Germplasm Enhancement Of Maize Research Articles

Resistance of germplasm enhancement of partially inbred maize lines to fall armyworm and corn earworm

ABSTRACT Maize (Zea mays L.) is an important crop worldwide. In the southern area of the United States the two major pests of maize are fall armyworm (Spodoptera frugiperda (J.E. Smith)) and corn earworm (Helicoverpa zea (Boddie)). Fall armyworm primarily attacks maize leaves and corn earworm damages the upper part of the ear. An effort to obtain new sources for resistance is conducted through Germplasm Enhancement of Maize (GEM) project. Plants were infested at the whorl growth stage with fall armyworm neonates and evaluated for leaf-feeding damage 14 days after infestation. Plants were evaluated for ear-feeding damage based on natural infestation by corn earworm. Leaf-feeding damage values in Tifton were lower than at Mississippi State. GEMN-0259, GEMS-0311, and GEMS-0312 exhibited similar resistance levels with the resistant check on leaf-feeding damage in Tifton. Ear-feeding ratings in Tifton and Mississippi State were similar. GEMS-0311 showed the most resistance to ear-feeding damage in both environments. GEMS-0311 with resistance to both insects could be used for new germplasm development.

Phenotypic and genome-wide association analyses for nitrogen use efficiency related traits in maize (Zea mays L.) exotic introgression lines.

Nitrogen (N) limits crop production, yet more than half of N fertilizer inputs are lost to the environment. Developing maize hybrids with improved N use efficiency can help minimize N losses and in turn reduce adverse ecological, economical, and health consequences. This study aimed to identify single nucleotide polymorphisms (SNPs) associated with agronomic traits (plant height, grain yield, and anthesis to silking interval) under high and low N conditions. A genome-wide association study (GWAS) was conducted using 181 doubled haploid (DH) lines derived from crosses between landraces from the Germplasm Enhancement of Maize (BGEM lines) project and two inbreds, PHB47 and PHZ51. These DH lines were genotyped using 62,077 SNP markers. The same lines from the per se trials were used as parental lines for the testcross field trials. Plant height, anthesis to silking interval, and grain yield were collected from high and low N conditions in three environments for both per se and testcross trials. We used three GWAS models, namely, general linear model (GLM), mixed linear model (MLM), and Fixed and Random model Circulating Probability Unification (FarmCPU) model. We observed significant genetic variation among the DH lines and their derived testcrosses. Interestingly, some testcrosses of exotic introgression lines were superior under high and low N conditions compared to the check hybrid, PHB47/PHZ51. We detected multiple SNPs associated with agronomic traits under high and low N, some of which co-localized with gene models associated with stress response and N metabolism. The BGEM panel is, thus, a promising source of allelic diversity for genes controlling agronomic traits under different N conditions.

GEM Project-Derived Maize Lines Crossed with Temperate Elite Tester Lines Make for High-Quality, High-Yielding and Stable Silage Hybrids

Maize silage is fundamental for high milk production in dairy farming. The incorporation of new genetic diversity into temperate maize germplasm has the potential to improve adapted cultivars, and it could be especially useful for improving the nutrition of silage varieties. The goal of this study is to assess the potential for lines from the Germplasm Enhancement of Maize (GEM) project to compete with commercial silage hybrids when crossed with elite temperate-adapted testers. We examined 35 GEM-derived hybrids along with five commercial checks in seven environments across three years in trials that were arranged in randomized complete block designs. Hybrids were compared based on their potential for conversion into animal productivity units: milk yield per hectare (Milk ha−1) and milk yield per ton of silage (Milk t−1). Broad phenotypic variation was observed for both traits, and the broad-sense heritability of Milk ha−1 and Milk t−1 were 0.24 and 0.31, respectively. Five out of six hybrids in the top 15%, based on a multi-trait stability index, were GEM-derived hybrids. The large proportions of phenotypic variance attributed to genotype by environment interactions (GEI) for quality traits suggests that local adaptation should be leveraged for silage breeding that make use of GEM-derived materials.

Genomic prediction for the Germplasm Enhancement of Maize project.

