Testcross Hybrids Research Articles

Registration of F1024 Sugarbeet Germplasm with Resistance to Sugarbeet Root Maggot

F1024 (Reg. No. GP‐272, PI 658654) sugarbeet (Beta vulgaris L.) germplasm with resistance to sugarbeet root maggot (Tetanops myopaeformis von Röder) was released by the USDA‐ARS and the North Dakota Agricultural Experiment Station, North Dakota State University, Fargo, ND on 15 Dec. 2009. F1024 was selected from a population formed by crossing F1016, a germplasm line resistant to root maggot, with a breeding line resistant to Cercospora leaf spot (CLS; caused by Cercospora beticola Sacc.). The population was subjected to three cycles of mass selection for resistance to root maggot followed by three cycles of selection among half‐sib families. Under natural root maggot infestations, F1024 had a damage rating of 2.1 (on a scale of 1 to 9, where 0 = no maggot feeding, and 9 = >75% of root surface with feeding scars), compared with an average of 6.1 for two susceptible commercial hybrids. In a 2009 CLS evaluation, F1024 had a significantly lower disease rating than the susceptible check, and the difference between F1024 and the resistant checks was not significant. The performance of testcross hybrids between the component half‐sib families of F1024 and a susceptible cytoplasmic‐male‐sterile line provided additional validation of the potential of root maggot–resistant hybrids in areas where root maggot is a perpetual threat.

Resistance of Early Generation Maize Inbred Lines and Their Hybrids to Maize Weevil [Sitophilus zeamais (Motschulsky)

Maize weevil [Sitophilus zeamais (Motschulsky)] causes important losses to maize (Zea mays L.) grain stored on‐farm by small‐scale farmers, thereby threatening food security throughout the developing world. We compared weevil resistance of S1, S2, S3, and S4 inbred lines with that of their testcross hybrids, studied gene action for resistance, and developed recommendations for weevil resistance breeding in maize. Weevil resistance was evaluated for lines and hybrids from two maize populations by infesting F2 grain samples with weevils and incubating the samples in a controlled temperature and humidity laboratory. Genotypic variation among lines and hybrids and both general and specific combining ability effects were significant for weevil resistance. Broad sense heritabilities were 62 and 50% for number of weevils emerged and 67 and 55% for grain weight loss among lines and hybrids, respectively. Weevil resistance of inbreds was generally not significantly correlated with, and thus could not be a useful predictor of, resistance of their hybrids. Grain weight loss from infestation and feeding was identified as a useful trait for weevil resistance breeding because of its moderate heritability, moderately high phenotypic correlation with number of weevils emerged (r = 0.55 to 0.77, p < 0.001), and ease of measurement. Useful genetic variation exists, and maize breeding programs can address weevil resistance with strategies that use both additive and nonadditive gene action.

A major terminal drought tolerance QTL of pearl millet is also associated with reduced salt uptake and enhanced growth under salt stress

The performance of a major quantitative trait locus (QTL) of terminal drought tolerance (DT) of pearl millet was assessed under salt stress. The test-cross hybrids of the QTL donor parent (drought tolerant, PRLT 2/89-33), QTL recipient parent (drought sensitive, H 77/833-2), and a set of six near isogenic lines introgressed with a terminal DT-QTL (QTL-NILs) were evaluated for germination and seedling emergence at 7 days after sowing (DAS) in Petri plates at four salinity levels, and at vegetative (24 DAS) and maturity stages at three salinity and alkalinity levels. Na+ and K+ accumulation, their compartmentation in different plant parts, and their effects on growth and yield parameters were evaluated. The DT-QTL donor parent and QTL-NILs accumulated less Na+ in shoot parts at seedling, vegetative and maturity stages, and also partitioned the accumulated Na+ more into nodes and internodes and less into leaves than the drought-sensitive recurrent parent. The pattern of reduced salt accumulation in the drought-tolerant parent and QTL-NILs was consistently associated with better growth and productivity in saline and alkaline treatments. It is concluded that the DT-QTL contributed by PRLT 2/89-33 exerted favourable effects on growth and productivity traits under salt stress by limiting Na+ accumulation in leaves.

