Leafy reduced-stature maize for short-season environments: Yield and yield components of inbred lines

Development of maize (Zea mays L.) types that produce leaf area and mature quickly would increase production of maize in mid- to short-season areas. The leafy (Lfy1) and reduced-stature (rd1) traits both make contributions to this end. However, these two traits have not previously been combined. Our objective was to evaluate the yield and yield components of non-leafy normal-stature (NLNS), leafy reduced-stature (LRS), non-leafy reduced-stature (NLRS), and leafy normal-stature (LNS) maize inbred lines. The two genes, ‘Lfy1’ and ‘rd1’, were incorporated into a series of inbred lines resulting in a range of canopy architectures. Ten variables were recorded for each of 30 inbred lines over three years. The 10 variables were: corn heat unit requirement from planting to tasselling, corn heat unit requirement from planting to silking, days between tasselling and silking, grain moisture content, husk dry weight, cob dry weight, ear length, maximum ear circumference, grain yield and ratio of grain yield to moisture content. Reduced-stature inbred lines reached anthesis more quickly than normal-stature inbred lines. Grain moisture content was less in reduced-stature inbred lines than normal stature trait groups. Leafy-reduced stature plants had the highest ratio of grain to moisture content and the lowest grain moisture content at harvest. Inbred lines containing the rd1 trait matured more rapidly than other trait groups. The LRS trait group yielded more than the other groups, and showed great potential for use in mid- to short-season environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Multi-attribute responses of maize inbred lines across managed environments

In maize (Zea mays L.) breeding programs, selection among and within segregating progenies is based mainly on indirect selection criteria. A better understanding of the environment influence on physiological attributes of maize inbred lines is important to the identification and selection of superior inbred lines as well as to successful hybrid seed production. In this study, the size and form of genotype (G) and genotype × environment (G × E) interaction effects for plant grain yield (PGY) and several physiological attributes were examined for 12 maize inbred lines grown in four managed environments, represented by two seasons (Y) and two nitrogen levels. Mixed model analysis revealed that the G effect was relatively high for attributes related to light capture, phenology, early biomass production, and numerical components of PGY. The G × E interaction effect explained most of the variability for PGY, harvest index (HI), and biomass production at maturity. Three-mode principal component analysis allowed us to: (1) describe the associations among multiple attributes across environments, (2) reveal the form of the main patterns of G × E interaction, (3) establish the importance of the genotype × year (G × Y) interaction for kernel number, HI, and biomass at maturity in determining PGY, (4) identify promising genotypes of high-PGY across environments, and (5) detect genotypes of similar response patterns for PGY but with a contrasting relative behavior for other attributes, which may permit the simultaneous selection for grain yield and desired secondary traits. Such selection results would contribute greatly in the identification of superior inbreds than selecting for grain yield alone.

Relationship between genetic markers and morphological traits in a maize inbred lines collection

Summary Genetic markers (isozymes), in addition to the traits recommended by the UPOV convention, have been used in France since 1989 to characterize the maize inbred lines submitted to registration. In the years 1989. 1990 and 1991, a total of 974 inbreds has been described according to this procedure. Relationships between genetic markers were investigated and underlined the occurrence of linkage disequilibria within the tested germplasm. These disequilibria appeared to depend strongly on the breeding history of the germplasm. In some cases, these disequilibria could be related to a major progenitor (foundation effect). Relationships between genetic markers and quantitative traits were also pointed out. High coefficients of determination (up to 60%) were in some cases observed at the within group level, and also appeared to be very dependent on the major progenitors of the group of interest. The consequences of these results for breeding and distinctiveness studies are discussed.     

Differences in reciprocal crosses of maize inbred lines diverse for protein content

Summary Forty-two crosses and their reciprocals in maize (Zea mays L.) involving inbred lines highly diverse for protein content were evaluated in four environments. Data were recorded on crude protein content of grain, protein yield, grain yield, 1000 kernel weight, dry matter content of ear at harves,, days to 50% slking, plant height, ear height, and early vigor. No significant variation due to reciprocal differences was observed for protein content and early vigor. For all other traits the variance component due to reciprocal x environment interactions was significant while the variance component due to reciprocal differences was significant only for kernel weight, dry matter content of ear, plant height, and ear height. The variance components due to nuclear differences and their interactions with environments were always highly significant and larger than the components due to reciprocal differences and reciprocal x environment interactions. The instability and low magnitude of reciprocal differences indicated that it might be difficult to exploit them commercially. However, seeing the presence of reciprocal differences for most of the traits studied, the evaluation of breeding materials for these differences seems to be important.The research work was supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 142.     

