Optimum allocation of test resources and comparison of alternative breeding schemes for hybrid maize breeding with doubled haploids

A major objective in hybrid maize breeding is the development of inbred lines with superior testcross performance. Inbred lines have commonly been derived in maize by recurrent selfing for five to six generations. The use of doubled haploids (DHs) enables the generation of completely homozygous lines in one step, representing a promising alternative to recurrent selfing. The implementation of the new DH technique in maize breeding requires an optimization of the entire breeding scheme in order to maximize progress from selection. The objectives of this study were to (i) compare selection gain (¢G) per breeding cycle with the probability of identifying superior genotypes with respect to the optimum allocation of test resources, (ii) evaluate several breeding schemes for an optimum use of the DH technique, (iii) determine the optimum number of test candidates and test locations as well the optimum type and number of testers for the different breeding schemes, and (iv) investigate the potential and limitations in the current DH technique in hybrid maize breeding. Monte Carlo simulations and numerical integration techniques were used to calculate the optimization criteria. The choice of G and the probability of identifying superior genotypes seems not to be crucial for the optimization of breeding schemes. The use of the new probability criterion supported the large optimum number of test locations determined by G. However, a larger impact of varying economic and quantitative-genetic parameters on the probability criterion than on ¢G was found, emphasizing their importance to maximize the chances of identifying a superior genotype. The use of Monte Carlo simulations for optimizing the allocation of test resources seems promising because of the possibility to calculate various optimization criteria for multi-stage selection in finite populations. However, the large computing power required for them can rapidly become prohibitive. Numerical integration techniques allow the calculation of G in multi-stage selection under the simplified assumption of infinite population size. The differences between finite and infinite population size were negligible for both, G and the optimum allocation of test resources. Thus, the simplifying assumption of infinite population size is justified as long as a tremendous reduction in computing time is warranted. Two-stage selection of DH lines was important to increase G and the probability of identifying superior genotypes, because it combines the evaluation of a large number of initial DH lines with the use of a large number of test locations. Consideration of an economic seed production indicated the necessity of separate breeding schemes for seed and pollen parent heterotic groups. For the pollen parent heterotic group, two-stage selection on testcross performance in both stages was most suitable, whereas for the seed parent heterotic group, line per se performance in the first stage followed by evaluation of testcross performance in the second stage was most appealing. The concentration of test resources on the most promising S1 families in early testing prior to DH production was superior to the evaluation of DH lines from the beginning of the selection process. The allocation of test resources was crucial to maximize G for a given scenario. Testers with broad genetic base allow a reduction of the number of testers in favor of an increased number of test locations and a largely increased G. An evaluation of progenies of each tester only in a single location instead of evaluating the progenies of each testers in all locations further increased G. With early testing prior to DH production, similar optimum numbers of testers and test locations were determined for evaluation of testcross performance of S1 families and DH lines within S1 families. This resulted in (i) a large optimum number of S1 families for the first stage and (ii) a small optimum number of S1 families but a large optimum number of DH lines within S1 families for the second stage. Current limitations in the DH technique with a low number of DH lines, which can be produced from a single maize plant, and high costs, affected the selection gain and the optimum allocation of test resources only marginally for breeding schemes with evaluation of DH lines from the beginning of the selection process. However, substantial improvements of the DH technique are required to realize the high potential of early testing prior to DH production in combination with a short cycle length. In conclusion, the optimum allocation of test resources is of utmost importance to increase selection gain under given economic resources. The implementation of DHs into maize breeding enables to shorten the length of the breeding cycle, but a careful evaluation of the breeding alternatives is required to maximize progress from selection.Die Entwicklung von Inzuchtlinien mit überlegener Testkreuzungsleistung ist eine der bedeutendsten Aufgaben in der Hybridmaiszüchtung. Üblicherweise werden Maisinzuchtlinien durch fortgesetzte Selbstbefruchtung in fünf bis sechs aufeinanderfolgenden Generationen hergestellt. Eine vielversprechende