Biomass Heterosis as the Basis for Grain and Stover Yield Heterosis in Arid Zone Pearl Millet Hybrids

ABSTRACTPearl millet [Pennisetum glaucum (L.) R. Br.] single cross hybrids, bred from high‐yielding parental lines in favorable environments, are not well adapted to northwest India's arid zone. The objectives of these experiments were (i) to measure grain and stover yield heterosis in testcrosses of six landrace‐based restorer populations and (ii) to understand how heterosis for biomass and harvest index (HI) affects heterosis for grain and stover yields in crosses of these populations. Six restorer populations and their testcrosses on 10 to 13 A1 male‐sterile lines were evaluated at two arid zone locations of northwest India. Heterosis was calculated as percentage advantage of testcross hybrids over parental restorer population. The range in individual testcross heterosis varied from −3% to +39% for grain yield and from −22% to +17% for stover yield. Variation in biomass heterosis accounted for 55% of the variation in grain yield heterosis and 84% of the variation in stover yield heterosis. Harvest index heterosis accounted for 38% of variation in grain yield heterosis and 13% of stover yield heterosis with a positive heterosis in HI resulting in negative heterosis in stover yield. The testcrosses with the highest biomass heterosis for each restorer population achieved 20% biomass heterosis level (combined with a mean 12% heterosis in HI), resulting in 32% grain yield heterosis and 18% stover yield heterosis. We concluded that heterosis for biomass was the major determinant of grain and stover yield heterosis in pearl millet.

Relationship between hybrid performance and AFLP based genetic distance in highland maize inbred lines

The objectives of this study were to determine the crossing performance of highland maize inbred lines for grain yield, days to silk and plant height; estimate genetic distance (GD) among the inbred lines and in association with tester parents, and to investigate the relationship of GD with hybrid performance and midparent heterosis (MPH). A total of 26 inbred lines were crossed with six (population and line) testers in a factorial-mating scheme. The F1’s and the parents were evaluated at five locations in Ethiopia. Nine amplified fragment length polymorphism (AFLP) primer pairs were used to genotype all the parents. The F1’s were found to vary widely for grain yield and other traits measured. Yield superiority of more than 30% over the best hybrid check was obtained for some testcross hybrids. Midparent heterosis on average was moderate for grain yield and, plant height. And for days to silking, MPH values were mostly negative. Mean GD values determined from the inbred lines by population tester (0.680) and line tester (0.661) combinations were not significantly different. Cluster analysis separated the tester parents from the corresponding inbred lines. AFLP grouping of the inbred lines was in agreement with their pedigree records. Genetic distances derived from the inbred lines × all testers and from the population testers’ sub-group were not positively correlated with hybrid performance and MPH for most traits. In contrast, correlations of GDs involving the line testers’ sub-group with F1’s and MPH were significantly positive but with low magnitude to be of predictive value.

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 e-insensitive Support Vector Machine Regression (e-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.

QTL mapping with near-isogenic lines in maize

A set of 89 near-isogenic lines (NILs) of maize was created using marker-assisted selection. Nineteen genomic regions, identified by restriction fragment length polymorphism loci and chosen to represent portions of all ten maize chromosomes, were introgressed by backcrossing three generations from donor line Tx303 into the B73 genetic background. NILs were genotyped at an additional 128 simple sequence repeat loci to estimate the size of introgressions and the amount of background introgression. Tx303 introgressions ranged in size from 10 to 150 cM, with an average of 60 cM. Across all NILs, 89% of the Tx303 genome is represented in targeted and background introgressions. The average proportion of background introgression was 2.5% (range 0-15%), significantly lower than the expected value of 9.4% for third backcross generation lines developed without marker-assisted selection. The NILs were grown in replicated field evaluations in two years to map QTLs for flowering time traits. A parallel experiment of testcrosses of each NIL to the unrelated inbred, Mo17, was conducted in the same environments to map QTLs in NIL testcross hybrids. QTLs affecting days to anthesis, days to silking, and anthesis-silk interval were detected in both inbreds and hybrids in both environments. The testing environments differed dramatically for drought stress, and different sets of QTLs were detected across environments. Furthermore, QTLs detected in inbreds were typically different from QTLs detected in hybrids, demonstrating the genetic complexity of flowering time. NILs can serve as a valuable genetic mapping resource for maize breeders and geneticists.

