Mid-parent Heterosis For Grain Yield Research Articles

Dissecting Bread Wheat Heterosis through the Integration of Agronomic and Physiological Traits.

Simple SummaryTo meet the challenge of feeding almost 10 billion people by 2050, wheat yield has to double by 2050. However, over the past 20 years, yield increase has slowed down and even stagnated in the main producing countries. Similar to what has been observed in maize, hybrids have been suggested as a solution to overcome yield stagnation in wheat. However, wheat heterosis, i.e., the fact that a progeny surpasses the performances of its parents, is still limited and poorly understood. To better characterize this phenomenon, we developed and phenotyped for physiological and agronomic traits 91 hybrids and their nineteen female and sixteen male parents. We showed that hybrids had a longer grain filling phase that led to bigger grains and an increased thousand kernel weight. This resulted in a better yield for 86% of hybrids compared to the average yield of their parents. In addition, hybrids appeared to be less affected by the negative correlation between protein content and yield compared to pure lines. These results shed light on the physiological bases underlying yield heterosis in wheat, paving new ways to breed for better wheat hybrids that can help to meet agriculture’s challenges.To meet the challenge of feeding almost 10 billion people by 2050, wheat yield has to double by 2050. However, over the past 20 years, yield increase has slowed down and even stagnated in the main producing countries. Following the example of maize, hybrids have been suggested as a solution to overcome yield stagnation in wheat. However, wheat heterosis is still limited and poorly understood. Gaining a better understanding of hybrid vigor holds the key to breed for better varieties. To this aim, we have developed and phenotyped for physiological and agronomic traits an incomplete factorial design consisting of 91 hybrids and their nineteen female and sixteen male parents. Monitoring the plant development with normalized difference vegetation index revealed that 89% of the hybrids including the five higher yielding hybrids had a longer grain filling phase with a delayed senescence that results in larger grain size. This average increase of 7.7% in thousand kernel weight translated to a positive mid-parent heterosis for grain yield for 86% of hybrids. In addition, hybrids displayed a positive grain protein deviation leading to a +4.7% heterosis in protein yield. These results shed light on the physiological bases underlying yield heterosis in wheat, paving new ways to breed for better wheat hybrids.

Effects of drought stress on grain yield, agronomic performance, and heterosis of marker-based improved provitamin-A maize synthetics and their hybrids

ABSTRACT Provitamin A-enriched maize (Zea mays L.) is an important complementary food staple for combating vitamin A deficiency (VAD) in high maize-producing and maize-consuming countries of sub-Saharan Africa (SSA). However, frequent drought is a major abiotic factor that retards maize growth, resulting in yearly fluctuations in grain yield. Development of provitamin A-enriched maize varieties resilient to recurrent drought stress could enhance and stabilize maize grain yield. This study was conducted to assess the effects of managed drought stress (MDS) on the performance and heterosis of some marker-based improved provitamin A maize synthetics and their varietal-cross hybrids. The maize synthetics and their varietal-cross hybrids, along with a drought-tolerant check (PVASYN13), were evaluated under MDS and well-watered (WW) conditions at Ikenne, Nigeria, for two years. Genotype and year effects were significant for grain yield and some agronomic traits under MDS and WW conditions. Grain yield was reduced by 56% under MDS. Grain yield was significantly correlated with days to anthesis, days to silking and anthesis-silking-interval under MDS but not under WW condition. Under MDS, three varietal-cross hybrids (PVASYNHGBC0/PVASYNHGAC0, PVASYNHGBC2/PVASYNHGAC0, PVASYNHGBC0/ PVASYNHGAC1) had similar grain yields and tolerance indices as the drought-tolerant check, whereas PVASYNHGBC1/PVASYNHGAC2 produced 12.5% more grain yield than the check. Three of the varietal-cross hybrids (PVASYNHGBC0/PVASYNHGAC0, PVASYNHGBC0/PVASYNHGAC1 and PVASYNHGBC1/PVASYNHGAC2) had significant mid-parent heterosis for grain yield under the two test conditions, and were recommended for developing drought-tolerant varieties to combat VAD in drought-prone environments of SSA.

