. Furthermore, to have a realistic chance of identifying a superior genotype, the probability P _k of identifying superior genotypes should be greater than 75%. For budgets between 200 and 5,000 field plot equivalents, P _k > 75% was reached only for genotypes belonging to the best 5% of the population. As the optimum allocation for P _k (5%) was similar to that for ΔG _k , the choice of the optimization criterion was not crucial. The production costs of DHs had only a minor effect on the optimum number of locations and on values of the optimization criteria. C. Friedrich H. Longin and H. Friedrich Utz contributed equally to this work.     

A general approach for the study of a population of testcross progenies and consequences for recurrent selection

Summary A model to study genetic effects at the level of a population of testcross progenies is presented. As there is no dominance for the testcross value, with the restriction of epistasis to pairs of loci, only additive x additive epistasis can contribute to the variance among progenies. To estimate the variance among progenies due to epistasis, it is necessary to have the population structured in families of full sibs, half sibs or S₁, with only a few plants per family tested in combination with the tester. Using a two-way mating design to produce the families, it is possible to estimate the variance due to additive x additive epistasis. The consequence of the presence of epistasis is studied at the level of recurrent selection for combining ability with the tester. It seems that epistasis itself does not change the efficiency of the breeding methods considered. However, when the population from intercrossing is structured in families, it could be efficient to use a combined selection when the heritability is very low. In this case it would be efficient to produce full-sib families (by single-pair matings) at the level of intercrossing. The best procedure is to produce such families at the same time as crossing with the tester. In comparison to the classical scheme of selection for combining ability with a tester, such a modification increases the efficiency of selection 41.1% if an off-season generation can be used.     

Hybrid maize breeding with doubled haploids: III. Efficiency of early testing prior to doubled haploid production in two-stage selection for testcross performance

Early testing prior to doubled haploid (DH) production is a promising approach in hybrid maize breeding. We (1) determined the optimum allocation of the number of S₁ families, DH lines, and test locations for two different breeding schemes, (2) compared the maximum selection gain achievable under both breeding schemes, and (3) investigated limitations in the current method of DH production. Selection gain was calculated by numerical integration in two-stage breeding schemes with evaluation of testcross progenies of (1) DH lines in both stages (DHTC), or (2) S₁ families in the first and DH lines within S₁ families in the second stage (S₁TC-DHTC). Different assumptions were made regarding the budget, variance components, and time of DH production within S₁ families. Maximum selection gain in S₁TC-DHTC was about 10% larger than in DHTC, indicating the large potential of early testing prior to DH production. The optimum allocation of test resources in S₁TC-DHTC involved similar numbers of test locations and test candidates in both stages resulting in a large optimum number of S₁ families in the first stage and DH lines within the best two S₁ families in the second stage. The longer cycle length of S₁TC-DHTC can be compensated by haploid induction of individual S₁ plants instead of S₁ families. However, this reduces selection gain largely due to the current limitations in the DH technique. Substantial increases in haploid induction and chromosome doubling rates as well as reduction in costs of DH production would allow early testing of S₁ lines and subsequent production and testing of DH lines in a breeding scheme that combines high selection gain with a short cycle length. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hybrid maize breeding with doubled haploids. IV. Number versus size of crosses and importance of parental selection in two-stage selection for testcross performance