Marker assisted breeding for transformability in maize

Corn lines with improved culturability and transformability were produced using Marker Assisted Breeding (MAB) to introgress specific regions from the highly transformable hybrid, Hi-II, into the elite line, FBLL that responds very poorly in culture. FBLL is a female inbred parental stiff-stalk line that has been used to produce a series of some of DEKALB’s historically best selling hybrids. Five unlinked regions important for culturability and transformability were identified by segregation distortion analysis and introgressed into FBLL to produce the highly transformable FBLL-MAB lines. Agrobacterium mediated transformation was used to screen the FBLL-MAB lines and select the most efficient lines for transformation using immature embryo explants. Two highly efficient transformation systems were developed using kanamycin and glyphosate as selective agents. To evaluate agronomics, two testcross hybrids were produced for each of the three lead FBLL-MAB lines. A 25-location, 3-replication yield trial was used to evaluate grain yield, yield stability, and agronomic characteristics of the hybrids. Yields were found to be 2–5% lower and more stable (across a diverse set of environments) among hybrids produced with the FBLL-MAB lines as compared to the same hybrids produced with FBLL.

Identification of Quantitative Trait Loci Across Recombinant Inbred Lines and Testcross Populations for Traits of Agronomic Importance in Rice

This study was conducted to determine whether quantitative trait loci (QTL) controlling traits of agronomic importance detected in recombinant inbred lines (RILs) are also expressed in testcross (TC) hybrids of rice. A genetic map was constructed using an RIL population derived from a cross between B5 and Minghui 63, a parent of the most widely grown hybrid rice cultivar in China. Four TC hybrid populations were produced by crossing the RILs with three maintaining lines for the widely used cytoplasmic male-sterile (CMS) lines and the genic male-sterile line Peiai64s. The mean values of the RILs for the seven traits investigated were significantly correlated to those of the F1 hybrids in the four TC populations. Twenty-seven main-effect QTL were identified in the RILs. Of these, the QTL that had the strongest effect on each of the seven traits in the RILs was detected in two or more of the TC populations, and six other QTL were detected in one TC population. Epistatic analysis revealed that the effect of epistatic QTL was relatively weak and cross combination specific. Searching publicly available QTL data in rice revealed the positional convergence of the QTL with the strongest effect in a wide range of populations and under different environments. Since the main-effect QTL is expressed across different testers, and in different genetic backgrounds and environments, it is a valuable target for gene manipulation and for further application in rice breeding. When a restorer line that expresses main-effect QTL is bred, it could be used in a number of cross combinations.

Recent Advances in Marker-Assisted Selection for Drought Tolerance in Pearl Millet

Pearl millet [Pennisetum glaucum (L.) R. Br.] is the staple cereal of the hottest, driest areas of the tropics and subtropics. Drought stress is a regular occurrence in these regions, making stress tolerance an essential attribute of new pearl millet cultivars. Recent breeding research has mapped several quantitative trait loci (QTLs) for components of grain and stover yield per se, as well as yield maintenance, under terminal drought stress conditions. We report here the evaluation of these QTLs as possible selection criteria for improving stress tolerance of an elite hybrid cultivar. Initial evaluations, based on hybrids made with topcross pollinators bred from lines selected directly from the mapping population, indicated an advantage to the QTL-based topcross hybrids. This advantage seemed to be related to a particular plant phenotype that was similar to that of the drought tolerant parent of the mapping population. Subsequent evaluations were based on testcross hybrids of drought tolerance QTL introgression lines in the background of the drought-sensitive parent of the mapping population, H 77/833-2. These introgression lines were bred by limited marker-assisted backcrossing of a putative major drought tolerance QTL into H 77/833-2 from the mapping population’s drought tolerant parent. Several of these QTL introgression lines had a significant positive general combining ability for grain yield under terminal stress and significantly out-yielded testcross hybrids made with the original recurrent parent both in unrelieved terminal drought stress and in gradient stress evaluations.