几种分子标记技术在玉米自交系类群划分中的应用

分子标记技术的开发利用推动了玉米育种的发展,概述了5种常用分子标记SSR、RFLP、RAPD、AFLP、SNP的原理及特点。综述了分子标记技术在玉米自交系类群划分中的应用。     

Genetic diversity of maize inbred lines in relation to downy mildew

A major emphasis in maize breeding in Asian countries has been the improvement for resistance to downy mildew, a serious disease that causes significant yield losses. A total of 102 inbred lines, including lines from Asian breeding programs, Mexico, USA and Germany, were analyzed with 76 SSR markers to measure diversity and investigate the effect of selection for downy mildew resistance. A mean polymorphism information content of 0.59, with a range of 0.14 to 0.83, was observed. Diversity at the gene level showed an average of 5.4 alleles per locus and a range of two to 16 alleles per locus, with a total of 409 alleles. About half of the alleles in the Asian lines had frequencies of 0.10 or less, and only 2% had frequencies > 0.80, indicating the presence of many alleles, and thus a high level of diversity. Some of the high-frequency alleles were in chromosomal regions associated with disease resistance. However, the frequencies of alleles in three SSR loci that are linked to a QTL for resistance to downy mildews in Asia were not significantly different in the subtropical/tropical Asian lines as compared to all the lines in the study. Lines from the US, Germany, and China, comprised three clusters of temperate maize(GS = 0.31), while those from India, Indonesia, Philippines, Thailand, Vietnam and CIMMYT comprised seven indistinct clusters of subtropical and subtropical maize (GS = 0.29). We conclude that maize breeding activity in Asia has not caused a decline in the overall amount of diversity in the region. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Population Density on the Vegetative Growth of Leafy Reduced-stature Maize in Short-season Areas

Maize hybrids which produce more leaves above the ear, with leaf area indices similar to conventional hybrids, which require fewer corn heat units to flowering and maturity, and tolerate higher population densities, should be better adapted for production in short season areas than currently available hybrids. Leafy reduced-stature maize hybrids, which have only recently been developed, have traits which address these criteria. The objective of this study was to evaluate the effects of different population densities (50 000, 100 000, 150 000, and 200 000 plants.ha⁻¹) on the vegetative growth of one leafy reduced-stature (LRS), one non-leafy reduced-stature (NLRS), and two conventional control hybrids (Pioneer 3979, < 2500 CHU, and Pioneer 3902, 2600–2700 CHU) at two locations. There were no differences among population densities for leaf number above the ear; however leaf area index increased as population density increased for all hybrids. The LRS hybrid had a greater average leaf number above the ear (2.7 and 2.0 more leaves than NLRS and the control hybrids, respectively). As a result the leaf area index value of LRS was much greater than the NLRS and similar to the conventional hybrids, but LRS matured substantially before the conventional hybrids. The LRS hybrid required fewer corn heat units to reach flowering and maturity and had more time for grain filling than the conventional hybrids. Therefore, LRS hybrids show promise for production in short season areas where maize cultivation is not economical due to shortness of growing season.     

Genetic characterization of 218 elite CIMMYT maize inbred lines using RFLP markers

Characterization of genetic diversity among maize inbred lines can facilitate organization of germplasm and improve efficiency of breeding programs. A set of 218 phenotypically diverse inbred maize lines developed at CIMMYT for hybrid production was characterized using 32 RFLP markers to: (1) analyze the genetic diversity present; (2) define potential heterotic groups based on clusters formed with marker data; and (3) identify the most representative testers for each potential heterotic group. Lines were clustered using five different genetic distance measurements to find consensus non-hierarchical clusters. Dendrograms were produced to study hierarchical classification within smaller groups of lines. A very high average allelic diversity was seen in this germplasm. Lines did not cluster based on phenotype, environmental adaptation, grain color or type, maturity, or heterotic response (as determined based on hybrid performance with testers), but lines related by pedigree usually did cluster together. Previously defined testers from opposite heterotic groups were not genetically differentiated, and did not represent well their heterotic group. Discrete clusters were difficult to find; thus, potential heterotic groups will be difficult to suggest using RFLP markers alone. However, suggestions on how to use molecular markers and cross performance information to refine heterotic groups and select representative testers are presented.     