Assessing Effect of Marker-Based Improvement of Maize Synthetics on Agronomic Performance, Carotenoid Content, Combining Ability and Heterosis

Vitamin A deficiency (VAD) is a serious problem in sub-Saharan Africa (SSA) and other parts of the world. Understanding the effect of marker-based improvement (MARS) of two maize synthetics (HGA and HGB) representing different heterotic groups on their agronomic performance, carotenoid content, and combining abilities could help identify suitable sources to develop divergent inbred lines for optimizing heterosis. This study involved three selection cycles each of the two synthetics and their nine varietal-cross hybrids together with a released check variety was conducted across four diverse locations in Nigeria in 2018 and 2019. Environment and hybrid effects were significant on grain yield and other agronomic traits as well as provitamin A content and other carotenoids. Genetic improvement per cycle of MARS in the parental synthetics was 15% for provitamin A, 25% for β-carotene and 26% for lutein in HGA and 4% for grain yield, 3% for zeaxanthin and 5% for α-carotene in HGB. Grain yield and agronomic traits of the two maize synthetics were controlled by additive and non-additive gene effects, while provitamin A content and other carotenoids were mainly controlled by additive gene effects. Some selection cycles which were high in grain yield and provitamin A content were identified as potential sources of new and divergent maize inbred lines in maize breeding programs. Some varietal-cross hybrids expressed significant mid-parent heterosis for grain yield and moderate mid-parent heterosis for provitamin A, β-carotene and xanthophylls. These hybrids could be commercialized at reasonable prices to small-scale farmers in rural areas that are most affected by vitamin A deficiency.

Biplot analysis of diallel crosses for yield and some of morphological traits in wheat

A study of combining ability for improving bread wheat yield and yield components was carried out via genotype plus genotype × environment (GGE) biplot. Six bread wheat genotypes were used to obtain F1 in a diallel cross matings at the Gachsaran Field Station, Dryland Agricultural Research Institute during 2012–2013 cropping seasons. Analysis of variance indicated significant effects of genotypes, general combining ability (GCA) and specific combining ability (SCA) for all of the studied traits. The highest values of plant height (108.15 cm), peduncle length (33.95 cm) and grain yield (265.5 g m−1) were obtained in g6 (DAMARA-6 ICW99-0427-8AP-0AP-0AP-3AP-0AP). GGE biplot analysis showed that g4 (Kouhdasht) and g6 (DAMARA-6 ICW99-0427-8AP-0AP-0AP-3AP-0AP) were as the best general combiners for days to maturity, plant height and grain yield, whereas g1 (Karim) had the highest GCA effects for spike length and thousand grain weight, and g3 (Aflak) showed the highest GCA effects in terms of peduncle length. Meanwhile, g6 showed large SCA effects for thousand grain weight and grain yield, while g4 had high SCA effects for spike length. g5 (PASTOR/2*SITTA//PBW343*2/KUKUNACMSS04Y00333S-099Y-099ZTM-099Y-099 M-3WGY-0B) had large SCA effects for peduncle length. Because of significant effects of mid-parent heterosis for grain yield, g4 × g6 was considered as the best possible cross for improvement of this trait. Based on the results of present study, GGE biplot indicated good potential for identifying suitable parents and heterotic crosses and the best hybrids according to diallel mating design.

Hybrid durum wheat: heterosis of grain yield and quality traits and genetic architecture of anther extrusion.

