Marianne Bänziger

Prediction of genetic values of quantitative traits in plant breeding using pedigree and molecular markers

The availability of dense molecular markers has made possible the use of genomic selection (GS) for plant breeding. However, the evaluation of models for GS in real plant populations is very limited. This article evaluates the performance of parametric and semiparametric models for GS using wheat (Triticum aestivum L.) and maize (Zea mays) data in which different traits were measured in several environmental conditions. The findings, based on extensive cross-validations, indicate that models including marker information had higher predictive ability than pedigree-based models. In the wheat data set, and relative to a pedigree model, gains in predictive ability due to inclusion of markers ranged from 7.7 to 35.7%. Correlation between observed and predictive values in the maize data set achieved values up to 0.79. Estimates of marker effects were different across environmental conditions, indicating that genotype × environment interaction is an important component of genetic variability. These results indicate that GS in plant breeding can be an effective strategy for selecting among lines whose phenotypes have yet to be observed.

Crops that feed the world 6. Past successes and future challenges to the role played by maize in global food security

Maize is one of the most important food crops in the world and, together with rice and wheat, provides at least 30% of the food calories to more than 4.5 billion people in 94 developing countries. In parts of Africa and Mesoamerica, maize alone contributes over 20% of food calories. Maize is also a key ingredient in animal feed and is used extensively in industrial products, including the production of biofuels. Increasing demand and production shortfalls in global maize supplies have worsened market volatility and contributed to surging global maize prices. Climatic variability and change, and the consequent rise in abiotic and biotic stresses, further confound the problem. Unless concerted and vigorous measures are taken to address these challenges and accelerate yield growth, the outcome will be hunger and food insecurity for millions of poor consumers. We review the research challenges of ensuring global food security in maize, particularly in the context of climate change. The paper summarizes the importance of maize for food, nutrition and livelihood security and details the historical productivity of maize, consumption patterns and future trends. We show how crop breeding to overcome biotic and abiotic stresses will play a key role in meeting future maize demand. Attention needs to be directed at the generation of high yielding, stress-tolerant and widely-adapted maize varieties through judicious combination of conventional and molecular breeding approaches. The use of improved germplasm per se will not, however, be enough to raise yields and enhance adaptation to climate change, and will need to be complemented by improved crop and agronomic practices. Faced with emasculated state extension provision and imperfect markets, new extension approaches and institutional innovations are required that enhance farmers’ access to information, seeds, other inputs, finance and output markets. Over the long-term, large public and private sector investment and sustained political commitment and policy support for technology generation and delivery are needed to overcome hunger, raise the incomes of smallholder farmers and meet the challenges of growing demand for maize at the global level.

The potential for increasing the iron and zinc density of maize through plant-breeding

The Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) maize-breeding programme has been focusing on identifying white-grained maize germplasm that has the potential to increase kernel iron and zinc concentrations, especially in sub-Saharan Africa. In addition, research at Cornell University has focused on traits such as multiple aleurone layer, which can increase kernel iron and zinc concentrations, and low phytic acid concentration, which holds promise for improving the bioavailability of iron and zinc. More than 1,400 improved maize genotypes and 400 landraces were grown and evaluated to assess grain iron and zinc concentrations. These materials represented all white-grained landraces that belonged to the core collection of CIM-MYTs germplasm bank, all white- and yellow-grained CIMMYT maize germplasm pools and populations, all white-grained materials that are currently in the active breeding programme of CIMMYT-Zimbabwe, and 57 white-grained maize cultivars currently grown in southern Africa. After a very thorough evaluation of the genetic variability of iron and zinc potentially available in white-grained tropical maize germplasm, promising genetic variability was found in both improved maize germplasm and landraces. One difficulty that maize breeders encounter is that grain iron and zinc concentrations are often correlated negatively with grain yield, which may result from the increased carbohydrate content of high-yielding materials, so that a given amount of iron and zinc is diluted. The multiple aleurone trait may be a fast track to overcome this effect. This trait is being introgressed into various materials in both the United States and southern Africa.

Breeding for low input conditions and consequences for participatory plant breeding: examples from tropical maize and wheat

Participatory plant breeding (PPB) has been suggested as an effective alternative to formal plant breeding (FPB) as a breeding strategy for achieving productivity gains under low input conditions. With genetic progress through PPB and FPB being determined by the same genetic variables, the likelihood of success of PPB approaches applied in low input target conditions was analyzed using two case studies from FPB that have resulted in significant productivity gains under low input conditions: (1) breeding tropical maize for low input conditions by CIMMYT, and (2) breeding of spring wheat for the highly variable low input rainfed farming systems in Australia. In both cases, genetic improvement was an outcome of long-term investment in a sustained research effort aimed at understanding the detail of the important environmental constraints to productivity and the plant requirements for improved adaptation to the identified constraints, followed up by the design and continued evaluation of efficient breeding strategies. The breeding strategies used differed between the two case studies but were consistent in their attention to the key determinants of response to selection: (1) ensuring adequate sources of genetic variation and high selection pressures for the important traits at all stages of the breeding program, (2) use of experimental procedures to achieve high levels of heritability in the breeding trials, and (3) testing strategies that achieved a high genetic correlation between performance of germplasm in the breeding trials and under on-farm conditions. The implications of the outcomes from these FPB case studies for realizing the positive motivations for adopting PPB strategies are discussed with particular reference for low input target environment conditions.

