Selection in segregating generations of autogamous species. I. Selection response for combined selection

Summary The response to selection is determined by the genetic and nongenetic variance, the selection intensity, and the size of the experiments. If, in pure line breeding, selection starts in segregating generations, the gain depends on the relation of additive to epistatic variance. The gain can be increased if an index is constructed using the information from relatives. From animal breeding this kind of selection is known as combined selection. In this paper the optimal number of families and subfamilies is determined that will maximise the selection response by combined selection for a fixed total size of experiments. The composition of the genetic variance has a imited influence on the optimal size of the progenies. If epistatic variance is important, then the number of F₂ families has to be reduced and the number of F₃ families and F₄ families must be increased. The same is true if the nongenetic variance is increased.     

Genetic analysis of drought stress response in rapeseed (Brassica campestris and B. napus). I. Assessment of environments for maximum selection response in grain yield

Summary Expectations of yield improvement in environments where drought was the major environmental factor limiting yields were studied in two species of rapeseed. Selection for yield in a drought stressed environment was predicted to be a more efficient selection stategy for yield improvement in dryland situations than selection in a more optimal environment, or selection based on a drought response index. The results indicate that selection for yield in a stressed environment are expected to lead to genetic advances in yield under optimal conditions as well as in a drought index. Selection under well watered conditions, on the other hand, was also expected to lead to correlated increases in yield in droughted environments but to decreases in the drought index. These results were found in both species of rapeseed grown in different water stress situations.The genetic advance in a drought response index was predicted to be greater in B. napus and marginally less in B. campestris if selection was practised for yield in a stressed environment rather than direct selection for the drought index. This was due to the higher heritability estimates in the stressed environments and the positive genetic correlations with yield.     

Geneticanalysis of drought stress response in rapeseed (Brassica campestris and B. Napus). II. Yield improvement and the application of selection indices

Summary The expected improvement in grain yield if selection was made for plant characters measured between flowering and maturity was evaluated in populations of rapeseed (Brassica campestris and B. napus) grown in a droughted environment. Drought was commenced at flowering in each species and measurements were made on plants at the commencement of the drought stress, during the stress treatment and at crop maturity.Substantial genetic and phenotypic variation was observed in yield as well as the different morphophysiological determinants of yield. In B. campestris no single parameter was found to be a suitable alternative selection criterion to yield since the correlated responses in yield if selection was for another character was lower than if selection was for yield alone. By the use of a selection index however, joint selection for yield, as well as harvest index, 1000 seed weight and seeds per pod, was expected to be 20% more effective than direct selection for yield under drought. In the B. napus population direct selection for flowering time or for harvest index was predicted to result in a genetic advance in yield equal to or greater than that obtained by direct selection for yield, whereas joint selection for flowering time and yield should result in a 16% greater yield increase. Selection for increased yield in these populations grown in droughted conditions is discussed.     

Specific adaptation and breeding for marginal conditions

Summary Breeding has been very successful in generating cultivars that in favorable environments, and together with large use of fertilizer and chemical control of weeds, pest and diseases, have increased agricultural production several fold. Today the environmental impact of high input agriculture in more favorable environments causes growing concern. By contrast, the impact of breeding in marginal environments has been elusive. The paper discusses evidence showing that the use of breeding principles developed for, and successfully applied, in favorable environments may be the main reason for the lack of breeding progress in marginal environments. Very little breeding work has actually been done in marginal environments, although the theory of correlated responses to selection indicates that selection conducted in good environments or in well-managed experiment stations is not expected to be very efficient when genotype by environment interactions of a cross-over type exist. The assumptions that heritability is higher under good conditions and that there is a carry-over effect of high yield potential are not supported by experimental evidence. If the target environment is below the cross-over point, selection has to be conducted for specific adaptation to that environment. The concept of wide adaptation has more a geographical than an environmental meaning, and it reduces genetic diversity and increases genetic vulnerability. Eventually the issue of genetic heterogeneity versus genetic uniformity is discussed in relation to specific adaptation to marginal environments.     