The Germplasm Enhancement of Maize (GEM) project was initiated in 1993 as a cooperative effort of public- and private-sector maize (Zea mays L.) breeders to enhance the genetic diversity of the U.S. maize crop. The GEM project selects progeny lines with high topcross yield potential from crosses between elite temperate lines and exotic parents. The GEM project has released hundreds of useful breeding lines based on phenotypic selection within selfing generations and multienvironment yield evaluations of GEM line topcrosses to elite adapted testers. Developing genomic selection (GS) models for the GEM project may contribute to increases in the rate of genetic gain. Here we evaluated the prediction ability of GS models trained on 6 yr of topcross evaluations from the two GEM programs in Raleigh, NC, and Ames, IA, documenting prediction abilities ranging from 0.36 to 0.75 for grain yield and from 0.78 to 0.96 for grain moisture when models were cross-validated within program and heterotic group. Predicted genetic gain from GS ranged from 0.95 to 2.58 times the gain from phenotypic selection. Prediction ability across program and heterotic group was generally poorer than within groups. Based on observed genomic relationships between GEM breeding lines and their tropical ancestors, GS for either yield or moisture would reduce recovery of exotic germplasm only slightly. Using GS models trained within program, the GEM programs should be able to more effectively deliver on its mission to broaden the genetic base of U.S. germplasm.

Identification of resistance for Phyllachora maydis of maize in exotic‐derived germplasm

AbstractTar spot of maize (Zea mays L.), caused by the obligate biotroph Phyllachora maydis Maubl., is an emerging disease in the United States and Canada, and the identification of sources of resistance for tar spot will enable the development of resistant hybrids. In 2019 and 2020, 25 accessions from the germplasm enhancement of maize (GEM) project containing exotic introgressions in elite backgrounds were evaluated in nine environments for tar spot severity. Environmental conditions had a major influence on disease development, as tar spot severity varied across locations with only four of the nine locations showing moderate to high levels of disease. In five environments, disease levels were low and disease severity data was not collected or used. Accessions were visually evaluated for tar spot during reproductive growth stages in three environments and during vegetative growth stages in one environment. There was a strong correlation between resistance to P. maydis across locations where accessions were evaluated in reproductive growth stages. Two accessions, GEMS‐0066 and GEMS‐0226, were the most resistant and could prove useful for tar spot resistance breeding. These accessions are publicly available and able to be directly used in breeding programs.

Genome-wide association studies of doubled haploid exotic introgression lines for root system architecture traits in maize (Zea mays L.)

Root system architecture (RSA) is becoming recognized as important for water and nutrient acquisition in plants. This study focuses on finding single nucleotide polymorphisms (SNPs) associated with seedling RSA traits from 300 doubled haploid (DH) lines derived from crosses between Germplasm Enhancement of Maize (GEM) accessions and inbred lines PHB47 and PHZ51. These DH lines were genotyped using 62,077 SNP markers, while root and shoot phenotype data were collected from 14-day old seedlings. Genome-wide association studies (GWAS) were conducted using three models to offset false positives/negatives. Multiple SNPs associated with seedling root traits were detected, some of which were within or linked to gene models that showed expression in seedling roots. Significant trait associations involving the SNP S5_152926936 on Chromosome 5 were detected in all three models, particularly the trait network area. The SNP is within the gene model GRMZM2G021110, which is expressed in roots at seedling stage. SNPs that were significantly associated with seedling root traits, and closely linked to gene models that encode proteins associated with root development were also detected. This study shows that the GEM-DH panel may be a source of allelic diversity for genes controlling seedling root development.

Germplasm enhancement of maize: a look into haploid induction and chromosomal doubling of haploids from temperate‐adapted tropical sources

AbstractThe allelic diversity (AD) project of the Germplasm Enhancement of Maize (GEM) programme utilized the double haploid (DH) breeding method to expedite development and release of lines derived from 300 exotic maize races. Using 18 races in this study, differential effects on haploid induction rates (HIRs) and doubling rates (DRs) by the recurrent parents PHB47 and PHZ51, the elevation that the race is traditionally grown at, and by the race itself were addressed in this study. Races from the AD project were grouped by elevation of their origin, high, middle or low altitude. Six races per elevation were randomly selected and backcrossed using both recurrent parents to generate 36 populations. Ten replications were randomized in a complete randomized design for two growing seasons. The recurrent parent effect was significant, with PHB47 having a higher HIR than PHZ51. Effect of elevation was significant with higher HIR associated with low‐elevation origin, and race also proved to be significant. Effects of elevation, recurrent parent and race were not significant for DR.