Physiological mechanisms contributing to the increased N stress tolerance of tropical maize selected for drought tolerance

Abstract An improved response of crop varieties to various stress factors may be associated to constitutive stress tolerance mechanisms that increase yield and yield stability. Increased leaf longevity, increased water and nutrient uptake, greater assimilate supply during grain filling, and increased grain and ear set have been associated with constitutive stress tolerance mechanisms in maize ( Zea mays L.). We examined tropical maize for adaptive changes associated with drought tolerance that are sustained under N stress and therefore may indicate constitutive stress tolerance mechanisms. Original and drought-tolerant selection cycles of four populations were evaluated in five experiments differing in N supply at Poza Rica, Mexico between 1992 and 1994. Selection for tolerance to mid-season drought stress consistently increased grain yield across N levels due to an increase in both the number of ears per plant and kernel weight. The number of ears per plant was associated with a shorter anthesis–silking interval (ASI) of drought-tolerant cycles. Reduction in ASI due to selection was greater under N stress as compared to well-fertilized conditions, however, it was not associated with either biomass or N accumulation of plants and ears around flowering. The N content of individual kernels did not change with selection and grain N concentration decreased. Greater kernel weights were likely the result of delayed leaf senescence and increased assimilate supply during grain filling. We conclude that decreased ear abortion and increased assimilate supply during grain filling of maize selected for tolerance to mid-season drought also provide tolerance to N stress and therefore may contribute to increased yield and yield stability.

Quantitative trait loci for yield and correlated traits under high and low soil nitrogen conditions in tropical maize

The first objective of this study was to map and characterize quantitative trait loci (QTL) for grain yield (GY) and for secondary traits under varying nitrogen (N) supply. To achieve this objective, a segregating F2:3 population previously developed for QTL mapping under water-limited conditions was used. The population was evaluated in Mexico under low N conditions in the dry winter season and under low and high N conditions in the wet summer season. From eight QTLs identified for GY under low N conditions, two were also detected under high N conditions. Five QTLs were stable across the two low N environments and five co-localized with QTLs identified for the anthesis-silking interval (ASI) or for the number of ears per plant (ENO) under low N conditions. The percentage of the phenotypic variance expressed by all QTLs for ASI and ENO was quite different when evaluated under low N conditions during the dry winter (40% for ASI and 22% for ENO) and the wet summer seasons (22% for ASI and 46% for ENO). The results suggest optimizing different breeding strategies based on selection index depending on the growing season. Good QTL colocalization was observed for ASI (four QTLs) and ENO (three QTLs) when looking at QTL identified under low N and water-limited conditions in the same population. The results suggest that that both secondary traits can be used in breeding programs for simultaneous improvement of maize against low N and drought stresses.

Secondary traits in parental inbreds and hybrids under stress and non-stress environments in tropical maize

Abstract Secondary traits other than grain yield (GY) have been successfully used to enhance the rate of genetic improvement for maize (Zea mays L.) populations under abiotic stresses. With increasing trend towards the development of hybrids in the tropics there is a need to understand the genetics of these traits in inbred–hybrid breeding systems. The objectives of this study were to estimate the general combining abilities for secondary traits and their relationship with GY in a group of tropical white inbred lines and their hybrids under stress and non-stress environments. Traits were measured in inbred lines and their hybrid combinations across stress and non-stress subtropical and tropical environments in Mexico. Hybrids showed earlier flowering, taller plants, more ears per plant (EPP), higher shelling percentage, slower leaf senescence, and higher leaf chlorophyll content than inbreds under all environments. La Posta Sequia (LP)-derived inbreds in general had desirable GCA for traits such as anthesis–silking interval (ASI), EPP, and leaf senescence but were more susceptible to foliar diseases. Conventionally bred maize lines had better standability, stay green, and resistance to diseases. Higher GY in lines and hybrids was associated with shorter ASI, earlier flowering, increased plant and ear height, increased EPP, increased shelling percentage, delayed senescence, and greater leaf chlorophyll concentrations. Under stress vs. non-stress conditions, we observed a higher variability for ASI and EPP, a higher inbred–hybrid correlation, and significant correlations between these traits and GY. Inbreds from LP appear to have high frequency of favorable dominant alleles for ASI and EPP.