Parental selection influences the gain from selection and the optimum allocation of test resources in breeding programs. We compared two hybrid maize (Zea mays L.) breeding schemes with evaluation of testcross progenies: (a) doubled haploid (DH) lines in both stages (DHTC) and (b) S₁ families in the first stage and DH lines within S₁ families in the second stage (S₁TC-DHTC). Our objectives were to (1) determine the optimum allocation regarding the number of crosses, S₁ families, DH lines, and test locations, (2) investigate the impact of parental selection on the optimum allocation and selection gain (ΔG), and (3) compare the maximum ΔG achievable with each breeding scheme. Selection gain was calculated by numerical integration. Different assumptions were made regarding the budget, variance components, correlation between the mean phenotypic performance of the parents and the mean genotypic value of the testcross performance of their progenies (ρ _P ), and the composition of the finally selected test candidates. In comparison with randomly chosen crosses, maximum ΔG was largely increased with parental selection in both breeding schemes. With an increasing correlation ρ _P , this superiority increased strongly, while the optimum number of crosses decreased in favor of an increased number of test candidates within crosses. Thus, concentration on few crosses among the best parental lines might be a promising approach for short-term success in advanced cycle breeding. Breeding scheme S₁TC-DHTC led to a larger ΔG but had a longer cycle length than DHTC. However, with further improvements in the DH technique and the realization of more than two generations per year, early testing of S₁ families prior to production of DH lines would become very attractive in hybrid maize breeding. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. T. Wegenast and C. F. H. Longin contributed equally to this work.     

Combined S2 and crossbred family selection in full-sib reciprocal recurrent selection

Summary A method (CRRS) that combines S₂ and crossbred family selection in full-sib reciprocal recurrent selection (FSRRS) is proposed. The method requires four generations per cycle in single-eared maize populations. Selection is based on performance of S₂ and full-sib families by applying selection index theory. Equations to estimate the coefficients included in the index are given. These estimates are functions of the genetic and phenotypic variances and covariances among and between the two kinds of families. Comparisons of FSRRS and CRRS under equivalent amount of effort show that CRRS has some advantage over FSRRS for low heritability of the trait being selected (e.g., maize yield) and when only one or two locations with two replications are involved in the selection experiment.Joint contribution: Institute Nacional de Investigaciones Agrarias, La Coruna, Spain; and Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, and Journal Paper No. J-10118 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011. Project 2194     

Trends in population parameters and best linear unbiased prediction of progeny performance in a European F2 maize population under modified recurrent full-sib selection

Recurrent selection is a cyclic breeding procedure designed to improve the mean of a population for the trait(s) under selection. Starting from an F₂ population of European flint maize (Zea mays L.) intermated for three generations, we conducted seven cycles of a modified recurrent full-sib (FS) selection scheme. The objectives of our study were to (1) monitor trends across selection cycles in the estimates of the population mean, additive and dominance variances, (2) compare predicted and realized selection responses, and (3) investigate the usefulness of best linear unbiased prediction (BLUP) of progeny performance under the recurrent FS selection scheme applied. Recurrent FS selection was conducted at three locations using a selection rate of 25% for a selection index, based on grain yield and grain moisture. Recombination was performed according to a pseudo-factorial mating scheme, where the selected FS families were divided into an upper-ranking group of parents mated to the lower-ranking group. Variance components were estimated with restricted maximum likelihood. Average grain yield increased 9.1% per cycle, average grain moisture decreased 1.1% per cycle, and the selection index increased 11.2% per cycle. For the three traits we observed, no significant changes in additive and dominance variances occurred, suggesting future selection response at or near current rates of progress. Predictions of FS family performance in Cn+1 based on mean performance of parental FS families in Cn were of equal or higher precision as those based on the mean additive genetic BLUP of their parents, and corresponding correlations were of moderate size only for grain moisture. The significant increase in grain yield combined with the decrease in grain moisture suggest that the F₂ source population with use of a pseudo-factorial mating scheme is an appealing alternative to other types of source materials and random mating schemes commonly used in recurrent selection.     

