Conventional Breeding Strategies Research Articles

An Overview of Methods for Assessment of Iron Bioavailability from Foods Nutritionally Enhanced Through Biotechnology

Iron deficiency and iron deficiency anemia continue to be significant public health problems worldwide. While supplementation and fortification have been viable means to improve iron nutriture of the population in developed countries, they may be less successful in developing regions for a number of reasons, including complexities in distribution and consumer compliance. Biofortification of staple crops, through conventional plant breeding strategies or modern methods of biotechnology, provides an alternative approach that may be more sustainable once initial investments have been made. Three types of biofortification strategies are being essayed, singly or in combination: increasing the total iron content of edible portions of the plant, decreasing the levels of inhibitors of iron absorption, and increasing the levels of factors that enhance iron absorption. Bioavailability is a key concept in iron nutrition, particularly for nonheme iron such as is found in these biofortified foods. An overview is presented of methods for evaluation of iron bioavailability from foods nutritionally enhanced through biotechnology.

Sustainability the food chain over genetic improvement of the quantity and quality of soybean grain

This study was aimed to evaluate level and stability of grain yield and grain quality as well as adaptability of exotic and domestic soybean cultivars, and identify the most favorable parental combinations as basis for realizing further genetic improvement of soybean. The research was conducted at the experimental field of the Agricultural Institute Osijek (Croatia) over 4-years period and consisted od 30 soybean cultivars: 15 cultivars originated from the soybean breeding program at the Institute Osijek (OS-cultivars) and 15 cultivars originated from the soybean breeding program at the University of Guelph (Canada) (CA-cultivars). OS-cultivars and CA-cultivars with high agronomic performance in grain yield, protein and oil content are identified as the most favorable parents to be used in hybridization for development new, genetically more diverse cultivars, improved in level and stability of quantity and quality of grain, as well as broad adaptability. Also, these results will be enable strategic incorporation of diversity from exotic Canadian germplasm into the domestic Croatian germplasm to develop new better cultivars. Genetic advance in quantity and quality of soybean grain considerably will contribute both further improving and increasing of soybean production, as well as production of safe product high nutritive and functional value for consumers, as well. Hence, genetic improvement of cultivars, based on conventional breeding strategies, could be characterized as an integral part of sustainable food chain.

The welfare implications of animal breeding and breeding technologies in commercial agriculture

The commercial applications in agriculture of new breeding technologies, as well as conventional breeding strategies, have the potential to influence animal welfare in both positive and negative ways. For example, the sexing of cattle semen might be used to reduce the number of unwanted male dairy calves provided that the technique had not been shown to produce adverse effects. On the other hand, inappropriate use of some breeding technologies may create new problems, or exacerbate welfare problems that may already have arisen within conventional livestock breeding. It is the impact of any breeding technology or strategy that is important to welfare, whether it is the quality of life of the offspring that is compromised, or whether it is the application of the technology itself that causes pain, distress or lasting harm to the subject animal. The aim of this paper is to provide clear and practical advice on the establishment of an appropriate framework within which developments in animal breeding and breeding technologies, and the outcome of such processes, may be considered, monitored and, where necessary, regulated. It builds on recommendations recently made by the FAWC to UK Government, and considers the wider public interest in these technologies. Finally, the response from stakeholders is considered in attempting to ensure that such controls do not render production systems non-competitive in a global livestock industry.