Hybrid durum has a promising yield potential coupled with good quality, but the efficiency of hybrid seed production must be improved. Hybrid breeding is a tremendous success story in many crops, but has not yet made a breakthrough in wheat, mainly due to inefficient hybrid seed production. In this study, we investigated the heterosis for grain yield and important quality traits in durum wheat of 33 hybrids built up from 24 parental lines, as well as the variation in anther extrusion and its genetic architecture in a vast collection of Central European elite durum lines. Average mid-parent heterosis for grain yield was 5.8%, and the best hybrids had a more than one ton per hectare higher grain yield than the best line cultivars. Furthermore, hybrids had a higher grain yield than lines at a given level of protein content or sedimentation value, underpinning their potential for a sustainable agriculture. However, seed set in our experimental hybrid seed production was low. We therefore evaluated 315 elite durum lines for visual anther extrusion, which revealed a large genetic variance and a heritability of 0.66. Results from association mapping suggest a mainly quantitative inheritance of visual anther extrusion with few putative QTL being identified, the largest one explaining less than 20% of the genotypic variance. Genome-wide prediction taking the four largest putative QTL into account yielded a mean cross-validated prediction ability of 0.55. Consequently, breeding for improved male floral characteristics is feasible in durum wheat, but should be mainly based on phenotypic selection.

Combining ability and heterosis of elite drought-tolerant maize inbred lines evaluated in diverse environments of lowland tropics

Estimates of combining abilities and heterosis of inbred lines are imperative for selection of suitable parents of maize hybrids. This study examined the combining ability of 24 drought-tolerant maize inbred lines, 12 each from International Centre for Maize and Wheat Improvement (CIMMYT) and International Institute of Tropical Agriculture (IITA). The lines were allotted into six groups each comprising four lines. The four lines in one group were used as females and crossed to the four lines in another group as males in six different sets using a North Carolina Design II mating scheme to generate 96 hybrids. The hybrids were evaluated together with four checks across six environments in the rainforest and savannah agro-ecologies of Nigeria between 2011 and 2012. The parental inbred lines were also evaluated in separate trial in each location. Significant hybrids × environment interaction was observed for grain yield and other measured traits. GCA effects accounted for 83.3% of the variation for grain yield at Bagauda, 78.1% at Saminaka, and 77.7% at Ikenne. GCA also contributed 91.1 and 80.0% to the variation observed for plant height and ear aspect, respectively, across the environments. Significant SCA × environment interaction detected for grain yield suggests that hybrids were not stable across test environments. Prediction of grain yield in hybrids using midparent values resulted in a R2 value of 0.13. Midparent heterosis for grain yield varied from 80 to 411%, with the top 36 hybrids recording >200%. Four CIMMYT (EXL02, EXL06, EXL04 and EXL16) and three IITA (ADL33, ADL41, and ADL32) inbred lines had positive and significant GCA effects for grain yield across environments. The novel alleles present in the CIMMYT lines will improve the adapted IITA germplasm in a new population for extracting new set of more productive inbred lines for developing adapted high yielding drought-tolerant maize hybrids.

Estimation of heterosis in yield and yield attributing traits in single cross hybrids of maize

A field experiment was conducted at National Maize Research Program, Rampur, Chitwan, Nepal during winter season from 6th October, 2015 to 5th March 2016 to estimate different heterosis on single cross maize hybrids . Thirteen maize hybrids were tested randomized complete block design with three replications. Hybrid namely RML-98/RL-105 gave the highest standard heterosis (57.5%) for grain yield over CP-666 followed by RML-4/NML-2 (32.6%), RML-95/RL-105 (29%) and RML-5/RL-105 (20.6%). The hybrid RML-98/RL-105 produced the highest standard heterosis (75.1%) for grain yield over Rajkumar followed by RML-4/NML-2(50.2%), RML-95/RL-105(46.6%), RML-5/RL-105 and (35.7%). Mid and better parent heterosis were significantly higher for yield and yield attributes viz. ear length, ear diameter, no of kernel row per ear, no of kernel per row and test weight. The highest positive mid-parent heterosis for grain yield was found in RML-98/RL-105 followed by RML-5/RL-105, RML-95/RL-105, and RML-4/NML-2. For the grain yield the better parent heterosis was the highest in RML-98/RL-105, followed by RML-5/RL-105, RML-95/RL-105, and RML-4/NML-2. These results suggested that maize production can be maximized by cultivating hybrids namely RML-98/RL-105, RML-5/RL-105, RML-95/RL-105, and RML-4/NML-2.