Hybrid maize breeding with doubled haploids: V. Selection strategies for testcross performance with variable sizes of crosses and S1 families

In hybrid maize (Zea mays L.) breeding, doubled haploids (DH) are increasingly replacing inbreds developed by recurrent selfing. Doubled haploids may be developed directly from S₀ plants in the parental cross or via S₁ families. In both these breeding schemes, we examined 2 two-stage selecting strategies, i.e., considering or ignoring cross and family structure while selection among and within parental crosses and S₁ families. We examined the optimum allocation of resources to maximize the selection gain ΔG and the probability P(q) of identifying the q% best genotypes. Our specific objectives were to (1) determine the optimum number and size of crosses and S₁ families, as well as the optimum number of test environments and (2) identify the superior selection strategy. Selection was based on the evaluation of testcross progenies of (1) DH lines in both stages (DHTC) and (2) S₁ families in the first stage and of DH lines within S₁ families in the second stage (S₁TC-DHTC) with uniform and variable sizes of crosses and S₁ families. We developed and employed simulation programs for selection with variable sizes of crosses and S₁ families within crosses. The breeding schemes and selection strategies showed similar relative efficiency for both optimization criteria ΔG and P (0.1%). As compared with DHTC, S₁TC-DHTC had larger ΔG and P (0.1%), but a higher standard deviation of ΔG. The superiority of S₁TC-DHTC was increased when the selection was done among all DH lines ignoring their cross and family structure and using variable sizes of crosses and S₁ families. In DHTC, the best selection strategy was to ignore cross structures and use uniform size of crosses.     

Correlations and comparisons of quantitative trait loci with family per se and testcross performance for grain yield and related traits in maize

Simultaneous improvement in grain yield and related traits in maize hybrids and their parents (inbred lines) requires a better knowledge of genotypic correlations between family per se performance (FP) and testcross performance (TP). Thus, to understand the genetic basis of yield-related traits in both inbred lines and their testcrosses, two F _2:3 populations (including 230 and 235 families, respectively) were evaluated for both FP and TP of eight yield-related traits in three diverse environments. Genotypic correlations between FP and TP, $ \hat{r}_{\text{g}} $ (FP, TP), were low (0–0.16) for grain yield per plant (GYPP) and kernel number per plant (KNPP) in the two populations, but relatively higher (0.32–0.69) for the other six traits with additive effects as the primary gene action. Similar results were demonstrated by the genotypic correlations between observed and predicted TP values based on quantitative trait loci positions and effects for FP, $ \hat{r}_{\text{g}} $ (M _FP, Y _TP). A total of 88 and 35 QTL were detected with FP and TP, respectively, across all eight traits in the two populations. However, the genotypic variances explained by the QTL detected in the cross-validation analysis were much lower than those in the whole data set for all traits. Several common QTL between FP and TP that accounted for large phenotypic variances were clustered in four genomic regions (bin 1.10, 4.05–4.06, 9.02, and 10.04), which are promising candidate loci for further map-based cloning and improvement in grain yield in maize. Compared with publicly available QTL data, these QTL were also detected in a wide range of genetic backgrounds and environments in maize. These results imply that effective selection based on FP to improve TP could be achieved for traits with prevailing additive effects.     

Allozyme frequencies, heterozygosity and genetic distances following S1 recurrent selection in two synthetic maize populations

 Three cycles of S₁ recurrent selection for yield were carried out in two synthetic maize populations, EPS6 from humid Spain and EPS7 from arid Spain. One-hundred S₁ lines were evaluated from each cycle of selection and the ten highest-yielding S₁ lines were recombined to produce the next cycle. Changes in variability and genetic distances in two synthetic maize populations, following three cycles of recurrent selection, recombining ten S₁ lines in each cycle, were determined. Isozyme analysis was performed on 125 seedlings per cycle of selection (four cycles in each of two populations). Regressions of each allozyme frequency on cycles of selection were performed, genetic distances between populations were determined, and simple correlations between genetic distances and heterosis were calculated. The average heterozygosity per locus was also calculated for each population. Regression analysis did not reveal any common trend between EPS6 and EPS7 for changes in allele frequencies presumably due to selection. The number of polymorphic loci, the mean alleles per locus, and the mean heterozygosity did not show any reduction in variability. Finally, selection did not affect genetic distances among cycles of selection. The agronomic evaluation of the selection program, after three cycles of selection, revealed that the genetic variance was not significantly reduced for most traits, and that the heterosis among cycles of selection of both populations had not changed significantly. The conclusions based on isozyme data supported the deductions made from agronomic data. Three cycles of selection neither caused relevant changes on variability nor on genetic distance among cycles of selection of both maize synthetic populations. These data did not indicate any basis for increasing the number of S₁ lines recombined for recurrent selection. Received: 28 April 1997 / Accepted: 23 June 1997     

