A comparison between single seed descent and early cross selection in wheat breeding

Summary Two selection procedures in wheat breeding were compared on the basis of their ability to supply high yielding inbred lines. The first procedure consists of an early selection between crosses in the F₃ generation, based on predictions of the cross mean and the between line variance. In the second procedure selection is postponed until the F₆, which is derived by single seed descent. The two procedures are evaluated in a two year test, using pseudo-lines of spring wheat. These pseudo-lines consist of mixtures of varieties and enable an estimation of the exact genetic parameters. In this way the accuracy of the predictions can be examined.In case of early selection, it appears that the predictions of the cross mean and especially the between line variance are very inaccurate. This is caused by the effects of plot size, intergenotypic competition and, to a lesser extent, dominance and/or epistasis. It results in an erroneous ranking of the crosses and the discarding of the potentially best cross. The F₆-SSD line estimates are much more accurate and thus the better lines are indeed selected. A first comparison between the two selection procedures therefore indicates a preference to the SSD method.     

Cross prediction in bread wheat germplasm using single seed descent lines

Populations of F₆ recombinant inbred lines, generated by single seed descent from a half diallel among eight bread wheat lines adapted to the East African highlands, were used to identify those crosses that were more likely to produce cultivars which combined resistance to yellow rust with improved yield. Crosses having the most resistant line as one parent offered the best prospect of success, particularly those which produced F₁hybrids exhibiting better parent heterosis. For plot grain yield there was a highly significant correlation between the observed and predicted rankings of the recombinant inbred line populations for the proportion of individual lines equalling or surpassing the target value. For yellow rust severity, however, this correlation was non-significant when a target value of zero was used. Adopting a slightly less stringent target of 0.25, coupled with the omission of two aberrant populations, increased this correlation significantly. The plant breeding implications of these results are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of pedigree selection and single seed descent for oil yield in linseed (Linum usitatissimum L.)

Summary Four populations of linseed derived from five parents were advanced from the F₂ to the F₆ generation by SSD and to the F₅ by pedigree selection. These populations were used to compare the efficiency of the two methods in order to provide superior genotypes with respect to grain yield and oil content, i.e. oil yield. The results showed minimum differences between pedigree and SSD lines for grain yield, where in only one cross the SSD lines were significantly superior to the pedigree lines. Since pedigree selection was carried out for both, seed yield and oil content, a positive response to selection was expected. However, early selection for yield, a character with low heritability, was not successful. On the contrary, significant differences for oil content were detected between the two groups of lines in three of the four crosses studied. In these cases the pedigree lines were superior to the SSD lines. These results demonstrate that strict selection in early generations for oil content, a character with comparatively high heritability, is feasible and successful in linseed. However, selection for seed yield should be postponed to later inbred generations. Consequently, in breeding for maximum oil yield of linseed a two-step selection procedure is recommended.Abbreviations PS pedigree selection - SSD single seed descent - TGW thousand grain weight     

Inter-genotypic plant competition in wheat under single seed descent breeding

Summary The effects of inter-genotypic competition were studied in wheat under conditions of single seed descent in the glasshouse. Four genotypes were grown mixed in diallelic combinations of two, making six mixtures and four component (pure) stands. It was found that genotypic differences in most cases over-rode initial effects of phenotypic differences in seed size, and played a dominant role in determining what plants, if any, were eliminated from the mixtures.Low genotypic competitive ability accounted for 8 to 14% losses, in a single generation, in representation in the population through failure to produce at least one fertile tiller. Howerver, under single seed descent where artificial selection assures retention if even only one seed is produced, the loss of weak competitors was slower than it might have been under natural selection in the field.     

On the selection for specific genes by single seed descent

Summary Single seed descent is an easy inbreeding strategy aimed at deferring selection in successive generations of a cross between two homozygous lines to that generation for which selection looks most successful. Different selection aims are discussed. For characters that are determined by a small number of (major) genes, numbers of plants and generations are tabulated that guarantee the breeder to achieve his aim with a given level of heterozygosity and a given probability.     