Prediction of hybrid performance in maize with a ridge regression model employed to DNA markers and mRNA transcription profiles.

BackgroundRidge regression models can be used for predicting heterosis and hybrid performance. Their application to mRNA transcription profiles has not yet been investigated. Our objective was to compare the prediction accuracy of models employing mRNA transcription profiles with that of models employing genome-wide markers using a data set of 98 maize hybrids from a breeding program.ResultsWe predicted hybrid performance and mid-parent heterosis for grain yield and grain dry matter content and employed cross validation to assess the prediction accuracy. Prediction with a ridge regression model using random effects for mRNA transcription profiles resulted in similar prediction accuracies than employing the model to DNA markers. For hybrids, of which none of the parental inbred lines was part of the training set, the ridge regression model did not reach the prediction accuracy that was obtained with a model using transcriptome-based distances.ConclusionWe conclude that mRNA transcription profiles are a promising alternative to DNA markers for hybrid prediction, but further studies with larger data sets are required to investigate the superiority of alternative prediction models.

Genetic variation in sorghum as revealed by phenotypic and SSR markers: implications for combining ability and heterosis for grain yield

AbstractHybrid breeding relies on selection of genetically unrelated and complementary parents for key traits. The objective of this study was to examine genetic variation and identify unique sorghum genotypes using phenotypic and simple sequence repeat (SSR) markers and to determine their relationships with combining ability and heterosis for grain yield. A total of 32 landraces and four cytoplasmic male sterile (CMS) lines were phenotyped using 25 agro-morphological traits and genotyped with 30 polymorphic SSR markers. The landraces were crossed with four CMS lines using a line × tester mating design. The 128 hybrids, 36 parentals and four check varieties were field-evaluated using a 12 × 14 alpha lattice design with three replications. General combining ability (GCA), specific combining ability (SCA) and heterosis for grain yield were determined. Genetic distance estimates ranged from 0.39 to 0.60 and 0.50 to 0.79, based on phenotypic and SSR markers, respectively. Landraces 72572, 75454, 200654, 239175, 239208, 244735A and 242039B and CMS lines ICSA 743 and ICSA 756 displayed positive and significant GCA effects for grain yield. Based on the SCA effects of yield, lines were classified into three heterotic groups aligned to the different cytoplasmic systems of testers. Lines with high GCA effects rendered hybrids with highly significant SCA effects with high mid-parent heterosis (MPH) for grain yield. Both marker systems were effective in demarcating sorghum genotypes that provided desirable cross-combinations with high combining ability effects and MPH for grain yield. The selected genotypes are recommended as potential parents for sorghum hybrid breeding in moisture stress environments.

Heterosis and combining ability of elite maize inbred lines under northern corn leaf blight disease prone environments of the mid-altitude tropics

Yields of maize (Zea mays L.) are remarkably low in sub-Saharan Africa and yet farmers have limited access to improved varieties. The objective of this study was to determine combining ability and heterosis of 18 elite maize inbred lines in environments prone to northern corn leaf blight (NCLB) disease in the mid-altitude tropics. Nine elite inbred lines were crossed as females with nine male lines using the North Carolina Design II mating scheme and 81 hybrids were generated. The hybrids, parents and three standard check varieties were evaluated using an alpha lattice design with two replications across seven environments in the mid-altitude sub-humid agro-ecologies in Ethiopia. The new top ranking hybrids displayed up to 250 % high-parent heterosis and mid-parent heterosis for grain yield and up to −25 % high-parent mid-parent heterosis for NCLB reaction. The specific combining ability (SCA) by site interaction was not significant, suggesting that the top yielding hybrids were relatively stable across environments. Inbred lines such as CML 395, 30H83-7-1, ILO’OE-1-9, 124-b (113), CML202, CML312, and Gibe-1-91 resulted in significant SCA effects for grain yield were selected as promising parents. Lines CML395 and ILO’OE-1-9 had negative and significant general combining ability (GCA) effects for NCLB reaction, implying the resistance of the inbred lines for NCLB. The most performing experimental hybrids such as CML-312 × CML395, CML197 × CML395, CML443 × DE-78-Z-126-3 had high SCA effects and produced mean grain yields of >8 t ha−1. The new hybrids may be used directly for production or as testers in hybrid development with resistance to NCLB.