Breeding maize as biogas substrate in Central Europe: I. Quantitative-genetic parameters for testcross performance

Biofuels have gained importance recently and the use of maize biomass as substrate in biogas plants for production of methane has increased tremendously in Germany. The objectives of our research were to (1) estimate variance components and heritability for different traits relevant to biogas production in testcrosses (TCs) of maize, (2) study correlations among traits, and (3) discuss strategies to breed maize as a substrate for biogas fermenters. We evaluated 570 TCs of 285 diverse dent maize lines crossed with two flint single-cross testers in six environments. Data were recorded on agronomic and quality traits, including dry matter yield (DMY), methane fermentation yield (MFY), and methane yield (MY), the product of DMY and MFY, as the main target trait. Estimates of variance components showed general combining ability (GCA) to be the major source of variation. Estimates of heritability exceeded 0.67 for all traits and were even much greater in most instances. Methane yield was perfectly correlated with DMY but not with MFY, indicating that variation in MY is primarily determined by DMY. Further, DMY had a larger heritability and coefficient of genetic variation than MFY. Hence, for improving MY, selection should primarily focus on DMY rather than MFY. Further, maize breeding for biogas production may diverge from that for forage production because in the former case, quality traits seem to be of much lower importance.     

Use of matroclinous haploid plants in recurrent selection in corn]

Two cycles of recurrent selection were performed in maize with the use of matroclinous haploids. Two synthetic populations, SP and SA, were improved. Each cycle consisted of two stages: (1) isolation of haploids from the synthetic populations and (2) growth of the haploids, pollination with pollen from diploid plants, and selection. The selection was performed for ear size in haploid plants. The mean gain in productivity in the synthetic populations SP and SA the per cycle was 16.48 and 20.98%, respectively. It is suggested that the high value of this index is related to the fact that haploid plants reveal useful genes with additive and epistatic effects. Natural selection may have played a part, too. The combination of artificial and natural selection in haploids resulted in a considerable gain in productivity in the synthetic populations to be improved.     

Relative changes in genetic variability and correlations in an early-maturing maize population during recurrent selection

Four cycles of S(1) family recurrent selection to improve grain yield and resistance to Striga hermonthica have been completed in TZE-Y Pop STR C(0.) In order to determine whether or not to continue with the recurrent scheme, it was desirable to evaluate the amount of residual genetic variance and associated parameters in the population. The objective of this study was to characterize the relative changes in the levels of the genetic variances, heritability estimates and genetic correlation coefficients, and to predict future gains from selection for grain yield, Striga resistance and other agronomic traits. Fifty S(1) families, derived from each cycle, were evaluated under Striga-infested and Striga-free conditions at Mokwa, Ikenne and Abuja, Nigeria, in 2005 and 2007. Under Striga infestation, genetic variances for grain yield, days to anthesis, plant height and Striga damage generally increased in the advanced cycles of selection. In contrast, the genetic variances for days to silk, anthesis-silking interval, ears per plant, ear aspect and number of emerged Striga plants decreased with selection. The advanced cycles of selection significantly out-yielded the original cycle in both research environments. Heritabilities for grain yield, Striga damage and number of emerged Striga plants were significantly greater than zero. The realized gains from selection for grain yield under Striga infestation (52?kg?ha(-1)?cycle(-1)) and Striga-free conditions (130?kg?ha(-1)?cycle(-1)) were remarkably lower than the predicted gains (350 and 250?kg?ha(-1?)cycle(-1), respectively). Adequate genetic variability exists in cycle 4 of the scheme to ensure future gains from selection.     