The use of general and specific combining abilities in a context of gene expression relevant to plant breeding

Many common traits are believed to be a composite reflection of multiple genetic and environmental factors. Recent advances suggest that subtle variations in the regulation of gene expression may contribute to quantitative traits. The nature of sequence variation affecting the regulation of gene expression either in cis (that is, affecting the expression of only one of the two alleles in a heterozygous diploid) or in trans (that is, affecting the expression of both alleles in a heterozygous diploid) is a key and usually unknown feature for the breeders. If the change in expression acts entirely in cis, then the structural gene can be treated as a candidate gene and a potential target for marker-assisted selection. Therefore, gene surveys for cis-regulatory variation are a first step in identifying potential targets for marker-assisted breeding. Here, we discuss in detail the “genome-wide analysis of allele-specific expression differences” (GASED) approach. The GASED approach was developed to screen for cis-regulatory variation on a genome-wide scale. In GASED, mRNA abundance is treated as if it were a quantitative phenotypic response variable, whose genetic between-F₁ hybrid variance is partitioned into additive and non-additive components. In plant breeding, this partitioning of the genetic variance is well known in the context of estimation of general and specific combining abilities for diallel crossing schemes. We demonstrate the GASED method using Arabidopsis thaliana data. The method can be used to screen for cis-regulatory variation in any crop species for which diallel crossing schemes are appropriate and genomic tools are available.     

Expected levels of heterozygosity in autotetraploid progeny populations

Summary A breeding program aimed at improving the seed yield in bean for processing was started. Six parents divergent in a range of agronomical characters were crossed according to diallel mating design. Cluster analysis performed on the composite cross population evidenced divergence between F1 hybrids and to maximize genetic recombination, the most different F1 hybrids were intercrossed. A Factor analysis was performed on 7 morphological and yield-determining traits of 15 hybrids F1 and parents. Three Factors, representing patterns of variables interpreted as productivity per plant, growth factor, and branching-productivity, were extracted. These results evidenced the inverse relationship between seed yield components (seeds/pod, pods/plant, and hundred seed weight): i.e. the selection for either variable is detrimental to the other. It may be possible to break this association if the plant construct assures high sink to satisfy all plant requirements: tall and large plants bearing numerous nodes, leaves and reproductive structures. Moreover, the selection practiced simultaneously on the yield components positively affected the seed yield trait. An index of selection and response to selection were estimated on the segregant populations. The realized gain in seed/yield reached after one cycle of selection (C1) was 6.5 g, about 20% over the base population (C0). The expected gain from selection based on the superior 5% plants in C2 families was 25.5% and approached the realized gain. The result of this paper gives evidence to support that the seed yield can be improved by an adequate selection intensity and selection index can be very useful when it is faced with the improvement of several traits at the same time.     

Variation of agronomic characters among and between guayule lines

Summary A selected group of 42 guayule (Parthenium argentatum Gray) lines were evaluated on the basis of various morphological characteristics and compared to a total population of 234 lines. The objective of this study was to determine the amount of variation that existed both within and among guayule lines over a 2-year period. The parental lines all reproduced apomictically, and as expected, many lines exhibited a high degree of uniformity for every measured character. However, many other lines were extremely variable for varying numbers of characters. This appears to be due to varying degrees and combinations of apomeiosis, thus establishing a long-term source of genetic variation. The high amount of variability existing among and within these guayule lines indicates that significant progress by selection for both rubber percentage and yield is feasible.     