Yield performance and stability of CMS-based triticale hybrids.

CMS-based triticale hybrids showed only marginal midparent heterosis for grain yield and lower dynamic yield stability compared to inbred lines. Hybrids of triticale (×Triticosecale Wittmack) are expected to possess outstanding yield performance and increased dynamic yield stability. The objectives of the present study were to (1) examine the optimum choice of the biometrical model to compare yield stability of hybrids versus lines, (2) investigate whether hybrids exhibit a more pronounced grain yield performance and yield stability, and (3) study optimal strategies to predict yield stability of hybrids. Thirteen female and seven male parental lines and their 91 factorial hybrids as well as 30 commercial lines were evaluated for grain yield in up to 20 environments. Hybrids were produced using a cytoplasmic male sterility (CMS)-inducing cytoplasm that originated from Triticumtimopheevii Zhuk. We found that the choice of the biometrical model can cause contrasting results and concluded that a group-by-environment interaction term should be added to the model when estimating stability variance of hybrids and lines. midparent heterosis for grain yield was on average 3 % with a range from -15.0 to 11.5 %. No hybrid outperformed the best inbred line. Hybrids had, on average, lower dynamic yield stability compared to the inbred lines. Grain yield performance of hybrids could be predicted based on midparent values and general combining ability (GCA)-predicted values. In contrast, stability variance of hybrids could be predicted only based on GCA-predicted values. We speculated that negative effects of the used CMS cytoplasm might be the reason for the low performance and yield stability of the hybrids. For this purpose a detailed study on the reasons for the drawback of the currently existing CMS system in triticale is urgently required comprising also the search of potentially alternative hybridization systems.

Short communication: Heterosis and combining ability of Nepalese yellow maize (Zea mays L.)

Nepalese yellow maize inbred lines were characterized for general combining ability (GCA) and specific combining ability (SCA) of their crosses and parents were identified having high combining ability for determining the heterotic effects on yield and other quantitative traits of hybrids. A line x tester mating design was used for making the crosses in the winter season of 2008 and the hybrids along with their parents and four checks were evaluated in Randomized Complete Block Design (RCBD) with three replications in the spring season of 2009 at the experimental farm of the National Maize Research Program, Chitwan. Significant variations were found among the treatments for grain yield and other traits which indicated presence of high magnitude of genetic variations among tested inbred lines. The cross between RML-32 and RML-17 produced the highest grain yield (15870 kg/ha) among the crosses. Hybrids such as RL-194 x RL-84 (10770 kg/ha), RML-21 x RML-17 (9270 kg/ha), RL-180 x RML-17 (9270 kg/ha) and L-1 x RL-84 (8785 kg/ha) were found superior grain yielder. Thirty-nine hybrids showed positive mid-parent heterosis for grain yield among 40 crosses. The highest positive mid-parent heterosis for grain yield was found 880% in cross between RML-32 and RML-17, followed by RL-98X RML-17 (507%), RL-103 x RML-17 (403%), PUTU-18 x RML- 17 (351%) and RL-180 x RL 84 (316%). Agronomy Journal of Nepal (Agron JN) Vol. 3. 2013, Page 172-180 DOI: http://dx.doi.org/10.3126/ajn.v3i0.9020