Heterosis in a Broad Range of Alfalfa Germplasm

Crosses between Medicago sativa L. subsp. sativa and subsp. falcata (L.) Arcang. produce progeny exhibiting heterosis for biomass yield. The purpose of this study was to determine which subsp. falcata germplasm produced the best hybrids in testcrosses with elite subsp. sativa material to guide future breeding efforts. Over 100 falcata genotypes from 40 populations were test crossed to four elite sativa populations. Testcross progeny and parental clones were grown for 2 yr in two locations and harvested three times per year to determine biomass yield. A broad range of testcross performance was observed, with mean heterosis values approximately zero. The highest yielding sativa–falcata hybrids were derived from European falcata germplasm. North American semi-improved falcata germplasm performed well in hybrid testcrosses. Preselection of parental falcata genotypes for autumn growth was associated with higher yielding testcross progeny. Positive heterosis was seen during the first harvest, but negative heterosis was often observed during second and, to a smaller extent, third harvests. Superior sativa–falcata hybrids were observed that showed good biomass yield and heterosis during all three harvests. Parental yield was least predictive of hybrid progeny yield during first harvest (h2 = 0.12). Heritability increased during second and third harvest to 0.31 and 0.33, respectively. Expected genetic gain per selection cycle is greater from progeny testing compared with simple recurrent phenotypic selection.

Assessing the relationship between endosperm type and grain yield potential in sorghum ( Sorghum bicolor L. Moench).

Sorghum hybrids with waxy endosperm (either homozygous or heterozygous waxy) have lower grain yield potential compared to non-waxy endosperm hybrids. The cause of this yield reduction is not known. From a genetic perspective, the yield reduction could be due to pleiotrophy or genetic linkage between the wx locus and other loci that influence grain yield. The specific cause of the relationship is important because an effective breeding program can alleviate the problem if it is due to linkage. The objective of this study was to determine whether linkage or pleiotrophy is causing the negative relationship between grain yield and waxy endosperm. From each of two F 2 breeding populations segregating for waxy endosperm, between 40 and 50 inbred lines were derived, with equal numbers of waxy and non-waxy endosperm lines. No selection for yield was practiced during the development of these lines. The lines from these two populations and a set of testcross hybrids (derived from one population) were evaluated in four environments in Texas from 1998 to 2000. Across all tests and environments, the combined yield of the waxy genotypes was 17% lower than non-waxy genotypes. While yields were lower in waxy genotypes, analysis of the individual inbred lines and hybrids revealed that several waxy inbred lines were not statistically different from the best non-waxy inbreds. These results imply that selection of high yielding waxy genotypes is possible, but a significant breeding emphasis on their development is required to effectively identify those genotypes.

Resistance of Maize Inbreds and their Hybrids to Fusarium Stalk Rot

 The resistance of 35 inbred lines and their 70 hybrids with two testers to stalk rot is described and the results of natural and artificial stalk infection are compared. A mixture of Fusarium spp. spores was used for artificial infections. The disease was scored for two years. Inbreds and hybrids differed significantly in resistance and infection types in both years. Generally, disease scores of hybrids were lower than of inbreds. No close association was found between lines and their testcross hybrids (r = –0.06 to 0.29) and between hybrids with the two testers (r = –0.04 to 0.38). The correlation coefficients of disease scores after natural and artificial infection were in inbreds and in hybrids highly significant (r = 0.45–0.87). The results suggest, that Fusarium stalk rot resistance should be tested in both inbreds and hybrids. We recommend for maize breeding, that tests with artificial infection should be followed by tests under natural infection.  

Marker-Assisted Selection for Resistance to Black Shank Disease in Tobacco.