Breeding methods and selection indices for improved tolerance to biotic and abiotic stresses in cool season food legumes

The objective of breeding for stress tolerance is to improve productivity for a target level of stress. If tolerance is viewed as resistance to change in productivity with increasing stress, productivity under stress depends not only on stress tolerance, but also on maximum productivity. Index selection theory indicates that selection in non-stress environments will be more effective than direct selection for productivity under stress whenever the correlation between the two types of environments exceeds the heritability of productivity under stress. With high genetic correlation, selection should be conducted within a level of stress that maximizes heritability. In cases where heritability under non-stress is much higher than under stress, an index combining data from stress and non-stress environments is expected to be more efficient than selection based on evaluation only within stress environments.Secondary traits will be useful in breeding for productivity under stress whenever they have high heritability and high genetic correlation with productivity under stress. For some abiotic stresses and many biotic stresses, heritability will be highest in the presence of stress and indirect or index selection will be of limited value.     

Genetic analysis and selection for wheat yield in drought-stressed and irrigated environments

Summary Wheat (Triticum aestivum L.) cultivars grown in the southern Great Plains of the U.S.A. are exposed to a wide range of moisture conditions due to large fluctuations in the amount and frequency of rainfall. Yield stability under those conditions is therefore a desirable trait for wheat breeders. Our primary objective was to quantify various genetic parameters for grain production in drought-stressed and irrigated environments. We also attempted to predict and measure yield responses when selection is practiced in either drought-stressed or irrigated environments, or both. Seventy F₂-derived lines from the cross, TAM W-101/Sturdy, were evaluated at Goodwell, OK, under irrigated and naturally drought-stressed conditions in 1987 and 1988. Genetic variance and heritability estimates were higher in the irrigated environment than in the drought-stressed environment. The genetic correlation coefficient for yields in the two environments was 0.20±0.16, indicating that selection of widely adapted genotypes requires testing in both environments. Based on the genetic variance/covariance structure of this particular population, the linear index which maximized the combined expected gain in both environments was 0.66Y₁ + 0.34Y₂, in which Y₁ and Y₂ are yields in the irrigated and drought-stressed environments. This index is not expected to apply across all populations; rather, it further supports the hypothesis that testing in either environment alone (drought stressed or irrigated) may not be most effective for increasing either mean productivity or yield under drought stress.     

Esterase 1 and Hordein C allelic and genotypic frequencies in CC XXXI,a male sterile barley population

Summary Seeds from the 5th, 6th, 12th and 18th generation of enforced outcrossing in CCXXXXI-B, a barley (Hordeum vulgare L.) population containing genetic male sterility, were assayed for genotypic distribution of Est 1 and Hor C alleles. The result suggest that natural selection was effective in altering allelic frequencies at both loci and that genotypic frequencies were often different from what would be expected if allelic frequencies solely determined genotypic frequencies. Departures of actual from expected genotypic frequencies occurred in that shifts in allelic frequencies were detected.Joint contribution of the USDA-ARS and published with the approval of the Director of the Montana Agric. Exp. Sta. Journal Series No. 2281.     

Can conventional breeding programmes provide onion varieties that are suitable for organic farming in the Netherlands?

Main stream commercial onion breeders do not select varieties for organic farming, but solely for conventional farming. Seed companies consider the organic market too small to justify investments in breeding for this sector. In order to study if their varieties also suit organic farmers’ needs we interviewed four Dutch commercial onion breeders on their breeding programme and selection criteria and compared the outcome with a variety profile composed of the priority traits of Dutch organic farmers. Breeders gave priority to the same storage and bulb quality traits that are demanded by organic farmers, because organic onions are exported to conventional supermarkets that apply the same quality standards to organic and conventional onions. However, organic farmers also need varieties that perform well in the field. Breeders give low priority to field selection. Furthermore, three of the four seed companies only breed hybrids. The cytoplasmic male sterility system used to produce these hybrids does not comply with organic principles. We conclude that at present breeders can provide varieties that meet organic farmers’ demands for storability and quality traits, but they should give higher priority to field selection to also improve required field traits. The latter will only occur, if in future the organic seed market will grow. If the organic sector wants varieties developed according to its own principles, it should either set up its own onion breeding programme or seek alliances with breeding companies that are prepared to harmonize their breeding methodology with the organic principles.     