High congruency of QTL positions for heterosis of grain yield in three crosses of maize

The genetic basis of heterosis in maize has been investigated in a number of studies but results have not been conclusive. Here, we compare quantitative trait loci (QTL) mapping results for grain yield, grain moisture, and plant height from three populations derived from crosses of the heterotic pattern Iowa Stiff Stalk Synthetic x Lancaster Sure Crop, investigated with the Design III, and analyzed with advanced statistical methods specifically developed to examine the genetic basis of mid-parent heterosis (MPH). In two populations, QTL analyses were conducted with a joint fit of linear transformations Z (1) (trait mean across pairs of backcross progenies) and Z (2) (half the trait difference between pairs of backcross progenies) to estimate augmented additive and augmented dominance effects of each QTL, as well as their ratio. QTL results for the third population were obtained from the literature. For Z (2) of grain yield, congruency of QTL positions was high across populations, and a large proportion of the genetic variance (~70%) was accounted for by QTL. This was not the case for Z (1) or the other two traits. Further, almost all congruent grain yield QTL were located in the same or an adjacent bin encompassing the centromere. We conclude that different alleles have been fixed in each heterotic pool, which in combination with allele(s) from the opposite heterotic pool lead to high MPH for grain yield. Their positive interactions very likely form the base line for the superior performance of the heterotic pattern under study.

Combining Ability, Genetic Distances, and Heterosis among Elite CIMMYT and IITA Tropical Maize Inbred Lines

Maize (Zea mays L.) breeding programs are increasingly using molecular tools to enhance the efficiency and speed of developing productive cultivars. As a preliminary step toward using each other's germplasm, 20 elite inbred lines, 10 each from CIMMYT and IITA, were crossed according to a modified North Carolina Design II and were genotyped using 62 simple sequence repeat (SSR) markers. The design used six sets of 25 hybrids, including all combinations among groups of five lines each from CIMMYT and IITA's respective heterotic groups A and B. The 150 single‐cross hybrids were evaluated in six environments in Zimbabwe and Nigeria during 2003 and 2004, with average grain yield ranging from 5.3 to 11.9 Mg ha−1 Combining ability analysis indicated that CML444 and CML312 can contribute to the breeding program of IITA, and that TZMI702 should be useful to CIMMYT. The SSR markers identified 209 alleles with an average 3.4 alleles locus−1 and revealed moderate genetic distances among the parent lines. Genetic distances were not significantly associated with hybrid performance, specific combining ability effects, or midparent heterosis for grain yield, but they effectively grouped the lines according to known pedigree relationships and approximately according to heterotic patterns used by CIMMYT and IITA. Although SSR‐determined genetic distances were not useful to predict hybrid grain yield, they could be helpful for preliminary classification and use of exotic or poorly characterized germplasm.

Estimates of heterosis and association of genetic distance with heterosis in durum wheat under different moisture regimes

The objectives of the present study were to evaluate heterosis for grain yield and yield components in durum wheat, and to assess the prediction potential of amplified fragment length polymorphism (AFLP) based and agronomic trait based genetic distances (GD and MD, respectively) to F1 performance, mid parent heterosis (MPH), and specific combining ability effects (SCA) under well-watered and moisture stress conditions. Six parental genotypes with different responses to moisture stress and their 15 F1 crosses were evaluated for their responses to moisture stress conditions in a glasshouse. Some cross combinations showed significant MPH for grain yield and yield components. The expression of heterosis for grain yield was greater under moisture stress conditions than under well-watered conditions. Cluster analysis of the parental lines based on agronomic performance under stress conditions was similar to cluster analysis result based on AFLP marker profiles. F1 performance was strongly correlated to both SCA effects and MPH under both stress and well water conditions. The correlation between SCA and MPH was very high under both treatment conditions for all traits. Correlation between GD and MD was significant only under stress conditions. Positive correlation was found only for the association between GD v. F1 performance and GD v. SCA effects for harvest index (HI) under well-watered conditions. None of the correlations between MD and SCA effects were significant. The absence of association between GD and heterosis for yield and most agronomic traits implied that heterozygosity per se diversity is not a good predictor of heterosis or F1 performance under both well-watered and stressed conditions.