Bulked segregant (BSA) and random amplified polymorphic DNA (RAPD) analyses were used to identify markers linked to the dominant black shank resistance gene, Ph, from flue-cured tobacco (Nicotiana tabacum) cv. Coker 371-Gold. Sixty RAPD markers, 54 in coupling and 6 in repulsion phase linkage to Ph, were identified in a K 326-derived BC1F1 (K 326-BC1F1) doubled haploid (DH) population. Thirty RAPD markers, 26 in coupling and 4 in repulsion phase linkage to Ph, were used to screen 149 K 326-BC2F1 haploid plants. Complete linkage between the 26 coupling phase markers and Ph was confirmed by screening 149 K 326-BC2F1 DH lines produced from the haploid plants in black shank nurseries. RAPD markers OPZ-5770 in coupling and OPZ-7370 in repulsion phase linkage were used to select plants homozygous for the Ph gene for further backcrossing to the widely grown flue-cured cultivar K 326. Black shank disease nursery evaluation of 11 K 326-BC4S1 lines and their testcross hybrids to a susceptible tester confirmed linkage between Ph and OPZ-5770. The results demonstrated the efficiency of marker-assisted selection for Ph using a RAPD marker linked in coupling and repulsion. Complete linkage between 26 RAPD markers and the Ph gene was confirmed in the K 326-BC5 generation, and RAPD phenotypes were stable across generations and ploidy levels. These RAPD markers are useful in marker-assisted selection for Ph, an important black shank resistance gene in tobacco.

Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. I. Biomass and grain yield.

To understand the genetic basis of inbreeding depression and heterosis in rice, main-effect and epistatic QTL associated with inbreeding depression and heterosis for grain yield and biomass in five related rice mapping populations were investigated using a complete RFLP linkage map of 182 markers, replicated phenotyping experiments, and the mixed model approach. The mapping populations included 254 F(10) recombinant inbred lines derived from a cross between Lemont (japonica) and Teqing (indica) and two BC and two testcross hybrid populations derived from crosses between the RILs and their parents plus two testers (Zhong 413 and IR64). For both BY and GY, there was significant inbreeding depression detected in the RI population and a high level of heterosis in each of the BC and testcross hybrid populations. The mean performance of the BC or testcross hybrids was largely determined by their heterosis measurements. The hybrid breakdown (part of inbreeding depression) values of individual RILs were negatively associated with the heterosis measurements of their BC or testcross hybrids, indicating the partial genetic overlap of genes causing hybrid breakdown and heterosis in rice. A large number of epistatic QTL pairs and a few main-effect QTL were identified, which were responsible for >65% of the phenotypic variation of BY and GY in each of the populations with the former explaining a much greater portion of the variation. Two conclusions concerning the loci associated with inbreeding depression and heterosis in rice were reached from our results. First, most QTL associated with inbreeding depression and heterosis in rice appeared to be involved in epistasis. Second, most ( approximately 90%) QTL contributing to heterosis appeared to be overdominant. These observations tend to implicate epistasis and overdominance, rather than dominance, as the major genetic basis of heterosis in rice. The implications of our results in rice evolution and improvement are discussed.

Overdominant epistatic loci are the primary genetic basis of inbreeding depression and heterosis in rice. II. Grain yield components.

The genetic basis underlying inbreeding depression and heterosis for three grain yield components of rice was investigated in five interrelated mapping populations using a complete RFLP linkage map, replicated phenotyping, and the mixed model approach. The populations included 254 F(10) recombinant inbred lines (RILs) derived from a cross between Lemont (japonica) and Teqing (indica), two backcross (BC) and two testcross populations derived from crosses between the RILs and the parents plus two testers (Zhong413 and IR64). For the yield components, the RILs showed significant inbreeding depression and hybrid breakdown, and the BC and testcross populations showed high levels of heterosis. The average performance of the BC or testcross hybrids was largely determined by heterosis. The inbreeding depression values of individual RILs were negatively associated with the heterosis measurements of the BC or testcross hybrids. We identified many epistatic QTL pairs and a few main-effect QTL responsible for >65% of the phenotypic variation of the yield components in each of the populations. Most epistasis occurred between complementary loci, suggesting that grain yield components were associated more with multilocus genotypes than with specific alleles at individual loci. Overdominance was also an important property of most loci associated with heterosis, particularly for panicles per plant and grains per panicle. Two independent groups of genes appeared to affect grain weight: one showing primarily nonadditive gene action explained 62.1% of the heterotic variation of the trait, and the other exhibiting only additive gene action accounted for 28.1% of the total trait variation of the F(1) mean values. We found no evidence suggesting that pseudo-overdominance from the repulsive linkage of completely or partially dominant QTL for yield components resulted in the overdominant QTL for grain yield. Pronounced overdominance resulting from epistasis expressed by multilocus genotypes appeared to explain the long-standing dilemma of how inbreeding depression could arise from overdominant genes.