An evaluation for genetic variation within maize inbred lines maintained by sib-mating and self-pollination

Summary Maintaining maize (Zea mays L.) inbred lines without genetic change is an important concern of maize breeders and seed producers. Long-time inbred lines, however, have been shown to be genetically unstable, and with selection pressure acting on the resulting genetic variation, these lines have evolved into different strains. The objective of this study was to compare maintenance of line integrity under reproduction by sib-mating and self-pollination in ear-to-row progenies. Ten lines, varying from 5 to 35 generations of previous ear-to-row selfing, were maintained for 11 successive generations under each method of reproduction. Alternate generations were compared by using a randomized complete-block design with each family of lines as a separate experiment. Data were collected for 10 plant and yield traits: pollen shed and silk emergence dates, plant and ear heights, tassel branch number, kernel row number, ear length and diameter, grain yield, and 300-kernel weight. Summarizing for F-tests over all traits and experiments, 30% were significant among sib-mated generations and 56% among selfed generations. Also, overall generation means of the two methods were different for 51% of the comparisons, and the selfed lines were less vigorous for 79% of the significant comparisons. It was concluded that sib-mating lessens the effect of genetic instability and that a reproduction system of sib-mating with intermittent generations of selfing may be more beneficial than continuous ear-to-row selfing for certain lines.Joint contribution: USDA-ARS, and Journal Paper No. J-11880 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011. Project No. 2194.     

Variation and inheritance of response to acetochlor among maize inbred lines and hybrids

Summary The variation of response to acetochlor was studied in a two-year experiment carried out by subjecting 18 maize (Zea mays L.) inbred lines to three herbicide rates (0, 2.5 and 5 l a.i./ha). In both years some inbred lines consistently exhibited an evident susceptibility, with symptoms consisting of the seedling curling up below the soil surface and causing impaired field emergence. The results were poor plant density and lower grain yield in comparison to control. In contrast, other lines showed a satisfactory level of tolerance.Then, to gather data on the inheritance of response to acetochlor, four tolerant inbreds (T) and four susceptible inbreds (S) were crossed to obtain four T×T, four S×S, four S×T and the corresponding four T×S two-way hybrids. These hybrids were studied together with parental lines by applying the same herbicide rates used in the previous trial. The S×S hybrids showed susceptibility to the herbicide and the T×T were tolerant, whereas the S×T and the T× S hybrids showed a tolerance very close to that of the T×T hybrids. No difference was found between S×T and the corresponding T×S hybrids as to herbicide response. On average, the 16 hybrids exhibited greater tolerance than the eight parental lines, with each hybrid group being more tolerant than its parental line group. These results indicate that tolerance to acetochlor is prevailingly dominant, that action of extranuclear genes should be ruled out, and that the level of plant vigour can affect herbicide reactiveness.     

Genetic control of kernel modification found in South African quality protein maize inbred lines

Summary The genetic control of endosperm modification in 12 opaque-2 maize (Zea mays L.) inbred lines was investigated by means of a diallel cross experiment conducted across two environments. Kernel vitreousness and kernel hardness were determined by partially dominant genes. Additive gene action was largely responsible for kernel modification. A favourable general combining ability for kernel vitreousness and kernel hardness was positively correlated with an accumulation of dominant kernel modifying genes. South African sources of endosperm modifiers have been found to be similar to those used in other quality protein maize breeding programmes. Certain inbred lines displayed sufficient genetic potential for use in a quality protein maize hybrid breeding programme.     

Genetic diversity in tropical maize inbred lines: heterotic group assignment and hybrid performance determined by RFLP markers

Tropical maize inbred lines, eight derived from a Thai synthetic population (BR‐105) and 10 from a Brazilian composite population (BR‐106), were assayed for restriction fragment length polymorphisms with 185 clone‐enzyme combinations. The aim of this study was to investigate genetic distances among tropical maize material and their relationship to heterotic group allocation and hybrid performance. Genetic distances (GDs) were on average greater for BR‐105×BR‐106 lines (0.77) than for BR‐106×BR‐106 (0.71) and for BR‐105×BR‐105 (0.69) lines. Cluster analysis resulted in a clear separation of BR‐105 and BR‐106 populations and was according to pedigree information. Correlations of parental GDs with single crosses and their heterosis for grain yield were high for line crosses from the same heterotic group and low for line combinations from different heterotic groups. Our results suggest that RFLP‐based GDs are efficient and reliable to assess and allocate genotypes from tropical maize populations into heterotic groups. However, RFLP‐based GDs are not suitable for predicting the performance of line crosses from genetically different heterotic groups.     