Alternative stellt die Technik der Erzeugung von Doppelhaploiden (DH) dar, mit deren Hilfe vollständig homozygote Linien in einem Schritt entwickelt werden können. Um den Zuchtfortschritt zu maximieren, erfordert die Einführung der DH-Technik in die Maiszüchtung eine Optimierung des gesamten Züchtungsgangs. Die Ziele unserer Studie waren: (i) die beiden Kriterien zur Bewertung des Zuchtfortschritts, nämlich den Selektionserfolg pro Zyklus und die Wahrscheinlichkeit, überlegene Genotypen zu identifizieren, hinsichtlich der optimalen Allokation von Testressourcen zu vergleichen, (ii) verschiedene Zuchtschemata für einen optimalen Einsatz der DH-Technik zu bewerten, (iii) die optimale Anzahl von Prüfkandidaten, Prüforten und Testern sowie den optimalen Testertyp zu bestimmen, und (iv) die Möglichkeiten und Grenzen der aktuellen DH-Technik zu untersuchen. Die Zielkriterien wurden mit Hilfe von Monte-Carlo-Simulationen und numerischen Integrationsverfahren berechnet. Die Wahl des Kriteriums zur Bewertung des Zuchtfortschritts, nämlich der Selektionserfolg vs. die Wahrscheinlichkeit, überlegene Genotypen zu identifizieren, hatte nur einen geringen Einfluß auf die Optimierung von Zuchtschemata. Die große Anzahl von Prüforten, die für den Selektionserfolg optimal war, wurde durch das Verwenden des Wahrscheinlichkeitskriteriums bestätigt. Allerdings wurde das Wahrscheinlichkeitskriterium stärker als der Selektionserfolg durch ökonomische und quantitativ-genetische Parameter beeinflusst, was deren Bedeutung für die Maximierung der Chancen, überlegene Genotypen zu identifizieren, unterstreicht. Monte-Carlo-Simulationen sind für die Optimierung der Allokation von Testressourcen geeignet, weil sie ermöglichen, verschiedene Zielkriterien für die Mehrstufenselektion in Populationen mit finiter Größe zu bestimmen. Allerdings kann der damit einhergehende hohe Rechenaufwand schnell zum begrenzenden Faktor werden. Der Selektionserfolg in der Mehrstufenselektion kann unter der vereinfachenden Annahme einer infiniten Populationsgröße mittels numerischer Integrationsverfahren bestimmt werden. Die Unterschiede in der optimalen Allokation von Testressourcen und dem Selektionserfolg zwischen den Berechnungen für finite und infinite Populationsgrößen waren vernachl¨assigbar klein. Somit ist die vereinfachende Annahme einer infiniten Populationsgröße gerechtfertigt, solange damit eine deutliche Reduktion der Rechenzeit verbunden ist. Der Selektionserfolg sowie die Wahrscheinlichkeit, überlegene Genotypen zu identifizieren, waren durch ökonomische und quantitativ-genetische Parameter nur begrenzt beeinflussbar. Dahingegen wurden beide Kriterien durch eine Zweistufenselektion von DH-Linien, bei der die Untersuchung einer großen Anzahl von Ausgangslinien mit dem Nutzen einer großen Anzahl von Prüforten kombiniert wird, beachtlich gesteigert. Die Berücksichtigung einer ökonomischen Saatgutproduktion erforderte die Verwendung unterschiedlicher Zuchtschemata für die heterotischen Gruppen der Saat- und Polleneltern. Für die Polleneltern war eine Zweistufenselektion auf Testkreuzungsleistung am besten geeignet, wohingegen sich für die Saateltern eine Selektion auf Linieneigenleistung in der ersten Selektionsstufe kombiniert mit einer Selektion auf Testkreuzungsleistung in der zweiten Selektionsstufe als überlegenes Zuchtschema erwies. Die Durchführung eines frühen Tests vor der DH-Produktion ermöglichte eine Konzentration der Testressourcen auf die viel versprechendsten S1-Familien, was der alleinigen Prüfung von DH-Linien während des gesamten Selektionsprozesses überlegen war. Der Nutzen von genetisch breiten Testern ermöglichte eine Reduktion der Testerzahl zu Gunsten einer gesteigerten Anzahl an Prüforten und eines stark erhöhten Selektionserfolgs. Die Untersuchung der Nachkommen jedes Testers an jeweils nur einem Ort anstelle der Prüfung aller Testkreuzungsnachkommen an allen Orten steigerte zusätzlich den Selektionserfolg. Vergleichbare Anzahlen von Testern und Prüforten waren für die Untersuchung von S1-Familien und DH-Linien optimal, wenn ein früher Test vor der Produktion von DH-Linien gemacht wurde. Dies führte dazu, dass in der ersten Selektionsstufe eine große Anzahl von S1-Familien, in der zweiten Selektionsstufe allerdings nur eine kleine Anzahl von S1-Familien mit jeweils einer großen Anzahl von DH-Linien innerhalb dieser S1-Familien optimal waren. Die Grenzen der aktuellen DH-Technik, insbesondere die geringe Anzahl von DH-Linien, die von einer Einzelpflanze produziert werden können, sowie die hohen Kosten beeinflussten den Selektionserfolg und die optimale Allokation der Testressourcen in Zuchtschemata, in denen ausschließlich DH-Linien getestet werden, kaum. Allerdings sind erhebliche Verbesserungen der DH-Technik nötig, um das große Potential des frühen Tests vor der DH-Produktion mit einer kurzen Zuchtzykluslänge zu vereinigen. Das Fazit ist: Die optimale Allokation der Testressourcen ist für die Maximierung des Selektionserfolgs unter gegebenen ökonomischen Rahmenbedingungen von außerordentlich großer Bedeutung. Die Einführung von Doppelhaploiden in die Maiszüchtung ermöglicht zwar eine Verkürzung der Zuchtzykluslänge, allerdings ist für eine Maximierung des Zuchtfortschritts die sorgfältige Abwägung verschiedener Zuchtalternativen von Nöten