Chickpea molecular breeding: New tools and concepts

Chickpea is a cool season grain legume of exceptionally high nutritive value and most versatile food use. It is mostly grown under rain fed conditions in arid and semi-arid areas around the world. Despite growing demand and high yield potential, chickpea yield is unstable and productivity is stagnant at unacceptably low levels. Major yield increases could be achieved by development and use of cultivars that resist/tolerate abiotic and biotic stresses. In recent years the wide use of early maturing cultivars that escape drought stress led to significant increases in chickpea productivity. In the Mediterranean region, yield could be increased by shifting the sowing date from spring to winter. However, this is hampered by the sensitivity of the crop to low temperatures and the fungal pathogen Ascochyta rabiei. Drought, pod borer (Helicoverpa spp.) and the fungus Fusarium oxysporum additionally reduce harvests there and in other parts of the world. Tolerance to rising salinity will be a future advantage in many regions. Therefore, chickpea breeding focuses on increasing yield by pyramiding genes for resistance/tolerance to the fungi, to pod borer, salinity, cold and drought into elite germplasm. Progress in breeding necessitates a better understanding of the genetics underlying these traits. Marker-assisted selection (MAS) would allow a better targeting of the desired genes. Genetic mapping in chickpea, for a long time hampered by the little variability in chickpea’s genome, is today facilitated by highly polymorphic, co-dominant microsatellite-based markers. Their application for the genetic mapping of traits led to inter-laboratory comparable maps. This paper reviews the current situation of chickpea genome mapping, tagging of genes for ascochyta blight, fusarium wilt resistance and other traits, and requirements for MAS. Conventional breeding strategies to tolerate/avoid drought and chilling effects at flowering time, essential for changing from spring to winter sowing, are described. Recent approaches and future prospects for functional genomics of chickpea are discussed.

THE GEOGRAPHICAL DISTRIBUTION OF ALLELIC DIVERSITY, A PRACTICAL MEANS OF PRESERVING AND USING MINOR ROOT CROP GENETIC RESOURCES

This paper addresses the preservation and use of minor root crop genetic resources, mostly aroids and yams. Conservation is fraught with difficulty: ex situ collections are expensive to maintain and methods for on-farm conservation have not been studied. Conventional breeding strategies present serious limitations when applied to these species. Furthermore, the evaluation and distribution of improved material are as problematical as its conservation. The similarities shared by these species regarding their domestication, breeding constraints and improvement strategies as well as farmers' needs, are briefly reviewed. Based on these biological constraints, we propose a practical alternative to current conservation and breeding strategies. This approach focuses on the geographical distribution of allelic diversity rather than localized ex situ and/or in situ preservation of genotypes. The practical steps are described and discussed. First, a core sample representing the useful diversity of the species is assembled from accessions selected for their diverse and distant geographic origins, wide genetic distances, quality, agronomic performances and functional sexuality. Second, the geographical distribution of this core sample, in vitro via a transit centre, allows the direct use of selected genotypes by farmers or for breeding purposes. Third, the distribution of genes is realized in the form of clones resulting from segregating progenies and, fourth, farmers select clones with local adaptation.

Fine mapping of the nematode resistance gene Mi-3 in Solanum peruvianum and construction of a S. lycopersicum DNA contig spanning the locus

Currently, the only genetic resistance against root-knot nematodes in the cultivated tomato Solanum lycopersicum (Lycopersicon esculentum) is due to the gene Mi-1. Another resistance gene, Mi-3, identified in the related wild species Solanum peruvianum (Lycopersicon peruvianum) confers resistance to nematodes that are virulent on tomato lines that carry Mi-1, and is effective at temperatures at which Mi-1 is not effective (above 30 degrees C). Two S. peruvianum populations segregating for Mi-3 were used to develop a high-resolution map of the Mi-3 region of chromosome 12. S. lycopersicum BACs carrying flanking markers were identified and used to construct a contig spanning the Mi-3 region. Markers generated from BAC-end sequences were mapped in S. peruvianum plants in which recombination events had occurred near Mi-3. Comparison of the S. peruvianum genetic map with the physical map of S. lycopersicum indicated that marker order is conserved between S. lycopersicum and S. peruvianum. The 600 kb contig between Mi-3-flanking markers TG180 and NR18 corresponds to a genetic distance of about 7.2 cM in S. peruvianum. We have identified a marker that completely cosegregates with Mi-3, as well as flanking markers within 0.25 cM of the gene. These markers can be used to introduce Mi-3 into cultivated tomato, either by conventional breeding or cloning strategies.