Genetic variation in testcrosses of European maize inbreds for resistance to the European corn borer and relations to line per se performance

AbstractThe European com borer (ECB) Ostrinia nubilalis H. is a major pest in World maize Zea mays L. production. Objectives of this study were to (1) investigate the genotypic variance and covariance in testcrosses of European flint and dent inbreds for ECB resistance and agronomic traits, and (2) estimate the correlation between line per se and testcross performance for ECB resistance traits. A total of 16 flint and 24 dent inbreds and their testcrosses with two testers from the opposite germplasm pool were evaluated in four and three German environments, respectively. Using artificial infestation with ECB larvae, resistance was assessed by damage rating of stalks, tunnel length in dissected stalks, and relative yield of infested plots compared with protected plots. Yield losses due to ECB damage in testcross hybrids amounted to 40%. Significant genotypic variances between flint and dent lines and high heritabilities were found for damage rating of stalks for both line per se and testcross performance. Heritabilities were low or intermediate for tunnel length and relative grain yield. Correlations between line per se and testcross performance were tight for the damage rating of stalks and moderate for tunnel length and relative yield in both flint and dent germplasm. For damage rating of stalks, per se performance of lines tested in a few environments can be used to predict their testcross performance. In contrast, assessment of testcross performance for tunnel length and relative yield requires evaluating testcrosses with several testers in multi‐environment trials.

Responses to Selection for Endosperm Hardness and Associated Changes in Agronomic Traits after Four Cycles of Recurrent Selection in Maize

Increased endosperm hardness of grain in maize (Zea mays L.) is desirable for dry milling, storage, and expert purposes. This study was conducted to evaluate changes in endosperm hardness resulting from four cycles of visual recurrent selection for increased hardness in two dent corn synthetics representative of U.S. Corn Belt germplasm, and to determine associated responses in other agronomic traits. Testcrosses of random S1 lines from each cycle of selection were grown in replicated trials at four locations in each of 2 yr for determination of grain hardness and evaluation of agronomic traits. Evaluation of grain hardness was through measurement of average kernel density and the Stenvert hardness test. Significant increase in endosperm hardness was limited to the first cycle of selection in both synthetics. No significant changes corresponding to altered endosperm hardness were detected for any of five agronomic traits measured in testcross hybrids from S1 lines representing each cycle of selection. Estimated genetic correlation coefficients indicated a negative, nonsignificant trend between grain yield and grain hardness. These results indicate increases in grain hardness can be achieved through selection in dent corn germplasm common to the U.S. Corn Belt. However, continual progress in recurrent selection may require quantification of hardness by measurements such as kernel density and the Stenvert hardness test, rather than simple visual judgment of relative hardness. Genetic correlation coefficients indicate selection progress can be simultaneously made for both grain yield and grain quality.

Culture Associated Variation in Maize Inbreds

AbstractThe extent of culture associated variation generated by a brief passage through tissue culture was evaluated in four maize inbred backgrounds: A641, H99, MS7I, and PA91. The progeny resulting from two generations of self‐pollination from the original regenerates were evaluated along with appropriate sexual controls in a replicated field experiment. Eighty regenerate lines and twenty controls were evaluated in each background. Data were collected for 22 traits and were subjected to analysis of variance, discriminant analysis, and principal component analysis. In 58 of 88 combinations of background and trait, the contrast of controls vs. regenerates was significant. The culture cycle increased the variation and changed the association between traits. Testcross hybrids involving these regenerate lines and controls were also evaluated with little or no significant variation or differences between regenerate hybrids and controls being detected.