Relationships among economic characters in lentil

Summary There are several reports of the association among lentil characters based on a few, relatively similar lines grown in single environments. Their results have often been inconsistent because of inadequate sampling of genotypes and environments. This study examined associations between economic characters in lentil over a wide range of genetic material and environments in West Asia. The first part of the study concerned correlations within large samples from the world lentil collection grown in two seasons, and the second part covered a smaller sample of genetic material from the collection sown over a wider range of conditions.The results showed similar phenotypic correlations over two seasons, contrasting in rainfall, in the world lentil collection. Similar genetic and phenotypic correlations were also shown by the smaller sample of germplasm over 10 environments. These results indicated low importance of covariance due to environmental and genotype-environment interaction effects. Seed yield was positively correlated with straw yield, indicating selection for either character will increase the other trait. The correlation between seed protein content and seed yield was small and negative, while the correlation of protein with straw yield was positive in sign. This suggests that the current ICARDA goals of selection for high seed and straw yield will not have a major correlated effect on seed protein content. There was a strong positive correlation between seed size and cooking time (r=0.96), therefore seed size can be used to predict cooking time.     

表型轮回选择和混合选择改良小麦群体效应的研究

在吴兆苏等(1984)创建的小麦基础群体中,采用不同的选择方案,对不育株和可育株分别进行两轮选择。结果表明两轮选择显著地降低了可育株和不育株群体的株高。可育株群体株高的下降速率大于不育株群体。单株产量和单株穗数没有得到显著的改良。相关性状穗长、千粒重等出现负向间接选择效应。轮回选择通过选株间的互交,维持了不     

Repetitive cycling and simple recurrent selection in traditional barley breeding programs

Summary A critical examination of three representative barley development programs from three barley producing areas of the world reveals that the evolution of each can be explained by the concepts of simple or phenotypic recurrent selection. Typically, each has utilized only 13 to 16 separate genetic sources in their program. Most of the sources were introduced over 50 years ago and have been progressively recombined through a simple recurrent selection process. The time period required for selection, testing, and reincorporation of improved genetic recombinants into the breeding program, or the completion of a cycle of recurrent selection, ranged from 6.5 to 10.5 years. If the major operative force in barley breeding is recurrent selection, then additional attention should be given to increasing the genetic base and reducing the time required to complete cycles of selection.     

Selection for yield in small plots of spring wheat

Summary Four selection experiments were sown: single-row plots with single row-spacing (20 cm), single-row plots with double row spacing (40 cm), three-row plots and six-row plots both with single row-spacing (20 cm). Selection entries were mimiced by 16 different varieties or advanced breeding lines, which were also sown in a yield trial. Each experiment was laid out as a 4-times replicated randomized block design. Row length was 2 m. Alley borders and border-rows of multiple-row plots were harvested separately to evaluate the effects of different harvesting procedures on the selection efficiency. Removal of alley borders was found to be disadvantageous, since the gain in precision was more than offset by the loss in sampled area. Wide spacing of single-row plots improved the selection efficiency in comparison with normal spaced single-row plots. In multiple-row plots the selection efficiency was not improved by harvesting only the central rows.For gross plot yield (= yield of net plot + yield of alley borders) the differences in selection efficiency between the various selection plot-types were explained on the basis of the genetic variance, the environmental variance and the coefficient of genetic correlation with farm yield as determined in the yield trial.     