Conventional breeding strategies to enhance the sustainability of <i>Musa</i> biodiversity conservation for endemic cultivars

Africa is one of the secondary centers of diversity for the genus Musa. There is a wide range of genetic diversity in the bananas found in the continent with different types dominating different ecological sub-regions. Banana production in Africa is affected by a number of diseases and pests. The development of resistant cultivars is considered to be the most effective approach to controlling diseases and pests. Banana breeding at the International Institute of Tropical Agriculture (IITA) involves crossing triploid (3x) landraces with diploid (2x) accessions to produce tetraploid (4x) hybrids. The 4x hybrids are crossed with improved 2x accessions to produce secondary triploids. In this study, the female fertility of 70 East African highland banana landraces was assessed. This was done through hand pollination with pollen from a wild diploid banana, Musa acuminata spp. burmannicoides Calcutta 4'. Results indicate that 50% of the landraces were female fertile and produced true seeds. Seeds were germinated by embryo rescue and the hybrids obtained were assessed to exhibit increased resistance to black sigatoka. However, bunch weights of the hybrids were generally lower than the control landrace, ‘Mbwazirume'. By crossing diverse genotypes, producing new populations and selecting improved genotypes, conventional breeding has the potential of increasing banana diversity at the farm level. It is thought that adoption of modern cultivars by farmers could lead to genetic erosion of landraces. However, studies in other crops showed that despite adopting modern cultivars farmers still maintain landraces. Both Ex-situ (off-site) and In-situ (on-site) conservation methods are important for Musa. In-situ conservation is appropriate for wild species in their natural habitats allowing the continuation of natural evolutionary processes and for landraces that were selected for important traits over many years by farmers. Wild species could also be conserved Ex-situ in field gene banks, In vitro culture and cryopreservation especially where their natural habitats are in prone to destruction. Field gene banks are more appropriate for breeders providing opportunities for continual assessment of the germplasm. Key Words: Accessions, black sigatoka, cryopreservation, hybrids, land races RESUME L'Afrique est un des centres secondaires de diversité pour le genre Musa. Il y a une large gamme de diversité génétique dans les bananes trouvées dans le continent avec des types différents dominant les différentes sous région écologiques. La production de banane en Afrique est affectée par un nombre des maladies et pestes. Le développement des variétés résistants est considéré être l'approche la plus effective pour contrôler les maladies et pestes. La reproduction de banane à l'Institut Internationale d'Agronomie Tropicale (IITA) implique le croisement triploïde (3x) des races de terre avec les accessions de diploïde (2x) pour produire les tétraploïdes hybrides (4x). Les hybrides 4x sont croisés avec les accessions 2x améliorés pour produire les triploïdes secondaires. Dans cette étude, la fertilité féminine des races de terre de banane de 70 régions de montagne de l'Afrique de l'est était évaluée. Ceci était faite à travers la pollination ouvrière avec le pollen d'un diploïde de banane sauvage, Musa acuminata spp. burmannicoides Calcuta 4'. Les résultats indiquent que 50% des races de terre étaient de fertilité féminine et ont produit des vraies graines. Les graines étaient germées par l'aide d'embryon et les hybrides obtenus étaient évalués pour exhiber l'augmentation de résistance au sigatoka noire. Cependant, les poids de bouquet des hybrides étaient généralement bas que les races de terre de contrôle, ‘Mbwazirume'. En croisant diverses génotypes, produisant des nouvelles populations et en sélectionnant des génotypes améliorés, la reproduction conventionnelle a le potentiel d'augmenter la diversité de banane au niveau de la ferme. Il est pensé que l'adoption des variétés modernes par les fermiers pourrait mener à l'érosion génétique de la race de terre. Cependant, les études sur les autres plantes ont montré que malgré l'adoption des variétés modernes les fermiers continuent à maintenir les races de terre. La conservation des deux méthodes Ex-situ (sur site) et In-situ (sur site) est importante pour Musa. La conservation In situ est appropriée pour les espèces sauvages dans leurs habitats naturels permettant la continuation du processus évolutionnaire naturel et pour les races de terre qui étaient sélectionnées pendant beaucoup d'années par les fermiers pour les traits importants. Les espèces sauvages pourraient aussi être conservées dans les banques des gènes sur terrain Ex-situ, en culture In vitro et en cryopréservation spécialement la où leurs habitats naturels sont en destruction prône. Les banques des gènes de terrain sont plus appropriées pour les reproducteurs pourvoyant les opportunités pour l'évaluation continuelle du germplasme. Mots Clés: Accessions, sigatoka noire, cryopréservation, hybrides, races de terre African Crop Science Journal Vol.12(1) 2004: 59-65

Maclean, J.L., Dawe, D.C., Hardy, B. and Hettel, G.P. (eds) Rice almanac. 3rd edn