Genetic analysis of tropical maize inbred lines for resistance to maize lethal necrosis disease

Maize lethal necrosis (MLN) disease is a recent outbreak in eastern Africa and has emerged as a significant threat to maize production in the region. The disease is caused by the co-infection of Maize chlorotic mottle virus and any member of potyviridae family. A total of 28 maize inbred lines with varying levels of tolerance to MLN were crossed in a half-diallel mating design, and the resulting 340 F₁ crosses and four commercial checks were evaluated under MLN artificial inoculation at Naivasha, Kenya in 2015 and 2016 using an alpha lattice design with two replications. The objectives of the study were to (i) investigate the magnitude of general combining ability variance (σ _GCA ² ) and specific combining ability variance (σ _SCA ² ) and their interaction with years; (ii) evaluate the efficiencies of GCA based prediction and hybrid performance by means of a cross-validation procedure; (iii) estimate trait correlations in the hybrids; and (iv) identify the MLN tolerant single cross hybrids to be used as female parents for three-way cross hybrids. Results of the combined analysis of variance revealed that both GCA and SCA effects were significant (P < 0.05) for all traits except for ear rot. For MLN scores at early and late stages, GCA effects were 2.5–3.5 times higher than SCA effects indicating that additive gene action is more important than non-additive gene action. The GCA based prediction efficiency for MLN resistance and grain yield accounted for 67–90% of the variations in the hybrid performance suggesting that GCA-based prediction can be proposed to predict MLN resistance and grain yield prior to field evaluation. Three parents, CKDHL120918, CML550, and CKLTI0227 with significant GCA effects for GY (0.61–1.21; P < 0.05) were the most resistant to MLN. Hybrids “CKLTI0227 × CML550”, “CKDHL120918 × CKLTI0138”, and “CKDHL120918 × CKLTI0136” ranked among the best performing hybrids with grain yield of 6.0–6.6 t/ha compared with mean yield of commercial check hybrids (0.6 t/ha). The MLN tolerant inbred lines and single cross hybrids identified in this study could be used to improve MLN tolerance in both public and private sector maize breeding programs in eastern Africa.     

Characterization of drought stress environments for upland rice and maize in central Brazil

Drought stresses arise when the combination of rainfall and soil water supply are insufficient to meet the transpiration needs of the crop. In the Cerrado region of Goiás state, Brazil, summer rainfall is typically greater than 1000 mm. However, drought stress can occur during rain-free periods of only 1–3 weeks, since roots are frequently restricted to shallow depths due to Al-induced acidity in deeper soil layers. If these droughts are frequent, then plant breeding programs need to consider how to develop suitable germplasm for the target population of environments (TPE). A crop simulation model was used to determine patterns of drought stress for 12 locations and >30 environments (6 years × 5–6 planting dates) for short and medium duration rice crops (planted in early summer), and for maize grown either as a 1st or 2nd crop in the summer cycle. Regression analysis of the simulations confirmed the greater yield impact in both crops of drought stress (quantified as the ratio of water-limited to potential transpiration) when it occurred around the time of flowering and early grain-filling. For rice, mild mid-season droughts occurred 40–60% of the time in virgin (0.4 m deep for rice or 0.5 m for maize) soils and improved (0.8 m for rice or 1.0 m for maize) soils, with a yield reduction of <30%. More severe reproductive and grain-filling stress (yield reductions of 50% for rice to 90% for maize) occurred less frequently in rice (<30% of time) and 1st maize crop (< 10% of time). The 2nd maize crop experienced the greatest proportion (75–90%) of drought stresses that reduced yield to <50% of potential, with most of these occasions associated with later planting. The rice breeding station (CNPAF) experiences the same pattern of different drought types as for the TPE, and is largely suitable for early-stage selection of adapted germplasm based on yield potential. However, selection for virgin soil types could be augmented by evaluation on some less-improved soils in the slightly drier parts of the TPE region. Similarly, the drought patterns at the maize research station (CNPMS) and the other maize screening locations are better suited to selection of lines for the improved soil types. Development of lines for the 2nd crop and on more virgin (acidic) soils would require more targeted selection at late planting dates in drier sites.     

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

Summary 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.Journal Paper No. J-9023 of the Iowa Agriculture and Home Economics Exp. Stn., Ames, Iowa. Project No. 1990.     