Entwicklung von Material und Strategien für eine nachhaltige Qualitätszüchtung von Winter-Emmer (Triticum diccoccum) für den ökologischen Landbau in Deutschland

47 Emmersorten wurden gemeinsam mit den im ökologischen Landbau wichtigsten Weizen- und Dinkelsorten an 8 Öko- und 7 konventionellen Standorten getestet. In den Projektversuchen wurden 10 Merkmale genauer bestimmt. Für alle Merkmale konnten signifikante genetische Varianzen festgestellt werden. Die Heritabilität der Merkmale Wuchshöhe, Ährenschieben, Gelbrost, Frost und Sedimentationsvolumen waren sehr hoch. Dahingegen wurden geringere Heritabilitäten für Ertrag, Proteingehalt sowie Resistenz gegenüber Blattflecken festgestellt. Es bietet sich also an, in den frühen Züchtungsgenerationen vor allem auf Standfestigkeit und Frosttoleranz zu selektieren. Ertrag und Proteingehalt sollten erst anhand mehrortiger Parzellenversuche bestimmt und selektiert werden. Es wurde eine hohe Korrelation bei allen Merkmalen zwischen den beiden Anbaualternativen ökologisch und extensiv konventionell festgestellt. Somit bietet sich an, das Emmerzuchtprogramm nur unter einer Anbaualternative zu führen und dabei indirekt auf die andere mit zu selektieren. Die besten Emmersorten sind vom Ertragsniveau, der Standfestigkeit sowie der Krankheitsresistenzen vergleichbar mit der alten aber sehr populären Dinkelsorte Oberkulmer Rotkorn. In der Qualitätsanalytik fielen insbesondere die Verfahren, die auf der Analyse der Glutenstruktur bzw. -qualität und dessen Kohäsivität zielen als schwierig bei Emmer-Mehlen auf. Diese Eigenschaften können möglicherweise auf die Abwesenheit der Puroindoline in dieser Art zurückgeführt werden. Zur weitergehenden Charakterisierung konnte ein Mikro-Backversuch speziell für Emmer-Mehle entwickelt werden. Anhand der Ergebnisse wurden 10 Emmer-Sorten mit guten bis sehr guten Backeigenschaften identifiziert. Alle Sorten sprechen auf eine eher schonende Teigbereitung positiv an. Die Teigbereitung sollte daher empfehlenswert bei Temperaturen um +20°C mit geringem mechanischem Energieeintrag erfolgen. Durch ein gering ausgeprägtes Gashaltevermögen und die vorliegende geringe Kohäsivität des Emmer-Glutens sollte auch die Gärzeit kurz gewählt werden

No correlation between amylase/trypsin-inhibitor content and amylase inhibitory activity in hexaploid and tetraploid wheat species

Wheat amylase/trypsin-inhibitors (ATI) are known triggers for wheat-related disorders. The aims of our study were to determine (1) the inhibitory activity against different α-amylases, (2) the content of albumins and globulins (ALGL) and total ATI and (3) to correlate these parameters in wholegrain flour of hexaploid, tetraploid and diploid wheat species. The amount of ATI within the ALGL fraction varied from 0.8% in einkorn to 20% in spelt. ATI contents measured with reversed-phase high-performance liquid chromatography (RP-HPLC) revealed similar contents (1.2–4.2 mg/g) compared to the results determined by LC-MS/MS (0.2–5.2 mg/g) for all wheat species except einkorn. No correlation was found between ALGL content and inhibitory activity. In general, hexaploid cultivars of spelt and common wheat had the highest inhibitory activities, showing values between 897 and 3564 AIU/g against human salivary α-amylase. Tetraploid wheat species durum and emmer had lower activities (170–1461 AIU/g), although a few emmer cultivars showed similar activities at one location. In einkorn, no inhibitory activity was found. No correlation was observed between the ATI content and the inhibitory activity against the used α-amylases, highlighting that it is very important to look at the parameters separately