Use of immature seed germination and single seed descent for rapid genetic gains in pigeonpea

Long duration required for generation advancement in pigeonpea [Cajanus cajan (L.) Millsp] is one of the major bottlenecks in realizing rapid genetic gains. Therefore, a technology for rapid generation turnover is warranted to facilitate the development of new cultivars and recombinant inbred lines. Breeding of early‐maturing cultivars has now opened up the possibility of rapid generation advance (RGA) in this crop. This paper reports the development of an RGA technology that integrates the germination of immature seeds with single seed descent method of breeding. The results showed that immature 35‐day‐old seeds can be used successfully to turn over a generation of pigeonpea with 100% seed germination. These way 3/4 successive generations can be grown within a year. The methodology presented in this study will accelerate the breeding process for breeding cultivars and develop rapidly the materials required for genomics research in pigeonpea.     

A theoretical study of the bulk breeding method

Summary The consequences of sampling losses that occur with the bulk breeding method are studied. The expected extinction rates are computed for various population sizes and various distributions of the number of seeds per plant (multiplication rates), using algebraic formulas in simple cases and computer simulations in more complex ones. Expected extinction rates are always high: they exceed 20 per cent after one generation. If the multiplication rate is not too variable, about 36, 50, 60 and 66 per cent of the lines are lost after 1, 2, 3 and 4 generations of bulk propagation respectively. The expected extinction rate increases almost linearly with the coefficient of variation of the multiplication rate after the first generation of bulk propagation. Random losses only partially add to sampling losses. Their effect is enhanced by a high coefficient of variation of the multiplication rate, but it is more and more reduced as generations advance.The selection resulting from sampling is also studied. It is shown that the response to selection is equal to the response that would be observed in a truncation selection procedure with an intensity equal to the coefficient of variation of the heritable component of the multiplication rate.Abbreviations BK bulk breeding - SSD single seed descent - HD haplodiploidization     

Selection for yield in wheat breeding

Summary If selection based on F₃ yield tests is to be effective, the yield tests must be successful in discriminating among yield genotypes. The available literature indicates that simple tests with limited or no replication are not very effective, although more extensive, replicated tests may be.Data from an experiment comparing F₃ yield tests with a single seed descent procedure showed that F₃ selection based on a two-replicate test with single seed descent procedure did not justify the extra work involved.     

Accelerated pedigree selection: An alternative to individual plant selection in the normal pedigree breeding method in the self-pollinated cereals

Summary Accelerated pedigree selection (APS) involves initial selection based on the assessment of lines rather than individual plants. These lines can be derived by accelerated generation procedures. Unlike single seed descent (SSD), line selection can begin in an early generation. This minimises the risk of differential mortality or sterility. Brief outlines of APS for spring and winter cereals are given. The length of the APS breeding cycle is shorter than that of either pedigree selection (PS) or SSD. In addition to higher efficiency of selection for yield, which must be offset against the need for greater resources, it is expected that APS should result in valuable genotypes being retained and better selection for desirable combinations of characters.     

The effectiveness of early generation (F3) yield testing and the single seed descent procedures in two cowpea (Vigna unguiculata (L.) Walp.) crosses

Summary Two breeding procedures were compared in two cowpea crosses. Both procedures were started from the same selected F₂ plants in each cross. For the early generation yield testing, the F₃ lines were yield tested and a pedigree and bulk methods followed in F₄ and F₅. Each F₅ line was bulked to provide seed for a yield test in F₆. In the Single Seed Descent (SSD) procedure, a single seed was taken from each F₂ plant to produce the F₃ generation. The procedure was repeated for the F₄ and F₅. The F₆ SSD lines and the F₆ yield testing lines were compared in yield tests.The results showed that differences in yielding ability of F₃ lines persisted over generations indicating that selection was effective. This was confirmed by the high significant correlations between F₃ yields and those of later generations which ranged from r=0.51^* to 0.85^**.The grain yields of lines derived by the single seed descent procedure were as good as those derived from early generation yield testing.Significant linear correlation between visual rating of F₃ and F₆ yields with actual yields revealed that it is possible to identify promising lines of cowpea visually.     