Genetic architecture of plant height in maize phenotype‐selected introgression families

AbstractThis study aimed at developing, characterizing and evaluating two maize phenotypic‐selected introgression libraries for a collection of dominant plant height (PHT)‐increasing alleles by introgressing donor chromosome segments (DCS) from Germplasm Enhancement of Maize (GEM) accessions into elite inbred lines: PHB47 and PHZ51. Different backcross generations (BC1‐BC4) were developed and the tallest 23 phenotype‐selected introgression families (PIFs) from each introgression library (PHB47 or PHZ51) were selected for single nucleotide polymorphism genotyping to localize DCS underlying PHT. The result shows that most PIFs carrying DCS were significantly (α = 0.01) taller than the respective recurrent parent. In addition, they contained larger donor genome proportions than expected in the absence of selection or random mating across all BC generations. The DCS were distributed over the whole genome, indicating a complex genetic nature underlying PHT. We conclude that our PIFs are enriched for favourable PHT‐increasing alleles. These two libraries offer opportunities for future PHT gene isolation and allele characterization and for breeding purposes, such as novel cultivars for biofuel production.

Registration of Maize Inbred Line GT888

Maize (Zea mays L.) inbred line GT888 (PL‐364, PI 670116) was developed and released by the USDA‐ARS in cooperation with the University of Georgia and in participation with the USDA Germplasm Enhancement of Maize (GEM) project. GT888 was derived from GEM population DK888:N11 (GEMN‐0177), which has 50% tropical and 50% temperate parentage. GT888 is a yellow dent inbred that was selected through a pedigree breeding program for improved resistance to aflatoxin contamination produced by the causal organism Aspergillus flavus (Link: Fr.). It was compared with three inbreds, Mp313E, Mp717, and Mp719, which are known standards that resist aflatoxin contamination. GT888 also exhibits heterosis when crossed to stiff stalk testers B14, B73, and LH132 and was assigned to the non‐stiff stalk group. GT888 tasseled and silked approximately 5 to 8 d later than a cohort of non‐stiff stalk standards.

Identifying and developing maize germplasm with resistance to accumulation of aflatoxins

Efforts to identify maize germplasm with resistance toAspergillus flavus infection and subsequent accumulation of aflatoxins were initiated by the US Department of Agriculture, Agricultural Research Service at several locations in the late 1970s and early 1980s. Research units at four locations in the south-eastern USA are currently engaged in identification and development of maize germplasm with resistance toA. flavus infection and accumulation of aflatoxins. The Corn Host Plant Resistance Research Unit, Mississippi State, MS, developed procedures for screening germplasm for resistance toA. flavus infection and accumulation of aflatoxins. Mp313E, released in 1990, was the first line released as a source of resistance toA. flavus infection. Subsequently, germplasm lines Mp420, Mp715, Mp717, Mp718, and Mp719 were released as additional sources of resistance. Quantitative trait loci associated with resistance have also been identified in four bi-parental populations. The Crop Protection and Management Research Unit and Crop Genetics and Breeding Research Unit, Tifton, GA, created a breeding population GT-MAS:gk. GT601, GT602, and GT603 were developed from GT-MAS:gk. The Food and Feed Safety Research Unit, New Orleans, LA, in collaboration with the International Institute for Tropical Agriculture used a kernel screening assay to screen germplasm and develop six germplasm lines with resistance to aflatoxins. The Plant Science Research Unit, Raleigh, NC, through the Germplasm Enhancement of Maize (GEM) Project provides to co-operators diverse germplasm that is a valuable source of resistance toA. flavus infection and accumulation of aflatoxins in maize.