Breeding maize lines for complete and partial resistance to maize streak virus (MSV)

Summary S₁ to S₅ inbred lines, derived from a maize population bred for its overall resistance to three tropical viruses, were screened for resistance to maize streak virus (MSV) by artificial plant infection using viruliferous leafhoppers. Symptoms were rated and intra-line frequency distributions studied for all pedigree inbred lines. Mortality due to MSV was very low among these inbreds. Symptoms appeared later, developed slower and were less severe than in the susceptible control hybrid. Results of a study of 500 S₁ and 93 S₂ lines suggested that resistance is under genetic control via a system involving loci with major genes (with dominance for resistance) controlling high to complete resistance, associated with a genetic system involving loci with minor genes controlling partial resistance. Lines expressing complete resistance to MSV were developed from 5 cycles of inbreeding and selection. The relevance of such complete and partial resistance is discussed.Abbreviations MRPS Mean Rating for Plants exhibiting Symptoms     

Phenotypic characterization and validation of provitamin A functional genes in early maturing provitamin A-quality protein maize (Zea mays) inbred lines

The number of drought and low-N tolerant hybrids with elevated levels of provitamin A (PVA) in sub-Saharan Africa could increase when PVA genes are optimized and validated for developed drought and low-N tolerant inbred lines. This study aimed to (a) determine the levels of drought and low-N tolerance, and PVA concentrations in early maturing PVA-quality protein maize (QPM) inbred lines, and (b) identify lines harbouring the crtRB1 and LcyE genes as sources of favourable alleles of PVA. Seventy early maturing PVA-QPM inbreds were evaluated under drought, low-N and optimal environments in Nigeria for two years. The inbreds were assayed for PVA levels and the presence of PVA genes using allele-specific PCR markers. Moderate range of PVA contents was observed for the inbreds. Nonetheless, TZEIORQ 55 combined high PVA concentration with drought and low-N tolerance. The crtRB1-3′TE primer and the KASP SNP (snpZM0015) consistently identified nine inbreds including TZEIORQ 55 harbouring the favourable alleles of the crtRB1 gene. These inbreds could serve as donor parents of the favourable crtRB1-3′TE allele for PVA breeding in maize.     

Marker-based screening of maize inbred lines using support vector machine regression

The phenomenon of heterosis is widely used in hybrid breeding programmes, despite the fact that no satisfactory molecular explanation is available. Estimators of quantitative genetic components like GCA and SCA values are tools used by the plant breeder to identify superior parental individuals and to search for high heterosis combinations. Obtaining these estimators usually requires the creation of new parental combinations and testing their offspring in multi-environment field trials. In this study we explore the use of ɛ-insensitive Support Vector Machine Regression (ɛ-SVR) for the prediction of GCA and SCA values from the molecular marker scores of parental inbred lines as an alternative to these field trials. Prediction accuracies are obtained by means of cross-validation on a grain maize data set from the private breeding company RAGT R2n. Results indicate that the proposed method allows the routine screening of new inbred lines despite the fact that predicting the SCA value of an untested hybrid remains problematic with the available molecular marker information and standard kernel functions. The genotypical performance of a testcross hybrid, originating from a cross between an untested inbred line and a well-known complementary tester, can be predicted with moderate to high accuracy while this cannot be said for a cross between two untested inbred lines.     

玉米杂交种及其亲本自交系的生化指纹鉴定

本试验以浙单9号等五个玉米杂交组合及其亲本自交系为材料,进行种子盐溶蛋白聚丙烯酰胺凝胶电泳等多种电泳鉴定方法的研究,以揭示玉米杂交种及其亲本自交系的“生化指纹”(biochenucal fingerprint),以及筛选出适合于玉米杂交种及其亲本自交系真实性和纯度鉴定的方法。结果表明,各供试玉米杂交种及其亲本自交系都具有相应的、唯一的种子盐溶蛋白聚丙烯酰胺凝胶电泳所显现的生化指纹。对于有些组合。玉米芽鞘和叶片绿色组织过氧化物酶同工酶电泳图谱存在阴极第4、第5酶带差异,因这两条酶带的差异稳定,并且重现性好,故能用过氧化物酶同工酶技术对其进行有效地鉴定。上述两种方法,尤其是前者,因技术要求不高,费用低,快速及重现性好等特点,能满足我国目前种子检验室日常玉米品种纯度快速测定工作的要求,具育良好的应用前景。