Prospects for Hybrid Breeding in Winter Triticale: I. Heterosis and Combining Ability for Agronomic Traits in European Elite Germplasm

Triticale (×Triticosecale Wittmack) (genomes AABBRR, 2n = 6x = 42) hybrid breeding and heterosis have received increased attention in recent years, but a comprehensive study is lacking. We investigated (i) the level of heterosis, (ii) the relative importance of general combining ability (GCA) vs. specific combining ability (SCA), (iii) correlations between GCA and line per se performance, (iv) trait correlations in parents and hybrids, and (v) prospects for hybrid breeding. Two hundred nine F1 hybrids of winter triticale, produced by a chemical hybridizing agent, together with their 57 female parents and five tester (male) lines were evaluated in six environments in Germany during the season 2001–2002. Midparent heterosis for grain yield averaged 10.3% and varied from –11.4 to 22.4%, whereas better‐parent heterosis averaged 5.0% and varied from –16.8 to 17.4%. Midparent heterosis was also positive for 1000‐kernel weight, number of kernels per spike, test weight, and plant height but negative for number of spikes per square meter, falling number, and protein concentration. GCA variance σ̂2GCA was more important than SCA variance σ̂2SCA for all traits except grain yield and protein concentration. For most traits, GCA × location and SCA × location interaction variances were small relative to σ̂2GCA and σ̂2SCA, respectively. Genetic correlations between midparent and hybrid performance and between GCA effects and line per se performance showed similar trends, being moderate for grain yield and protein concentration and higher for the other traits. We concluded that grain yield heterosis in winter triticale crosses from parents in the current European germplasm pool is adequate to justify continuing research on hybrid breeding. By selecting parents for combining ability and establishing genetically diverse heterotic groups, a midparent grain yield heterosis of 20% could presumably be surpassed. Further information is needed on F1 seed production and the cytoplasmic male sterility system.

Heterotic Patterns among Flint Maize Populations

Most maize (Zea mays L.) hybrids cultivated in Europe follow the heterotic pattern European flint × Corn Belt dent. However, at present, no commercial hybrids have been obtained from European flint inbreds exclusively. A flint × flint heterotic pattern could be an alternative to use in breeding programs to obtain early maturity, high‐yielding hybrids, potentially useful for producing high quality flours. The objective of this work was to search for heterotic patterns within flint germplasm adapted to temperate conditions. Ten open‐pollinated flint maize cultivars were crossed in a diallel design without reciprocals and evaluated in four environments in northwestern Spain. Cultivar effects explained most of the sum of squares for traits analyzed, except for yield. Crosses yielded 30% more than parental cultivars. Yield of crosses ranged from 3.94 to 5.61 Mg ha−1 Midparent heterosis values ranged from 2.5 to 64.7%. The crosses ‘Gallego’ × ‘Basto/Enano Levantino’ and Basto/Enano Levantino × ‘Longfellow’ showed high heterosis and the best averages for grain yield. On the basis of these results, these crosses could be included in a recurrent reciprocal selection breeding program to increase heterosis among the cultivars and improve their hybrids. A cluster analysis made with midparent heterosis for grain yield as a measure of distance revealed that the flint germplasm probably had similar origins and were related to the ‘Northern Flint’ race. Some differences among these cultivars were detected. ‘Norteño Largo’ was closely related to Northern Flint, while Basto/Enano Levantino was different from the rest of the cultivars. It was concluded that the development of hybrids combining good yield and the typical agronomic characteristics of flint maize is feasible.