Extensive genetic diversity and low linkage disequilibrium within the COMT locus in maize exotic populations

The caffeic acid 3-O-methytransferase (COMT) gene is a prime candidate for cell wall digestibility improvement based on the characterization of brown midrib-3 mutants. We compared the genetic diversity and linkage disequilibrium at this locus between exotic populations sampled within the Germplasm Enhancement of Maize (GEM) project and 70 inbred lines. In total, we investigated 55 exotic COMT alleles and discovered more than 400 polymorphisms in a 2.2kb region with pairwise nucleotide diversity (π) up to 0.017, much higher than reported π values of various genes in inbred lines. The ratio of non-synonymous to synonymous SNPs was 3:1 in exotic populations, and significantly higher than the 1:1 ratio for inbred lines. Selection tests detected selection signature in this gene in both pools, but with different evolution patterns. The linkage disequilibrium decay in exotic populations was at least four times more rapid than for inbred lines with r2>0.1 persisting only up to 100bp. In conclusion, the alleles sampled in the GEM Project offer a valuable genetic resource to broaden genetic variation for the COMT gene, and likely other genes, in inbred background. Moreover, the low linkage disequilibrium makes this material suitable for high resolution association analyses.

Evaluation of fall armyworm resistance in maize germplasm lines using visual leaf injury rating and predator survey

After examining ear-colonizing pest resistance, 20 maize lines from the USDA-ARS Germplasm Enhancement of Maize (GEM) Program were evaluated for whorl-feeding fall armyworm (FAW) (Spodoptera frugiperda) resistance using 4 maize inbred lines as the resistant and susceptible controls. Both FAW injury ratings at 7- and 14-d after infestation, and predator abundance and diversity at whorl stage (V6-V8) were recorded in 2009 and 2010. The survey of the diversity and abundance of predators in each experimental plot were conducted 7 d after the FAW infestation. Of the 20 germplasm lines examined, 3 of them (i.e., entries 9, 15, and 19 that were derived from tropical maize germplasm lines were originated from Uruguay, Cuba, and Thailand, respectively) were identified as the best FAW-resistant germplasm lines using the leaf injury ratings and predator survey data. In addition, the abundance and diversity of the predators were greater in 2010 than in 2009, which might have caused the low level of the FAW injury ratings on all lines examined in 2010. The 2-year data showed that the FAW injury ratings were negatively correlated to the predator abundance and diversity, which is also influence by genotype × environment interactions. The findings suggested that tropical germplasm is an important source of native resistance to the FAW and the corn earworm. At the same time, the maize genotype × environment interaction (e.g., predator attractiveness, and varying weather conditions) should be included in the multiple-year evaluations of insect and disease resistance of maize germplasm lines under field conditions.

Review: Plant Resistance to Agriculture Insect Pests

Competition and interaction among organisms in nature results to selection for traits which confer added advantage. As a result of such interactions between plants and their natural enemies, plants have developed defensive mechanisms, both chemical and physical, to minimize pest damage. Intensive researches were carried out to identify and isolation the gene responsible for resistance plant to insects’ infestation which is used in breeding programs for crops improvement. It can be summarized as follows: 1- Corn ear worm, Helicoverpa zea (Boddie) can’t cause damage to certain corn strain due to maysin, a C-glycosyl flavone, and related compounds in the silks that inhibit corn earworm larval growth. Scientist can identify corn chromosome regions associated with silk maysin concentration, and then used it in producing new hybrid.2- Other insect such as the fall army worm, Spodoptera frugiperda can’t cause damage to resistance corn due to some factors as high hemicelluloses content, low protein content and leaf toughness. They can also identify a gene code which is controlled by the resistance traits of corn to the fall army worm. 3- However, the European corn borer, Ostrinia nubilalis (Hubner) can’t cause damage to certain corn varieties due to high concentrations of 2,4-dihydroxy-7- methoxy-1,4-benzoxazin-3-one (DIMBOA) in the leaves of mid-whorl stage plants. Scientist indicated that resistance to the European corn borer was identified in Peruvian corn germplasm and the GEM (Germplasm Enhancement of Maize) corn collection, then make backcross to produce new resistance strain. 4- Other crops as tomato infested by Tuta absoluta (Meyrick ( but some tomato varieties as Lycopersicon esculentum can resist it due to antixenosis factor which interrupted tomato leaf miner from feed, oviposition and shelter . 5- Russian wheat aphid, Diuraphis noxia (Mordvilko), can’t cause damage to some wheat and barley and others crop due to antibiosis and antixenosis factors. Six sources of resistant wheat, each governed by a different, single gene, have been identified.6- Two-spotted spider mites, Tetranychus urticae (Acari: Tetranychidae) and Whiteflies, Bemisia tabaci (Hemiptera: Aleyrodidae) are herbivorous pests. Additional whitefly infestation to spider-mite infested plants resulted in a reduced attraction of predatory mites (Phytoseiulus persimilis). This interference was shown to result from the reduction in (E)-β-ocimene emission from plantsinfested by both spider mites and whiteflies.7- Transgenic crops expressing a protein from the bacterium Bacillus thuringiensis Berliner (Bt) were carried out into potato, maize, cotton and soybean to be reduced their insect infestation.

