The use of risk aversion in plant breeding; Concept and application

Summary Variance analysis is used to measure stability (inter-temporal) and adaptability (over space) components of variance with multilocation-multiyear sorghum yield data from India. Adaptability and stability were highly correlated. Only the stability component is relevant for farmers in their adoption decision. Measures of farmer's risk aversion are used to rank genotypes according to preferences which take account both of yield and stability. Since yield differences were large and risk aversion moderate, preference based rankings did not differ markedly from yield based rankings.     

Basic information for the use of primary trisomics in genetic and breeding research

Summary An attempt is made to arrange the basic information needed for using primary trisomics in localizing single and complementary dominant and recessive genes, accounting for random complete chromatid and random chromosome association, for heterozygosity of the trisomics and for female and male transmission rates. The expected ratios are given separately for the di-, tri- and tetrasomic group in each population. Also the ratios in the total populations have been calculated for different rates of female and male transmission of the extra chromosome. Both the backcross procedure and that of selfing trisomic F₁ plants are considered. Finally the use of trisomics for studying quantitative characters is discussed in connection with the breeding and incompatibility system of the crops and in relation to the frequent morphological differences among trisomics as well as between trisomics and their disomic counterpart.     

Genetic diversity inHippophae L. and its use in plant breeding

Summary In the last decade,Hippophae species, particularlyH. rhamnoides L., has received special attention in several countries for its multiple uses. The plant produces edible fruit with high nutrient and medicinal values and is characterized by its nitrogen-fixing symbiosis. It is widely distributed on the Eurasian continent (27°–69° N). Wide genetic variation is the basis of its distribution and provides good opportunities for selection and breeding.Hippophae is extremely variable in height, from a small bush less than 50 cm to a tree more than 20 m high. Phenological observations have shown a clear gradient of growth rhythm and plant size corresponding to the geographical distribution from north to south inH. rhamnoides. Studies on vitamin C concentration in the fruit have revealed significant differences between and within natural populations. The fruit size varies from 4 to 60 g/100 berries, the fruit color, from yellow, orange to red, and the shape, from flat, round, oval to cylindrical. Isozyme studies have shown that the mean number of alleles per locus per population is 2.1 and the percentage of polymorphic loci was 40.3% (at 0.95 standard). There is large genetic diversity residing within and between subspecies and species in the genusHippophae.     

The concept of varietal ability in plant breeding

Summary At the population level, the general varietal ability of a genotype is defined as the expectation of all possible varieties of a given type with this genotype. Specific varietal abilities of order 1, 2, 3..., are defined as interactions from the combined effect of 2, 3, 4... parents in varietal combinations. Genetic effects for varietal value and for the criterion of test are introduced. They allow an expression of the covariance between the value of a genotype according to the testing system and the value of its progeny from random mating in varietal combination. So a general expression of the genetic advance in the varietal value of a population improved by recurrent selection can be given whatever the type of variety and the testing system. The genetic advance is also considered at the level of the varieties. Some examples of prediction formulae for varietal abilities are given for hybrids, synthetics and lines. The problem of prediction of the ability of a genotype to give lines with the best general varietal abilities is also considered.Communication given at the 3rd meeting of the Eucarpia section Biometrics in Plant Breeding, Cambridge, April 4–7, 1978, and in a revised version on the occasion of the Professor Schnell's 65th birthday, Stuttgart, University of Hohenheim, May 19, 1978.     

Correlations in plant breeding

Even small correlations between seedling and adult characters can be used for seedling selection. Even small correlations between desirable adult characters alter considerably the necessary progeny size.     

The genetics of plant-virus interactions: implications for plant breeding

Summary Host resistance is the main means of control of plant virus diseases. This paper reviews the genetics of resistance and matching virulence. Theoretical models of basic compatibility between plant species and their viruses, and of resistance, are described and used to predict features of resistance genetics, and mechanisms. These predictions are compared with a survey of known examples of resistance.Resistance is mainly controlled at a single genetic locus, although more complex systems are known. About half of the resistance alleles studied were dominant, the remainder were either incompletely dominant or recessive. Doubt is cast on the reliability of assessing resistance genotypes (numbers of loci and dominace relationships) from distant phenotypic measurements such as symptom severity or plant growth. A model is proposed to reconcile apparent inconsistencies between genotype and phenotype.Dominant resistance alleles are strongly associated with virus localising mechanisms normally involving local lesions. Incompletely dominant and recessive alleles allow spread of the virus, but inhibit multiplication or symptom development. Fully recessive alleles may be associated with complete immunity.Most resistance genes in the survey had been overcome by virulent virus isolates with dominant localising resistance alleles especially vulnerable. Comparatively few resistance genes have proved exceptionally durable. Acquisition of virulence can be associated with loss of general pathogenic fitness, but in some cases this can be restored by further selection of the virus in resistant hosts.Virulence/avirulence determinants have been mapped to individual base changes in different functional regions of the viral genome. A virus may contain several virulence determinants and may develop a stable gene-for-gene relationship with a host having several resistance genes. It may be possible to design robust, oligogenic resistance systems which will be difficult for the virus to overcome.     