Heterotic Patterns among Flint Maize Populations

Most maize (Zea mays L.) hybrids cultivated in Europe follow the heterotic pattern European flint × Corn Belt dent. However, at present, no commercial hybrids have been obtained from European flint inbreds exclusively. A flint × flint heterotic pattern could be an alternative to use in breeding programs to obtain early maturity, high-yielding hybrids, potentially useful for producing high quality flours. The objective of this work was to search for heterotic patterns within flint germplasm adapted to temperate conditions. Ten open-pollinated flint maize cultivars were crossed in a diallel design without reciprocals and evaluated in four environments in northwestern Spain. Cultivar effects explained most of the sum of squares for traits analyzed, except for yield. Crosses yielded 30% more than parental cultivars. Yield of crosses ranged from 3.94 to 5.61 Mg ha−1 Midparent heterosis values ranged from 2.5 to 64.7%. The crosses ‘Gallego’ × ‘Basto/Enano Levantino’ and Basto/Enano Levantino × ‘Longfellow’ showed high heterosis and the best averages for grain yield. On the basis of these results, these crosses could be included in a recurrent reciprocal selection breeding program to increase heterosis among the cultivars and improve their hybrids. A cluster analysis made with midparent heterosis for grain yield as a measure of distance revealed that the flint germplasm probably had similar origins and were related to the ‘Northern Flint’ race. Some differences among these cultivars were detected. ‘Norteño Largo’ was closely related to Northern Flint, while Basto/Enano Levantino was different from the rest of the cultivars. It was concluded that the development of hybrids combining good yield and the typical agronomic characteristics of flint maize is feasible.

Diallel Analysis of Winter Wheat at Two Nitrogen Levels

Price reduction and environmental concerns advocate a lower use of nitrogen (N) fertilizer on the wheat (Triticum aestivum L.) crop. It is a common hypothesis that hybrids would be more valuable in stressed environments such as limited fertilizer conditions. The objective of this study was to assess heterosis and combining ability at two N levels. Seven winter wheat cultivars were used to produce a 7 × 7 diallel cross without the reciprocals. The 21 F1 hybrids and parental lines were tested in replicated plots over 2 yr without N fertilizer (NO) or with 150 kg N ha -1 (N+). The diallel analysis was conducted according to Griffing with year, genotype, and treatment as fixed effects. Mid-parent heterosis for grain yield was +12.2%** at NO and +8.9%** at N+ in 1997 and +1.7% ns at NO and -0.4% ns at N+ in 1998. This was directly related to high mid-parent heterosis for above-ground dry weight in 1997 (+11.2%** at NO and +10.9%** at N+) and low heterosis in 1998 (+1.2% ns at NO and + 0.0% ns at N + ). General (GCA) and specific (SCA) combining ability effects were always significant. The GCA/SCA ratio ranged from 3.6 to 14.8. The GCA x N level interaction was generally significant indicating different parental contributions at low or high N levels. The SCA x N level interaction was never significant. There was a tendency toward higher GCA/SCA ratio at NO than at N+. The choice of parents will be dependent upon the N level under which the new hybrids will be grown.

Genetic diversity in the parental lines and heterosis of the tropical rice hybrids

Genetic diversity within and between the maintainer (B) and restorer (R) lines used in hybrid breeding programs of the Philippine was investigated with information from analysis of pedigree record, quantitative traits and SSR assays. Mean coefficients of coancestry were calculated as 0.11 within R lines, 0.27 within B lines, and 0.04 between R and B lines, indicating greater diversity among R lines than among B lines, and the significant divergence between B and R lines. These results are consistent with those obtained from quantitative trait analysis and SSR marker assays. Relative gene diversity for 37 random SSR markers averaged 0.20 within B lines,0.28 within R lines, and 0.52 between two groups of lines. There were no consistent associations among various genetic diversity measures. Random sets of SSR marker and pedigree based diversity measures had no significant correlation with mid-parent heterosis for grain yield and biomass, indicating that prediction of heterosis for complex traits based on these two genetic diversity estimates is difficult.