This book is the third in the series called Rice Almanac published by the International Rice Research Institute, Los Banos, Philippines. It will serve as a standard source book for the most important economic activity on earth. The Almanac provides fully updated rice production and consumption statistics from the top ten rice‐producing countries, in addition to 54 other countries from Afghanistan to Venezuela. The book is composed of chapters on the importance of rice, the rice plant and its ecology, international issues, international rice research and development, rice around the world (covering 64 countries) and a country‐wise rice‐related database. Important conversion factors and rice facts provided at the end of the book provide valuable information, which is seldom available in other books. However, this part needs updating since it lacks a number of units and conversion factors, especially from India. A particularly useful feature of the Almanac is the listing of the names and locations of rice‐related database networks, together with a description of each one’s particular area of specialization and a contact e‐mail address. The book addresses, in an effective manner, various international issues important to rice, such as looming regional water crises, global climate change, biotechnology, intellectual property rights (IPR), functional genomics, crop modelling, nutritional rice, sustaining food security and biosafety. A chapter on international rice research and development deals with the mandate and programmes of the three leading CGIAR centres: the International Rice Research Institute based in the Philippines (the foremost global rice research centre), the West Africa Rice Development Association in Cote d’Ivoire (responsible for rice in that region) and the Centro International de Agricultura Tropical in Colombia, which undertakes rice research relevant to Latin America and the Caribbean. The relevance of rice research among national and international institutes has been emphasized by the establishment of a number of rice consortia. This has led to a host of benefits including the speeding of transfer of information and advance research innovations, quick resolution of problems, the enablement of scientific collaborations across political borders and economic barriers, and the sharing of scarce research resources among the rice‐growing countries. An attempt has been made for the first time to establish a rice publication network between IRRI, CIAT, WARDA and FAO, along with collaborating National Agricultural Research and Extension Systems (NARES) from different countries, in order to establish a shared and up‐to‐date database on rice production systems. The major achievements of the three CGIAR centres in enhancing productivity and sustainability of rice‐growing areas through collaborative research programmes are outlined, and their impact on improving the livelihood of the rural poor living in fragile environments has been well summarized. Prospects for the improvement of rice productivity in different environments through the development of inter‐specific varieties (NERICAs), C4 rice, transgenic approaches, aerobic rice, and hybrid technology have received proper recognition in this book. Graphical presentation of the data and descriptions of research approaches, including some of the protocols, enables the reader to get a quick insight into the subject. A sizeable portion of the book covers information about rice‐growing environments, production constraints and opportunities, research developments in the rice sector, adoption of modern technology, external trade, and basic statistics from the 64 rice‐growing countries of Asia, Africa, Latin America, Europe, Australia and United States of America. However, the information on the availability and adoption of popular high‐yielding modern and traditional rice varieties from different countries which possess a high degree of tolerance to abiotic and biotic stresses needs to be updated. As the ‘Encyclopaedia of rice databases’, which I perceive this volume to be, the third edition of Rice Almanac will be of great help in updating information relating to rice production systems and in structuring research activities befitting different rice environments across the globe. It may also be helpful in formulating collaborative groups working on different aspects of rice research, depending upon the common needs of the agro‐ecosystems. Over all, this is an excellent reference book that is nicely presented. The book has a potential market worldwide for the scientists working on various aspects of rice research, including genetic engineering and biotechnology, gene mapping and marker‐aided selection, in addition to the conventional breeding and management strategies for sustainable improvement of rice yields.

Chimerism in grapevines: implications for cultivar identity, ancestry and genetic improvement.

In the course of DNA profiling of grapevine cultivars using microsatellite loci we have occasionally observed more than two alleles at a locus in some individuals and have identified periclinal chimerism as the source of such anomalies. This phenomenon in long-lived clonally propagated crops, such as grapevine, which contains historically ancient cultivars, may have a role in clonal differences and affect cultivar identification and pedigree analysis. Here we show that when the two cell layers of a periclinal chimera, Pinot Meunier, are separated by passage through somatic embryogenesis the regenerated plants not only have distinct DNA profiles which are different from those of the parent plant but also have novel phenotypes. Recovery of these phenotypes indicates that additional genetic differences can exist between the two cell layers and that the Pinot Meunier phenotype is due to the interaction of genetically distinct cell layers. It appears that grapevine chimerism can not only modify phenotype but can also impact on grapevine improvement as both genetic transformation and conventional breeding strategies separate mutations in the L1 and L2 cell layers.