Association of progeny variance and genetic distances among parents and implications for the design of elite maize breeding programs

Choice of crosses is crucial for a successful and sustainable management of breeding programs. Our objectives were to (1) investigate the association between the Rogers’ distances among parents and the genetic variance within their crosses (σ2 within) in elite maize breeding germplasm, (2) study whether this association can be improved selecting trait-specific markers, and (3) evaluate the consequences to implement the usefulness criterion based on Rogers’ distances on the optimum choice of crosses. Testcross performance of eleven segregating crosses with a total of 930 progenies was evaluated in six environments for grain yield (GY) and grain moisture content (GMC). Moreover, the 930 genotypes were fingerprinted with 425 polymorphic SNP markers. Our findings revealed that working within a heterotic group, σ2 within increased with increasing Rogers’ distances among the parents. This was more pronounced for GY (rP = 0.55 P < 0.1) compared to GMC (rP = 0.17). Selecting trait specific markers, which were associated with putative QTL affecting these traits, led for GY to a decrease in the association between σ2 within and Rogers’ distances among the parents. Consequently, using for GY a regression model based on Rogers’ distances estimated with an unselected set of markers allows a rough implementation of the usefulness criterion in maize breeding programs. Our model calculations suggested that implementing the usefulness criterion will facilitate a broadening of the diversity of elite maize breeding pools by counterbalancing a reduction in parental performance with an increase in σ2 within

Estimation of quantitative genetic and stability parameters in maize under high and low N levels

AB It is important to breed maize (Zea mays L) cultivars with high performance under variable N levels. We studied the effect of N levels and estimated quantitative genetic parameters for grain yield, quality, and other traits, and examined stability of performance for grain yield in diverse Chinese maize germplasm. From 2006 to 2008, each year 20 and in total 30 maize hybrids, including commercial hybrids currently grown in this region and other ex¬perimental hybrids as well as high-oil hybrids, were tested using nine environments (location-year combinations) in North China Plain. In each environment, two replicated trials were grown: one under high N application rate (HN, 225 kg N ha-1) and the other under low N application rate (LN, no N fertilization). Compared to HN, grain yield was significantly reduced (35.6%) under LN level, as well as kernel number per ear, 1000-kernel weight, plant and ear heights, and protein concentration. In the analysis over environments under each N level, genotypic variance was significant and heritability was high for all traits. In the analyses across N levels and environments, genotypic variance was significant for all traits and larger than the genotype × N and/or environment interaction variance components except for protein concentration. In stability analyses across N levels, hybrids differed for their linear response to environments, and some showed dissimilar response under HN and LN levels. Our results indicated that breeding maize adapted to variable N levels is feasible with the Chinese germplasm available in the summer breeding programs in North China Plain. Multi-environment tests are required to identify hybrids with high grain yield under variable N conditions, and examining yield stability separately under HN and LN would be useful

Optimum allocation of resources for QTL detection using a nested association mapping strategy in maize

In quantitative trait locus (QTL) mapping studies, it is mandatory that the available financial resources are spent in such a way that the power for detection of QTL is maximized. The objective of this study was to optimize for three different fixed budgets the power of QTL detection 1 − β* in recombinant inbred line (RIL) populations derived from a nested design by varying (1) the genetic complexity of the trait, (2) the costs for developing, genotyping, and phenotyping RILs, (3) the total number of RILs, and (4) the number of environments and replications per environment used for phenotyping. Our computer simulations were based on empirical data of 653 single nucleotide polymorphism markers of 26 diverse maize inbred lines which were selected on the basis of 100 simple sequence repeat markers out of a worldwide sample of 260 maize inbreds to capture the maximum genetic diversity. For the standard scenario of costs, the optimum number of test environments (Eopt) ranged across the examined total budgets from 7 to 19 in the scenarios with 25 QTL. In comparison, the Eopt values observed for the scenarios with 50 and 100 QTL were slightly higher. Our finding of differences in 1 − β* estimates between experiments with optimally and sub-optimally allocated resources illustrated the potential to improve the power for QTL detection without increasing the total resources necessary for a QTL mapping experiment. Furthermore, the results of our study indicated that also in studies using the latest genomics tools to dissect quantitative traits, it is required to evaluate the individuals of the mapping population in a high number of environments with a high number of replications per environment