Two-locus theory in recurrent selection for general combining ability in maize

Summary Two-locus theory for recurrent selection for general combining ability in maize was developed. The theory featured: (a) recombination of the selfed progeny of selected parents; and (b) linkage disequilibrium in the initial gametic array. The theory indicated: (a) that initial linkage disequilibrium exerts a permanent influence upon selection progress; (b) that interposition of one or more generations of random mating before each cycle reduces the permanent effect in ensuing cycles; and (c) that random mating done before initiation of selection is more efficient in removing the influence of linkage disequilibrium on selection progress than random mating done between subsequent cycles.     

Molecular mapping of quantitative trait loci for three kernel-related traits in maize using a double haploid population

Kernel size and kernel weight are important factors possibly involved in the determination of grain yield in maize, so identifying the genetic basis of kernel-related traits provides insights into the breeding of high-yield maize varieties. Kernel length (KL), kernel width (KW) and hundred kernel weight (HKW) were evaluated in three various planting conditions for the 240 field-grown double haploid (DH) lines derived from the single-cross hybrid Xianyu335. Variations in KL, KW and HKW were observed among DH lines, and all three traits showed a broad sense heritability of 76%. A total of 964 single nucleotide polymorphisms (SNPs) from the MaizeSNP3072 chip was utilised to create a high-density genetic map of 1546.4 cM and to identify quantitative trait loci (QTLs). Using composite interval mapping, a total of five, seven and five QTLs have been mapped for KL, KW and HKW, respectively. qkl1-2 and qkl4-1 explained 17.8% and 14.2% of the phenotypic variation in KL, respectively, and the other three QTLs contributed 3.2–4.0%. The phenotypic variation explained (PVE) of seven QTLs responsible for KW ranged from 3.3 to 9.5%. Three QTLs for HKW, qhkw1, qhkw5 and qhkw10 each explained more than 10% of the phenotypic variation, and qhkw4 and qhkw9 accounted for 3.0% and 6.0%, respectively. Due to their detection in multiple planting environments, the loci mapped here appear to be potential targets for the improvement of maize grain yield.     

Marker-assisted selection and evaluation of high oil in vivo haploid inducers in maize

Doubled haploid technology, which is used to rapidly purify genetic resources, is one of the key technologies in modern maize breeding. In a previous study, the major quantitative trait locus qhir1, which influences in vivo haploid induction, was narrowed down to a 243-kb region, which made it feasible to use marker-assisted selection (MAS) for inducer development. Recently, a new method was developed for haploid identification using oil content (OC). The objective of this study was to develop high oil inducer lines using MAS of the qhir1 locus. We constructed an F₂ population, two backcross populations that were backcrossed to the inducer CAU5 (BC₁F₁-CAU5) and the high oil inbred line GY923 (BC₁F₁-GY923), respectively, which was derived from the cross GY923 × CAU5, and subjected continuous selfing to develop high oil inducer lines. In each cycle, three different parameters including kernel OC, marker genotype at qhir1 and haploid induction rate (HIR) were used for pedigree selection. Three candidate high oil inducer lines were developed, with an OC of approximately 8.5 %, an HIR of approximately 8 % and superior agronomic performance, which are suitable values for the application of these lines to haploid identification by OC. Our results confirm the notion that HIR selection combined with MAS for qhir1 is an effective approach to haploid inducer breeding. In addition, we determined that the accuracy of haploid identification by OC is influenced by the female germplasm resource and the high oil inducer and that appropriate critical points for OC can balance the false discovery rate and false negative rate.     