Identification of research to improve the efficiency of breeding strategies for white clover in Australia - a review

A major challenge faced by today’s white clover breeder is how to manage resources within a breeding program. It is essential to utilise these resources with sufficient flexibility to build on past progress from conventional breeding strategies, but also take advantage of emerging opportunities from molecular breeding tools such as molecular markers and transformation. It is timely to review white clover breeding strategies. This background can then be used as a foundation for considering how to continue conventional plant improvement activities and complement them with molecular breeding opportunities. In this review, conventional white clover breeding strategies relevant to the Australian dryland target population environments are considered. Attention is given to: (i) availability of genetic variation, (ii) characterisation of germplasm collections, (iii) quantitative models for estimation of heritability, (iv) the role of multi-environment trials to accommodate genotype-by-environment interactions, (v) interdisciplinary research to understand adaptation to dryland environments, (vi) breeding and selection strategies, and (vii) cultivar structure. Current achievements in biotechnology with specific reference to white clover breeding in Australia are considered, and computer modelling of breeding programs is discussed as a useful integrative tool for the joint evaluation of conventional and molecular breeding strategies and optimisation of resource use in breeding programs. Four areas are identified as future research priorities: (i) capturing the potential genetic diversity among introduced accessions and ecotypes that are adapted to key constraints such as summer moisture stress and the use of molecular markers to assess the genetic diversity, (ii) understanding the underlying physiological/morphological root and shoot mechanisms involved in water use efficiency of white clover, with the objective of identifying appropriate selection criteria, (iii) estimation of quantitative genetic parameters of important morphological/physiological attributes to enable prediction of response to selection in target environments, and (iv) modelling white clover breeding strategies to evaluate the opportunities for integration of molecular breeding strategies with conventional breeding programs.

MOLECULAR AND CONVENTIONAL BREEDING STRATEGIES FOR SEEDLESS CITRUS

Seedlessness is a desirable trait in Citrus. We are using conventional breeding methods and molecular techniques to generate new seedless varieties for the Australian citrus industry. Amongst the conventional breeding strategies, diploid hybridisation relies on parents that will transmit parthenocarpic ability, male/female sterility, and pollen incompatibility. Candidate genes to achieve seedless fruit using molecular techniques include those that induce fruit formation in the absence of fertilisation (parthenocarpy), and genes that directly target seed destruction after fertilisation to leave an imperceptible residue. Parthenocarpy occurs naturally in navel orange and Satsuma mandarin but it is not known how it is initiated and controlled at the molecular level. We are using Arabidopsis to study parthenocarpic fruit formation. An Arabidopsis mutant, fruit without fertilisation (fwf), has been identified that forms a seedless fruit (silique) in the absence of pollination. The ovules degenerate in parthenocarpic fwf fruit, and fruit size approaches that of a seeded silique. The fwf fruit forms all of the fruit tissues observed in seeded siliques. It shatters open at maturity, indicating that the seedless fruit matures normally. It is a recessive mutation for which the chromosomal location is known and gene cloning is in progress. Some Citrus cultivars that are normally seedy can set seedless fruit if pollination is prevented. In such cultivars, seeded fruits are often produced because insect-mediated pollination is difficult to avoid. Introduction of a male sterility gene that prevents fertile pollen formation should allow parthenocarpic fruit formation. We have introduced a male sterility gene into West Indian lime. We have also constructed genes that inhibit seed formation in Arabidopsis and tobacco soon after fertilisation and the initiation of seed development. This was done because it is not yet possible to control parthenocarpy effectively. Not all Citrus cultivars possess a parthenocarpic ability, and male-sterile plants are still capable of being cross-pollinated and thus subjected to seed set. The constructs target destruction of various seed tissues early in development, resulting in minimal seed development that leaves behind a soft trace of seed in the fruit. We have observed that in different seed types specific tissues need to be targeted in order to reduce seed size effectively. These genes are also being evaluated in West Indian lime. We are awaiting flowering and fruit set to evaluate the efficacy of their action. Improvement of Citrus in Australia using gene biotechnology began in 1992, and the first transgenic West Indian lime plants containing marker genes were regenerated in. 1996. Transgenic plant lines containing the developed genes have since been generated and individual lines are being mass propagated for use in field trials under the guidelines of the Genetic Manipulation Advisory Committee (GMAC). Our molecular work complements our hybridisation strategies for seedlessness and forms part of the Australian national Citrus breeding and evaluation program supported with funds from the Australian citrus industry via levies collected from growers.