A comparison between single plant plots and five plant plots for the initial selection stage of a potato breeding programme

Summary The efficiency of single plant selection in the initial selection stage of a potato breeding programme was examined. A random sample of potato clones was grown in single plant plots and also in tworeplicates of five plant plots. After harvest, each plot was visually assessed by three potato breeders; total tuber weight and number of tubers per plant were also recorded, from which mean tuber weight was calculated. The error variances of the single plant plots were found to be significantly greater than from the five plant plots for total tuber weight, mean tuber weight and number of tubers per plant, but not significantly different for breeders' preference. Coefficients of correlation between single and five plants plots were significantly greater than zero for all traits examined, but they were lower than the corresponding correlations between the two replicates of five plant plots. Although a random sample of clones was examined, each clone had been assessed for breeders' preference the previous year. When the clones were grouped according to the previous years preference scores, it was found that the correlation coefficients between single and five plant plots for breeders' preference were inversely related, in magnitude, to the mean preference score of each group. It was concluded that single plant selection was generally ineffective, particularly when only the better clones, from the previous years assessment, are examined.     

Identifying superior parents in a potato breeding program using cross prediction techniques

Cross prediction techniques were applied to data collected from over 600 hybrid combinations of potato (Solanum tuberosum) and analyzed to determine the potential of using early generation cross prediction techniques to identify superior parental clones. Performance of parental lines based on parameters collected in early generations were compared with the observed frequencies of desirable recombinants with a common parent in the latter stages of a breeding program. Results showed that value of parents, in their ability to produce desirable recombinants in breeding programs, can be predicted using univariate cross prediction techniques. This type of information can be available from early generation progeny trials and could easily be incorporated into a practical potato breeding scheme. This revised version was published online in August 2006 with corrections to the Cover Date.     

Genomics-assisted breeding: The next-generation wheat breeding era

Common wheat provides approximately 20% of the total dietary calorie intake of human beings. Recent technological advances in whole-genome sequencing and their application in wheat and its progenitor species provide new opportunities to uncover the genetic variation of wheat traits and to accelerate the traditional breeding (TB) strategies in the context of genomics-assisted breeding (GAB). Integration of TB, marker-assisted selection (MAS) and genomic selection (GS) with high-density SNP markers is expected to accelerate the breeding process and to further enhance genetic gain. With the assistance of the next- or third-generation sequencing technologies and high-throughput phenotyping platforms, GAB can now realistically be considered in the following area: (i) genome sequencing and high-quality assembly to uncover new variations, (ii) whole-genome sequence-based association studies, (iii) gene function (or functional gene) identification and (iv) integration of whole genomic breeding information, utilizing multi-omics data and different breeding strategies. We argue that GAB is becoming the preferred strategy in pursuit of new wheat cultivars with superior traits on high yielding, high nutritional quality, climate-resilience and so on.     

A comparison of yield and yield component selection in winter wheat

Summary Three populations of winter wheat were formed by crossing Avrora to Sage, TAM W-101, and Danne. Approximately 10% of the F₂ plants from these crosses were selected for high and low levels of number of tillers per plant, number of kernels per spike, 1000-kernel weight, and grain yield. Forty-eight solid seeded F₃ lines obtained from the selected F₂ plants were then selected for high and low expressions of yield components and grain yield. Realized heritabilities were estimated. Indirect responses of yield to yield component selection and direct response to selection for grain yield were measured. Heritabilities were low for tiller number, number of kernels per spike and kernel weights but were high or intermediate for grain yield when selection occurred in the F₂ generation. When selection was practiced in the F₃ generation, heritabilities for tiller number and yield were low, but were intermediate to high for number of kernels per spike and kernel weight and high heritabilities were found for kernel weight. Selection for kernel weight often increased grain yield; however, direct selection for grain yields was usually as effective.Journal article no. J-4488 of the Oklahoma Agri. Exp. Stn., Stillwater, Oklahoma 74074.     

Honeycomb selection for yield in early generations of spring wheat

Summary The effectiveness of the honeycomb selection method for yield in spring wheat (Triticum aestivum L.) was evaluated using progenies from two wheat crosses, Glenlea x NB131 and Glenlea x Era. Honeycomb selection was carried out in the F₂ and F₃ generations, grown at the University of Manitoba in the summers of 1980 and 1981, respectively. In both generations, divergent selection was made for both high and low yield. Plants selected in the F₃ generation were entered in an F₄ yield test in the summer of 1982. Results of the experiment showed that honeycomb selection for yield in the F₂ and F₃ generations was effective in identifying parents of high- and low-yielding lines. F₃ plants from highyielding F₂ selections gave higher yields than those from low-yielding F₂ selections by 11.5% and 13.0% for Glenlea x NB131 and Glenlea x Era crosses, respectively. The F₄ yield test showed that high yielding selections from both crosses significantly outyielded by 8.9% low yielding selections and by 14.4% the unselected composite lines. It is concluded that the honeycomb selection method can be used for early generation selection in spring wheat.     