Diallel Analysis of Diverse Maize Germplasm Lines for Resistance to Aflatoxin Accumulation

ABSTRACTIn this study, diverse, novel maize (Zea mays L.) germplasm from the Germplasm Enhancement of Maize (GEM) Project was screened to determine the combining ability and inheritance of ear rot resistance and resistance to aflatoxin accumulation in hybrid combinations. Nine GEM germplasm lines and six known resistant and susceptible inbred lines were selected as parents of a diallel cross that was evaluated over 2 yr at Mississippi State, MS, and 1 yr at College Station, TX. Resistant lines Mp717 and Mp494 each had negative and significant general combining ability effects, and were frequently present as parents in the hybrid combinations with the lowest aflatoxin accumulation. Of the GEM germplasm lines evaluated, GEMN‐0130 was the best source of aflatoxin resistance, with significant and negative GCA values at all three site years. The GEM lines exhibiting significant and negative GCA effects for aflatoxin accumulation in at least one site year include two GEM germplasm lines, GEMS‐0030 (2 yr) and GEMS‐0074 (1 yr), and a nonreleased GEM source derived from the tropical Cuban accession, CUBA117 (1 yr). The GEM germplasm lines significantly contributing to reduced ear rot in at least two out of three environments included CUBA117, GEMS‐0030, GEMS‐0002, and GEMN‐0130. These results demonstrate the GEM Project is a useful and untapped source of diverse germplasm for resistance to Aspergillus flavus ear rot and aflatoxin accumulation.

Swimming in the Breeding Pool: Partnering for Conservation of Plant Genetic Resources through Crop Germplasm Enhancement

Substantial and sustainable increases in productivity of all crops are needed to meet the predicted demand for food, feed, fibres, flowers, fuels, fun, feed-stocks and pharmaceuticals of this 21st century bio-based economy. Plant breeding is vital for protecting the yield gains made to date, and for further increasing the genetic yield potential of all crops. As a result of the Green Revolution, global productivity of the main food staples steadily rose since the 1960s. Such achievements ensued from crop genetic enhancement partnerships. They are models for illustrating partnering for exchange, evaluation, release and use of plant genetic resources worldwide. These partnerships include national agricultural research institutes and international agricultural research centres. For many decades the global wheat yield increased due to an effective International Wheat Improvement Network (IWIN), which deployed cutting-edge science alongside practical multi-disciplinary applications, resulting in the development of bred-germplasm that has improved food security and the livelihoods of farmers in the developing world. IWIN operates field evaluation trials in more than 250 locations of 100 countries for testing breeding wheat lines across many environments. The International Network for Genetic Evaluation of Rice (INGER) has become an integral component of rice breeding programmes: every year partners provide about 1000 genetically diverse breeding lines, which have been grown in about 600 experiment stations from 80 countries. The Latin American Maize Programme (LAMP) has assessed national germplasm, facilitated the exchange of genetic resources across the continent, and its core subset has been made available to encourage further use in broadening maize genetic resources. For example, the Germplasm Enhancement of Maize (GEM) project has used LAMP-selected landraces in crosses with elite temperate maize lines provided by North American private companies, to introgress useful genetic diversity into US maize germplasm, with the aim to broaden the genetic base of “corn-belt” hybrids.