Genetic Gain for Soybean Seed Yield by Three Recurrent Selection Methods

This study compared genetic gain for soybean [Glycine max (L.) Merr.] seed yield associated with three strategies of recurrent selection in soybean population AP6. For the strategy of a two‐stage test and one intermating generation (2ST‐1IG), 100 F4‐derived lines from 45 single‐cross populations were evaluated in each of three maturity classes, and the 10 highest yielding lines were intercrossed to begin the next cycle. The conventional breeding strategy (CB‐2ST‐IIG) consisted of testing 100 F4‐derived lines from 10 single‐cross populations in each maturity class and intermating 10 selected lines to initiate the next cycle. For the strategy of a two‐stage test and three intermating generations (2ST‐3IG), 100 F4‐derived lines were tested in the midseason maturity class, and 10 selected lines were intermated three generations to form the population for the next cycle. Composites of equal numbers of seeds of the parents for six cycles of 2ST‐1IG and CB‐2ST‐1IG, four cycles of 2ST‐31G, and the individual parents of the most advanced cycles were grown at three Iowa locations during 2 yr. The genetic gain per cycle for seed yield was significantly greater for 2ST‐1IG (53.8 kg ha−1) with one generation of intermating between cycles than for 2ST‐3IG (3.5 kg ha−1) with three intermating generalions. The genetic gain per cycle was greater for 2ST‐1IG than for CB‐2ST‐1IG, although the differences were only significant for the early maturity class. The superior genetic gain for 2ST‐1IG indicated that selection from the largest numbero f single‐cross matingsp ossible should maximize yield improvement from recurrent selection in soybean.

Resistance of Transgenic Hybrid Squash ZW-20 Expressing the Coat Protein Genes of Zucchini Yellow Mosaic Virus and Watermelon Mosaic Virus 2 to Mixed Infections by Both Potyviruses

The resistance of three transgenic yellow crookneck squash lines expressing the zucchini yellow mosaic virus (ZYMV) and/or watermelon mosaic virus 2 (WMV 2) coat protein (CP) genes was investigated under field conditions. Resistance was evaluated under high disease pressure achieved by mechanical inoculations of ZYMV and WMV 2 and natural challenge inoculations by aphid vectors. Assessment of infection rates was based on visual monitoring of symptom development, enzyme-linked immunosorbent assays (ELISA), infectivity assays, and analysis of virus occurrence in fruits. Horticultural performance was evaluated by counting the number of mature fruits per plant and estimating their fresh fruit weight The transgenic line ZW-20B expressing both the ZYMV and WMV 2 CP genes showed excellent resistance in that none of the plants developed severe foliar symptoms, although localized chlorotic dots or blotches appeared on some leaves. In contrast, the two transgenic lines expressing only single CP genes, either the ZYMV or WMV 2 CP gene, developed severe symptoms by the end of the trial period, as did the nontransformed control lines. Differences between transgenic squash lines were even more dramatic in fruit quality: All transgenic ZW-20B fruits were symptomless, while all fruits from the two transgenic lines with single CP genes showed symptoms, and thus were unmarketable. ZW-20B is a hybrid of commercial quality, and its high level of resistance to ZYMV and WMV 2 should enable the production of yellow summer squash in areas where both potyviruses are prevalent This transgenic yellow crookneck squash line is especially valuable in light of the difficulties of obtaining similar resistance by conventional breeding strategies.