Genetic advance for chip colour in potatoes

Summary The objective of this study was to investigate the response to selection for chip colour after harvest (CH), storage at 12.8°C (CR) and at 3°C (CC) in three hybrid populations. Population 1 was derived from crossing ND860-2 (cold chipper) with F58089 (regular chipper), Population 2 was obtained from crossing ND860-2 with Russette (nonchipper), and Population 3 was derived from crossing Russette with F58089. Eighty-five to ninety-six random clones for each population plus ten check cultivars were planted in 1991 at two locations in East Canada. For CH, Population 3 had the highest predicted gain. No genetic variation for this trait was detected in Population 1. The potential genetic advance by selection within Population 1, as measured by the predicted mean of the selected clones, however, was similar to the others because Population 1 has a higher mean. For CR, Populations 2 and 3 had similar expected response estimates. Population 1 had no genetic variation for CR but showed similar potential advance to Population 2 and higher than Population 3. For CC, Population 2 had the highest predicted gain. The predicted means of selected clones of Populations 1 and 2 were higher than that of Population 3. Consequently, Populations 1 and 2 had greater potential for improvement for CC than Population 3.     

Inheritance of some components of yield in the ‘deli dura variety’ of oil palm

Summary Genetic and environmental parameters are estimated by variance analysis of numerical data obtained from two programmes of plant breeding: biparental matings and Comstock and Robinson's North Carolina Model 1. The yield component total weight of fruit bunches and its subcomponents, average bunch weight and total bunch number are considered. Two experiments, involving different families, within the structure of each design are analysed. Good agreement is reached between the findings both within and between experiments and designs, and also from character to character. Additive genetic variation appears virtually to be absent whereas dominance effects (which as notationally defined also include epistasis) are of high magnitude even in relation to the environmental contribution. This suggests that further rapid selection progress within this variety might not be immediately likely and that past selection may have caused a fixation of additive variation. However, large errors are attached to all parameter estimates. Analysis of the data year by year rather than over years indicates that there is a fall in the relative magnitude of genetic to total phenotypic variation with increasing palm age and some explanations for this phenomenon are suggested.     

Genetic Variation in Nitrogen Uptake and Utilization in Spring Rape (Brassica napus L.) and its Exploitation Through Selection

Field and controlled environment studies revealed significant genetic variation in nitrogen uptake and utilization within an F2 population of spring rape. However, narrow-sense heritabilities of measures of these characters were consistently low, and were less than that of seed yield in the field. Flowering time in the field had a much higher heritability than seed yield and the expected yield response to selection for this character was 34 % greater than the response to direct yield selection. An even greater yield response was expected from selection based on an index combining measurements of flowering time, nitrogen uptake and nitrogen utilization. The expected yield response to selection for an index incorporating these characters and seed yield was SI % greater than that for direct yield selection.     

Response to mass selection and estimation of heritability for oil content in Dimorphotheca pluvialis

Three populations of Dimorphotheca pluvialis (L.) Moench were subjected to mass selection for increased oil content. After three cycles of selection, at a selection intensity of 10%, in all three populations a significant increase in oil content was observed. The average gain in oil content per selection cycle varied from 0.5% to 1.2%. Realised heritabilities ranged from 0.15 to 0.58. It is likely that the observed differences in response and heritability for oil content can be attributed to different genetic constitutions of the populations used. Though heritabilities are specific for populations, selection procedures and environmental circumstances, the observed values for the realised heritabilities indicate that a substantial part of the phenotypic variance is additive genetic variance. Therefore, continued (mass) selection for increased oil content will be effective in Dimorphotheca. This revised version was published online in July 2006 with corrections to the Cover Date.     

旅大红骨玉米种质掌状扁形果穗的遗传研究

以玉米自交系丹717×丹340组成的6世代群体为材料,利用P1、P2、F1、F2、BC1和BC2 6世代联合分析法,研究旅大红骨种质掌状扁形果穗性状的遗传规律.结果表明:该性状的次数频率分布在F2分离世代呈近似“m”状的双峰分布,B1群体基本表现为直线型分布,B2群体表现为2个峰值的类似正态分布.通过AIC值进行适合性检验,该性状符合2对主基因+加-显-上位性B-1-1模型,2对主基因的加性效应占据主导地位且互作效应明显,加性效应值分别为2.42和1.86,主基因的遗传率介于46.2％～68.8％之间,在自交进代过程有逐渐下降的趋势,育种中应注重早代试材的筛选,或进行回交选育以提高育种值.