Evaluation of Corn Germplasm Lines for Multiple Ear-Colonizing Insect and Disease Resistance

Ear-colonizing insects and diseases that reduce yield and impose health threats by mycotoxin contaminations in the grain, are critical impediments for corn (Zea mays L.) production in the southern United States. Ten germplasm lines from the Germplasm Enhancement of Maize (GEM) Program in Ames, IA, and Raleigh, NC, and 10 lines (derived from GEM germplasm) from the Texas Agricultural Experiment Station in Lubbock, TX, were examined in 2007 and 2008 with local resistant and susceptible controls. Four types of insect damage and smut disease (Ustilago maydis) infection, as well as gene X environment (G X E) interaction, was assessed on corn ears under field conditions. Insect damage on corn ears was further separated as cob and kernel damage. Cob penetration rating was used to assess corn earworm [Helicoverpa zea (Boddie)] and fall armyworm [Spodoptera frugiperda (J.E. Smith)] feeding on corn cobs, whereas kernel damage was assessed using three parameters: 1) percentage of kernels discolored by stink bugs (i.e., brown stink bug [Euschistus serous (Say)], southern green stink bug [Nezara viridula (L.)], and green stink bug [Chinavia (Acrosternum) hilare (Say)]; 2) percentage of maize weevil (Sitophilus zeamais Motschulsky)-damaged kernels; and 3) percentage of kernels damaged by sap beetle (Carpophilus spp.), "chocolate milkworm" (Moodna spp.), and pink scavenger caterpillar [Pyroderces (Anatrachyntis) rileyi (Walsingham)]. The smut infection rates on ears, tassels, and nodes also were assessed. Ear protection traits (i.e., husk tightness and extension) in relation to insect damage and smut infection also were examined. Significant differences in insect damage, smut infection, and husk protection traits were detected among the germplasm lines. Three of the 20 germplasm lines were identified as being multiple insect and smut resistant. Of the three lines, entries 5 and 7 were derived from DKXL370, which was developed using corn germplasm from Brazil, whereas entry 14 was derived from CUBA117.

Genetic characterization of a core set of a tropical maize race Tuxpeño for further use in maize improvement.

The tropical maize race Tuxpeño is a well-known race of Mexican dent germplasm which has greatly contributed to the development of tropical and subtropical maize gene pools. In order to investigate how it could be exploited in future maize improvement, a panel of maize germplasm accessions was assembled and characterized using genome-wide Single Nucleotide Polymorphism (SNP) markers. This panel included 321 core accessions of Tuxpeño race from the International Maize and Wheat Improvement Center (CIMMYT) germplasm bank collection, 94 CIMMYT maize lines (CMLs) and 54 U.S. Germplasm Enhancement of Maize (GEM) lines. The panel also included other diverse sources of reference germplasm: 14 U.S. maize landrace accessions, 4 temperate inbred lines from the U.S. and China, and 11 CIMMYT populations (a total of 498 entries with 795 plants). Clustering analyses (CA) based on Modified Rogers Distance (MRD) clearly partitioned all 498 entries into their corresponding groups. No sub clusters were observed within the Tuxpeño core set. Various breeding strategies for using the Tuxpeño core set, based on grouping of the studied germplasm and genetic distance among them, were discussed. In order to facilitate sampling diversity within the Tuxpeño core, a minicore subset of 64 Tuxpeño accessions (20% of its usual size) representing the diversity of the core set was developed, using an approach combining phenotypic and molecular data. Untapped diversity represents further use of the Tuxpeño landrace for maize improvement through the core and/or minicore subset available to the maize community.