Biotechnology of Seed Crops: Genetic Engineering of Seed Storage Proteins

Table 1. Amino acid composition of total maize and soybean seed protein and purified storage protein fractions.z During their development, seeds accumulate large amounts of storage, or reserve, products that are metabolized during germination and the early stages of seedling growth. Energy reserves include complex carbohydrates (starch) and lipids, while reserves of N, C, and S are accumulated in specific storage proteins. The uses of seed products by man are innumerable, ranging from fermentation of maize starch for alcohol production to refinement of soybean protein for use in infant formula. Agronomic crops, such as maize, wheat, oats, and soybeans, are grown because they generate large amounts of seed reserves per unit of cultivated land. A major concern of the agricultural industry is increasing yields of seed components to enhance the value of these crops. During the past century, plant breeders have been quite successful in developing new cultivars of seed crop plants that give higher yields of seed components than older ones. Advances have been made not only with respect to increasing the content of starch, oil, and protein in the seed, but also, in some cases, in altering the quality of these components. For example, naturally occurring genetic variation has been exploited in the development of maize cultivars with modified starch content (Nelson, 1979; Shannon and Garwood, 1984) and increased or altered oil content (Alexander 1988, 1989; Weber, 1987). In a similar manner, the composition of soybean oil has been altered through selection to decrease the content of linolenic acid, resulting in a product with a more desirable flavor (Fehr, 1989; Hymowitz, 1987). The synthesis of carbohydrates and lipids involve complex metabolic processes, and although a great deal is known about these pathways, it remains to be determined how recent advances in genetic engineering technology will aid in the modification of these processes to enhance seed quality. For example, alteration of the activities of one or two enzymes in the starch biosynthetic pathway might allow for the accumulation of desirable intermediates of starch biosynthesis. Such a situation may, however, result in a serious imbalance in the overall carbon flow in the seed, which could affect factors such as grain yield and seed viability. Conventional selection and breeding strategies may, therefore, be more efficient than the use of genetic engineering in the development of crop cultivars possessing seed carbohydrates and lipids of desirable composition (Alexander, 1988). An important aspect of seed quality that is directly amenable to manipulation through genetic engineering involves the nutritional characteristics of seed proteins. In most cases, seed proteins of any one crop do not contain sufficient quantities of certain amino acids that are essential in the diet of humans and other monogastric animals (Nelson, 1969). Generally, cereals are most limiting in lysine and tryptophan, and legumes are deficient in the sulfur amino acids methionine and cysteine (Table 1). The economic advantages for increasing the nutritional quality of seed proteins are enormous: estimates of total protein production for each of two major seed crops in the United States, maize and soybeans, exceed 16 million tons annually (Wilson, 1987). Development of nutritionally balanced seed proteins in high-production crops like maize and soybean would benefit all areas of the agricultural industry concerned with grain production for human and livestock consumption. Plant breeders have had only limited success in improving the nutritional quality of seed proteins, primarily because genes encod-

Yield Improvement in a Soybean Population by Utilizing Alternative Strategies of Recurrent Selection1

Maximum genetic gain from recurrent selection requires an efficient and effective strategy for selection of superior individuals from a population and intermating these selections to form a new population. This research was conducted to compare genetic response to alternative strategies of recurrent selection for seed yield in soybean [Glycine max (L.) Merr.]. Four strategies initiated in the AP6 soybean population represented a factorial arrangement of one or two stages of replicated yield testing before selection of superior F4‐derived or S2‐derived lines and one or three generations of intermating between cycles. A fifth strategy, representing a conventional breeding program, was initiated in a population formed by sampling the first of three generations of intermating used to synthesize AP6. The conventional breeding strategy utilized a two‐stage yield test of F4‐derived lines obtained from a limited number of single crosses each cycle. The response to selection was determined by evaluating composites of the parents of each cycle at three locations during 2 yr. The number of cycles completed for the strategies varied from two to five. The average genetic gain per cycle was 9.6 ± 1.1 g m−2 for the conventional breeding strategy, 6.0 ± 1.1 g m−2 for the twostage test and one intermating, 5.5 ± 1.8 g m−2 for the two‐stage test and three intermatings, 1.8 ± 0.6 g m−2 for the one‐stage test and one intermating, and 0.3 ± 1.1 g m−2 for the one‐stage test and three intermatings. The ranking of the five strategies for genetic gain per year was the same as for genetic gain per cycle. The results indicated that multiple generations of intermating did not enhance genetic gain and that the two‐stage test, as conducted in this study, was superior to the one‐stage test for yield improvement.