RECENT ADVANCES IN BREEDING MAIZE FOR DROUGHT AND SALINITY STRESS TOLERANCE

Maize production losses due to drought and salinity prominently affect economies and the livelihoods of millions of people, given the global and regional importance of maize and its pronounced susceptibility to these stress factors. Climate change and accelerating competition for irrigation water are expected to further increase the need for adaptive strategies. There is vast evidence for genetic approaches being able to significantly improve the drought and salinity tolerance of maize. Field-based breeding approaches have resulted in average breeding gains of around 100 kg ha-1 yr-1 under drought conditions, and there are first reports on transgenic drought and salinity tolerance mechanisms increasing maize grain yields under laboratory and field conditions. Drought and salinity tolerance are based on complex genetic systems and successful genetic enhancement programs need to consider gene-by-gene, gene-by environment and gene-by-developmental stage interactions. In the case of drought, field-based and transgenic approaches have resulted in the improvement of diverse and potentially additive tolerance mechanisms. Increasing yields and yield stability of maize in the face of climate change and scarcity of irrigation water will therefore likely be the most successful if complementary investments in field-based and transgenic breeding approaches are being made

Environmental classification of maize-testing sites in the SADC region and its implication for collaborative maize breeding strategies in the subcontinent

When evaluating genotypes, it is efficient and resourceful to identify similar testing sites and group them according to similarity. Grouping sites ensures that breeders choose as many variable sites as possible to capture the effects of genotype-by-environment (GE) interactions. In order to exploit these interactions and increase testing efficiency and variety selection, it is necessary to group similar environments or mega-environments. The present mega-environments in the Southern African Development Community (SADC) countries are confounded within each country, which limits the exchange of germplasm among them. The objective of this study was to revise and group similar maize-testing sites across the SADC countries that are not confounded within each country. The study was based on 3 years (1999–2001) of regional maize yield trial data and geographical information systems (GIS) parameters from 94 sites. Sequential retrospective (Seqret) pattern analysis methodology was used to stratify testing sites and group them according to their similarity and dissimilarity based on mean grain yield. The methodology used historical data, taking into account imbalances of data caused by changes over locations and years, such as additions and omission of genotypes and locations. Cluster analysis grouped regional trial sites into seven mega-environments, mainly distinguished by GIS parameters related to rainfall, temperature, soil pH, and soil nitrogen with an overall R2 = 0.70. This analysis provides a challenge and an opportunity to develop and deploy maize germplasm in the SADC region faster and more effectively.

Recent advances in breeding for drought tolerance in maize

Drought is an important cause of instability of national maize grain yields and of the food supply and economy of small-scale maize-based farming systems in the tropics. Water shortages affect maize yields throughout the crop cycle, but most severely at flowering and to a lesser degree at establishment. Maize is often grown in environments thought to be better suited to sorghum and millet, yet because farmers in these areas persist with maize, researchers have an obligation to help stabilize maize production. Recurrent selection, which focused mainly on increasing tolerance to drought during flowering and grainfilling, has successfully increased grain yield over a wide range of moisture regimes. Gains ranged from 80-108 (mean 94) kg ha−1 yr−1 when selecting among full-sib (FS) families to 73–144 (mean 111) kg ha−1 yr−1 when selecting among S, families, and were observed when selections were evaluated at a drought-induced yield level of 1.5–2.4 ton ha−1. This represents annual gains in severely droughted environments of >5%. Under well-watered conditions where yields ranged from 5.6-8.0tonha−1, gains from the same two selection schemes ranged from 38–108 (mean 73)kgha−1 yr−1 and from 27–89 (mean 59)kgha−1 yr−1, respectively. Gains in yield were mainly due to increased numbers of grains per plant, and were associated with a reduced anthesis-silking interval (ASI) and an increased harvest index. These changes are consistent with increased assimilate partitioning to the ear at flowering, a phenomenon that will need to be addressed by maize models that focus on genotypic responses to stress. Little change was observed in plant water status or foliar senescence rates. Drought-tolerant varieties have shown similar gains when evaluated under low N conditions, suggesting that partitioning of N to the ear has also been improved. The challenge now is to develop drought-tolerant hybrids. Conventionally improved populations have provided a lower frequency of drought-tolerant hybrids than their counterparts with a history of improvement for drought tolerance. Molecular marker-based linkage maps of ASI, an easily observed indicator of drought tolerance at flowering, suggest that this trait could be efficiently transferred to elite inbred lines using marker-assisted backcrossing techniques. Yields of drought-tolerant varieties, lines and hybrids, when grown under well-watered conditions, have shown clearly that drought tolerance does not come at the cost of yield potential. The key to efficient improvement for drought-tolerance lies in the choice of elite adapted germplasm, use of carefully managed drought stress, and selection based on a minimum data set comprising anthesis date, ASI, ears per plant and shelled grain yields. This results in stabilized and improved yield for the maize component of maize-based cropping systems exposed to mid-season and terminal drought in highly variable rainfall environments.