Dry matter and relative sugar yield from enzymatic hydrolysis of maize whole plants and cobs

With 1 figure and 6 tablesAbstractThe objective of this work was to determine the potential of germplasm from the Germplasm Enhancement of Maize (GEM) programme for use as a biofuel feedstock, relative to commercial grain and silage hybrids. Eighteen maize genotypes including GEM varieties and commercial checks were evaluated in a 2‐year field study for dry matter yield, moisture at harvest and sugar produced by hydrolysis of whole plants and cobs. There were no significant correlations between any of the traits measured, suggesting that it should be possible to improve yield with quality traits using a selection index. A brown midrib variety was in the top significance group for hydrolysis traits, underscoring the impact of this mutation on the digestibility of lignocellulosic biomass. Commercial varieties tended to have superior dry matter yield, while several GEM lines ranked highly for sugar produced by hydrolysis of whole plants. Selection indices that take both sugar produced by hydrolysis and dry matter yield into account produced rankings of the germplasm used in this study that were markedly different than rankings based on either trait alone.

The strategy and potential utilization of temperate germplasm for tropical germplasm improvement: a case study of maize (Zea mays L.)

The organization of maize (Zea mays L.) germplasm into genetically divergent heterotic groups is the foundation of a successful hybrid maize breeding program. In this study, 94 CIMMYT maize lines (CMLs) and 54 United States germplasm enhancement of maize (GEM) lines were assembled and characterized using 1,266 single nucleotide polymorphisms (SNPs) with high quality. Based on principal component analysis (PCA), the GEM lines and CMLs were clearly separated. In the GEM lines, there were two groups classified by PCA corresponding to the heterotic groups “stiff stalk” and “non-stiff stalk”. CMLs did not form obvious subgroups by PCA. The allelic frequency of each SNP differed in GEM lines and CMLs. In total, 3.6% alleles (46/1,266) of CMLs are absent in GEM lines and 4.4% alleles (56/1,266) of GEM lines are absent in CMLs. The performance of F1 plants (n = 654) produced by crossing between different groups based on pedigree information was evaluated at the breeding nurseries of two CIMMYT stations. Genomic estimated phenotypic values of plant height and days to anthesis for a testing set of 45 F1 crosses were predicted based on the training data of 600 F1 crosses using a best linear unbiased prediction method. The prediction accuracy benefitted from the adoption of the markers associated with quantitative trait loci for both traits; however, it does not necessarily increase with an increase in marker density. It is suggested that genomic selection combined with association analysis could improve prediction efficiency and reduce cost. For hybrid maize breeding in the tropics, incorporating GEM lines which have unique alleles and clear heterotic patterns into tropically adapted lines could be beneficial for enhancing heterosis in grain yields.

Comparison of Conventional, Modified Single Seed Descent, and Doubled Haploid Breeding Methods for Maize Inbred Line Development Using Germplasm Enhancement of Maize Breeding Crosses

ABSTRACTBreeding crosses from the Germplasm Enhancement of Maize (GEM) project between exotic accessions and elite Corn Belt Dent inbreds provide a unique opportunity for broadening the genetic base of the U.S. maize (Zea mays L.) crop by incorporating favorable exotic alleles in elite genetic backgrounds. Genetic gains in the GEM program could be enhanced by identifying more efficient breeding procedures for this purpose. We compared four breeding methods, conventional Germplasm Enhancement of Maize (CG), conventional mass (CM), modified single seed descent (MSSD), and doubled haploid (DH) methods, for their relative capacity to produce superior maize inbred lines and hybrids from three GEM breeding crosses: (ANTIG01 × N16) × DE4, (AR16035 × S02) × S09, and (DKXL212 × S09) × S43b. Testcross evaluations of 50 selected lines from each combination of cross and breeding method were conducted across multiple environments, generally revealing no differences among breeding methods. The MSSD and CM methods had higher proportions of the top 10 lines for grain yield and yield:moisture ratio in the second year testcross reevaluations of about the top 28% of lines. Hybrids from MSSD lines tended to have higher grain moisture, but MSSD hybrids with high yield:moisture ratio could still be identified. The MSSD method may be an effective and efficient alternative to both pedigree and DH methods for developing high performing lines from the GEM breeding crosses as it requires relatively fewer resources.