The development and use of microsatellite markers for genetic analysis and plant breeding with emphasis on bread wheat

In recent years, a variety of molecular markers, based on microsatellites or simple sequence repeats (SSRs) have become the markers of choice, thus necessitating their development and use in a variety of plant systems. In this review, the basic principles underlying different hybridization-based (oligonucleotide fingerprinting) and PCR based approaches (STMS, MP-PCR, AMP-PCR/ ISSR/ ASSR, RAMPs/ dRAMPs, SAMPL), making use of microsatellites, have been outlined. Different methods for enrichment of genomic libraries for microsatellites have also been outlined. Relevant literature on the subject, giving a summary of results obtained using each approach, has been reviewed and critically discussed. The review also includes a discussion on literature, which deals with the use of microsatellites in genome mapping, gene tagging, DNA fingerprinting, characterization of germplasm and cytogenetics research. Special emphasis has been laid on the genome of bread wheat, where the work done in the authors' own laboratory has also been briefly reviewed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Wheat breeding for end-product use

High grain yield is the primary objective of most wheat breeding programs around the world. In some countries, for example Australia and Canada, a new wheat cultivar must meet a prescribed level of quality before it can be registered for commercial production. For most traditional uses, wheat quality derives mainly from two interrelated characteristics: grain hardness and protein content. Grain hardness is a heritable trait but it can be strongly affected by abnormal weather conditions such as excessive rainfall during the harvest period. Protein content is weakly heritable and strongly dependent on environmental factors such as available soil nitrogen and moisture during the growing season. In addition, each end-use requires a specific 'quality' in the protein. Quality is determined by the molecular structure of the major proteins of flour which, in turn, controls the interactions of the proteins during the breadmaking process. Durum wheats have the hardest grain texture and are usually high in protein content. They are especially suited to the production of pasta because of their highly vitreous grain (high milling yield of semolina), unique combination of storage proteins for good cooking quality of pasta, and high yellow pigment content required for attractive appearance of cooked product. All three characteristics are highly heritable and can be readily improved by conventional breeding. Recent research has shown that the presence of γ-gliadin 45 is a reliable marker of good cooking quality. This marker is now used for screening early generation material in many durum wheat breeding programs. Common (hexaploid) wheats cover a wide range of grain hardness and protein content. The hardest wheats of this class, generally highest in protein, are used for pan bread. Considerable progress has been achieved in research of the molecular properties of flour proteins that are required for highest bread quality. The key protein component in this regard is glutenin. Segregating breeding populations can be screened by electrophoresis or high performance liquid chromatography for the presence of desirable glutenin subunits. Common wheats of medium hardness and lower protein content are used for other types of bread and noodles. Wheats with softest texture and lowest protein are used for cakes and cookies. In some end-uses, e.g., Chinese-type noodles, starch quality is important together with protein quality; this feature should be taken into consideration in developing a screening strategy for wheats for this application. Screening tests that reflect end-use requirements for most of the known products are available, and should be applied in testing wheats according to intended use. This revised version was published online in August 2006 with corrections to the Cover Date.     

The use of synthetic octoploids in strawberry breeding

Summary A number of methods of producing synthetic octoploids from Fragaria species of lower levels of ploidy are described. Six synthetic octoploids were produced from various combinations involving one hexaploid species, two tetraploid species and four diploid species. Three of the synthetic octoploids are male fertile and were used successfully as male parents in crosses with octoploid strawberry cultivars.     

Family selection in plant breeding

Summary Plant breeding programmes rarely take explicit practical account of the two sources of genetic variance, namely between and within full-sib families, even though existence of these two sources of variation has long been recognised. This paper refers to inbred and clonal crops, not to outbred, seed-propagated species. Theory suggests that the two variances should be of similar size, sometimes very similar. Good comparisons have never been made because variance within families is laborious to estimate. It is proposed that sets of families be assayed as to means in formal trials and only the best few, judged against standards as having potential for superior segregates, should be exploited thoroughly. The calculation as to approximate equality of genetic variances between and within families is important. The vast majority of families should probably be discarded without further ado, and at considerable economy. Sensible decision-making requires an economic component in order to exploit the trade-off between the cost of the initial trial and families discarded without further cost. The object of this paper is a critical review of practical principles, not a general review of a large and diffuse literature.     

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.     

A database management system for information retrieval and documentation of experiments for plant breeders

Summary An existing database system for seed collections has been augmented by programs providing plans for field experiments, record sheets and adhesive labels for seed packets. Information on accession names and parentage is picked up from the database. At the end of the season information on material seleeted in the trials is automatically added to the database.     

Linear variance models for plant breeding trials

Most plant breeding trials involve a layout of plots in rows and columns. Resolvable row–column designs have proven effective in obtaining efficient estimates of treatment effects. Further improvement may be possible by postblocking or by inclusion of spatial model components. This study reviews options for augmenting a baseline row–column model by the addition of spatial components. The models considered are different variants of the linear variance model in one and two dimensions. Usefulness of these options is assessed by analysing a number of field trials from plant breeding and variety testing.     

The population genetics of plant pathogens and breeding strategies for durable resistance

The durability of disease resistance is affected by the evolutionary potential of the pathogen population. Pathogens with a high evolutionary potential are more likely to overcome genetic resistance than pathogens with a low evolutionary potential. We will propose a set of guidelines to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. Under our model of pathogen evolution, the two most important parameters to consider are reproduction/mating system and gene/genotype flow. Pathogens that pose the greatest risk of breaking down resistance genes are those that possess a mixed reproduction system, with at least one sexual cycle per growing season and asexual reproduction during the epidemic phase, and a high potential for gene flow. The lowest risk pathogens are those with strict asexual reproduction and low potential for gene flow. We will present examples of high- and low-risk pathogens. Knowledge of the population genetic structure of the pathogen may offer insight into the best breeding strategy for durable resistance. We will present broad guidelines suggesting a rational method for breeding durable resistance according to the population genetics of the pathogen. This revised version was published online in August 2006 with corrections to the Cover Date.