The Global Durum Wheat Panel (GDP): An International Platform to Identify and Exchange Beneficial Alleles

Representative, broad and diverse collections are a primary resource to dissect genetic diversity and meet pre-breeding and breeding goals through the identification of beneficial alleles for target traits. From 2,500 tetraploid wheat accessions obtained through an international collaborative effort, a Global Durum wheat Panel (GDP) of 1,011 genotypes was assembled that captured 94-97% of the original diversity. The GDP consists of a wide representation of Triticum turgidum ssp. durum modern germplasm and landraces, along with a selection of emmer and primitive tetraploid wheats to maximize diversity. GDP accessions were genotyped using the wheat iSelect 90K SNP array. Among modern durum accessions, breeding programs from Italy, France and Central Asia provided the highest level of genetic diversity, with only a moderate decrease in genetic diversity observed across nearly 50 years of breeding (1970-2018). Further, the breeding programs from Europe had the largest sets of unique alleles. LD was lower in the landraces (0.4 Mbp) than in modern germplasm (1.8 Mbp) at r 2 = 0.5. ADMIXTURE analysis of modern germplasm defined a minimum of 13 distinct genetic clusters (k), which could be traced to the breeding program of origin. Chromosome regions putatively subjected to strong selection pressure were identified from fixation index (F st ) and diversity reduction index (DRI) metrics in pairwise comparisons among decades of release and breeding programs. Clusters of putative selection sweeps (PSW) were identified as co-localized with major loci controlling phenology (Ppd and Vrn), plant height (Rht) and quality (gliadins and glutenins), underlining the role of the corresponding genes as driving elements in modern breeding. Public seed availability and deep genetic characterization of the GDP make this collection a unique and ideal resource to identify and map useful genetic diversity at loci of interest to any breeding program

Breeding for increased nitrogen-use efficiency: a review for wheat (T. aestivum L.)

Nitrogen fertilizer is the most used nutrient source in modern agriculture and represents significant environmental and production costs. In the meantime, the demand for grain increases and production per area has to increase as new cultivated areas are scarce. In this context, breeding for an efficient use of nitrogen became a major objective. In wheat, nitrogen is required to maintain a photosynthetically active canopy ensuring grain yield and to produce grain storage proteins that are generally needed to maintain a high end-use quality. This review presents current knowledge of physiological, metabolic and genetic factors influencing nitrogen uptake and utilization in the context of different nitrogen management systems. This includes the role of root system and its interactions with microorganisms, nitrate assimilation and its relationship with photosynthesis as postanthesis remobilization and nitrogen partitioning. Regarding nitrogen-use efficiency complexity, several physiological avenues for increasing it were discussed and their phenotyping methods were reviewed. Phenotypic and molecular breeding strategies were also reviewed and discussed regarding nitrogen regimes and genetic diversity

Hybrid wheat: quantitative genetic parameters and heterosis for quality and rheological traits as well as baking volume

KEY MESSAGE: Heterosis effects for dough quality and baking volume were close to zero. However, hybrids have a higher grain yield at a given level of bread making quality compared to their parental lines. Bread wheat cultivars have been selected according to numerous quality traits to fulfill the requirements of the bread making industry. These include beside protein content and quality also rheological traits and baking volume. We evaluated 35 male and 73 female lines and 119 of their single-cross hybrids at three different locations for grain yield, protein content, sedimentation value, extensograph traits and baking volume. No significant differences (p &lt; 0.05) were found in the mean comparisons of males, females and hybrids, except for higher grain yield and lower protein content in the hybrids. Mid-parent and better-parent heterosis values were close to zero and slightly negative, respectively, for baking volume and extensograph traits. However, the majority of heterosis values resulted in the finding that hybrids had higher grain yield than lines for a given level of baking volume, sedimentation value or energy value of extensograph. Due to the high correlation with the mid-parent values (r &gt; 0.70), an initial prediction of hybrid performance based on line per se performance for protein content, sedimentation value, most traits of the extensograph and baking volume is possible. The low variance due to specific combining ability effects for most quality traits points toward an additive gene action requires quality selection within both heterotic groups. Consequently, hybrid wheat can combine high grain yield with high bread making quality. However, the future use of wheat hybrids strongly depends on the establishment of a cost-efficient and reliable seed production system