水稻轮回选择群体XTBG-HP1表型遗传多样性分析

该研究基于4个陆稻群体及172个水稻品种或杂交组合,构建了水稻多亲本隐性核不育轮回选择群体XTBG-HP1,并经过4次轮回重组,采用16个表型性状对其进行了遗传多样性分析。结果表明:(1)该群体14个数量性状符合正态分布,各表型均存在极端性状个体。(2)数量性状变异系数范围为0.08～0.41,均值为0.20; Shannon-Wiener多样性指数范围为0.72～1.92,均值为1.50。(3)群体在株型与产量构成因子性状方面有显著的相关性,对株型的选择可以实现产量性状的改良。(4)剑叶长、每穗粒总数、千粒重、穗长、粒长、一次枝梗数、有效穗数、剑叶宽、二次枝梗数、抽穗期10个性状可作为群体综合评价指标。(5)剑叶长、二次枝梗数、每穗粒总数3个表型性状具有较高的遗传变异、丰富的遗传多样性及与综合得分F值相关系数较高。综合以上结果发现,后期群体进行基因挖掘、品种改良以及优良育种材料的选育可以基于剑叶长、二次枝梗数及每穗粒总数3个表型性状,同时要充分利用群体株型与产量构成因子性状间的显著相关性。此外,该研究群体中极端单性状或综合得分F值较高的个体,可进一步用于品种选育。     

Breeding maize as biogas substrate in Central Europe: II. Quantitative-genetic parameters for inbred lines and correlations with testcross performance

Breeding maize for use as a biogas substrate (biogas maize) has recently gained considerable importance. To optimize hybrid breeding programs, information about line per se performance (LP) of inbreds and its relation to their general combining ability (GCA) is required. The objectives of our research were to (1) estimate variance components and heritability of LP for agronomic and quality traits relevant to biogas production, (2) study correlations among traits as well as between LP and GCA, and (3) discuss implications for breeding of biogas maize. We evaluated 285 diverse dent maize inbred lines in six environments. Data were recorded on agronomic and quality traits, including dry matter yield (DMY), methane fermentation yield (MFY), and their product, methane yield (MY), as the main target trait. In agreement with observations made for GCA in a companion study, variation in MY was mainly determined by DMY. MFY, which showed moderate correlation with lignin but only weak correlation with starch, revealed only low genotypic variation. Thus, our results favor selection of genotypes with high DMY and less focus on ear proportion for biogas maize. Genotypic correlations between LP and GCA [r _g (LP, GCA)] were highest (≥0.94) for maturity traits (days to silking, dry matter concentration) and moderate (≥0.65) for DMY and MY. Multistage selection is recommended. Selection for GCA of maturity traits, plant height, and to some extent also quality traits and DMY on the level of LP looks promising.     

Biochemical and genetic analyses of N metabolism in maize testcross seedlings: 1. Leaves

Key message

Aside from the identification of 32 QTL for N metabolism in the seedling leaves of a maize testcross population, alanine aminotransferase was found to be a central enzyme in N assimilation.

Abstract

Excessive application of nitrogen (N) fertilizer to grow commercial crops like maize is a cause of concern because of the runoff of excess N into streams and rivers. Breeding maize with improved N use efficiency (NUE) would reduce environmental pollution as well as input costs for the farmers. An understanding of the genetics underlying N metabolism is key to breeding for NUE. From a set of 176 testcrosses derived from the maize IBMsyn10 population grown in hydroponics, we analyzed the youngest fully expanded leaf at four-leaf stage for enzymes and metabolites related to N metabolism. Three enzymes, along with one metabolite explained 24% of the variation in shoot dry mass. Alanine aminotransferase (AlaAT) stood out as the key enzyme in maintaining the cellular level of glutamate as it alone explained 58% of the variation in this amino acid. Linkage mapping revealed 32 quantitative trait loci (QTL), all trans to the genomic positions of the structural genes for various enzymes of N assimilation. The QTL models for different traits accounted for 7–31% of the genetic variance, whereas epistasis was generally not significant. Five coding regions underlying 1-LOD QTL confidence intervals were identified for further validation studies. Our results provide evidence for the key role of AlaAT in N assimilation likely through homeostatic control of glutamate levels in the leaf cells. The two QTL identified for this enzyme would help to select desirable recombinants for improved N assimilation.