Agronomic performance of anther‐derived doubled haploid and single seed descent lines in crosses between primary and secondary winter triticale

Octoploid (8x) and hexaploid (6x) primary triticales (xTriticosecale Wittm.) can be used as crossing parents with secondary 6x triticales to enlarge the genetic basis of a breeding programme or introgress traits. Doubled haploid (DH) production permits to develop homozygous lines more rapidly from a segregating generation than other breeding methods such as single seed descent (SSD). Both anther‐derived DH and SSD lines were produced from reciprocal cross‐combinations between 8x and 6x primary and 6x secondary triticales. Field experiments of DH and SSD lines were conducted in three environments as two‐replicate lattices to measure seven agronomic traits. A tendency for higher grain yield, taller plants and a higher 1000‐kernel weight of SSD lines compared with DH lines was found. Significant genetic variation for all traits in both breeding methods was revealed, indicating their suitability to select superior genotypes. Hexaploid and even more so 8x primary triticales can profitably be included as crossing parents in a commercial breeding programme. In such crosses, the primary triticales should be used as the male parents if followed by DH method.     

The effect of low frequency random crossing on heterozygosity and frequency of recombinant homozygotes in lines developed by single seed descent

Summary This study examines the effects of random cross-fertilization on genotypic frequencies during the development of lines by the method of single seed descent, using a combination of generation matrix methods and equilibrium prediction equations for two linked loci. Results show that heterozygosity declines rapidly in early generations, as in the case of complete selfing, and declines to an equilibrium limit that is relatively insensitive to the degree of linkage and almost entirely determined by the frequency of crossing. The overall frequency of recombinant homozygotes is reduced by crossing but the frequency increases in successive generations to an equilibrium limit which is independent of the level of linkage and depends entirely on the frequency of crossing. The approach to the limiting frequency of recombinant homozygotes is extremely slow, in terms of generations, for closely linked loci and low frequency of crossing. The study then concentrates on the frequency structure of the F₈ generation to cover the time-span typically involved in practical breeding programmes.     

Comparison of selection procedures in breeding for seed yield in segregating sesame populations

Summary Four hundred sesame (Sesamum indicum L.) lines selected by five methods from five crosses were tested for yield and other agronomic characters in F₆ and F₇. More high yielding lines were isolated by the bulk method and in the cross involving two local cultivars when two seasons means and performance in each individual season were considered. The cross involving two exotic cultivars did not produce a single line superior to local recommended variety. The 80 highest yielding lines recorded a 10.8% increase in yield compared to the mean of 400 lines. Thirty five lines were among the top 20% both seasons with a 13.2% increase in yield. Highly significant seasonal and population effects were recorded. There were no significant differences among the methods of selection although the yield differences among lines were highly significant. Interactions of seasons × populations, seasons × methods, populations × methods and seasons × populations × methods were highly significant. The results indicate that simple and less expensive bulk method may be used with equal success as the pedigree, single seed descent or early generation testing methods when selecting for yield in segregating sesame populations. The importance of inclusion of a high yielding, locally adapted cultivar in the crossing programmes for yield breeding of sesame has also been revealed.Abbreviations BM Bulk method - EGT Early generation testing - G × E Genotype × environment - MSS Modified single seed descent - PM Pedigree method - SSD Single seed descent     

The relationship between powdery mildew (Sphaerotheca fuliginea) resistance and leaf chlorosis sensitivity in cucumber (Cucumis sativus) studied in single seed descent lines

Summary The genetic relation between powdery mildew resistance and sensitivity for leaf chlorosis of glasshouse cucumber was investigated. The powdery mildew resistant, leaf chlorosis sensitive hybrid variety Profito was crossed with the powdery mildew susceptible, non chlorosis sensitive hybrid variety Corona. Forty four F₆ inbred lines of this cross, produced by single seed descent, were tested for powdery mildew resistance (PMR) and leaf chlorosis sensitivity (LCS). PMR and LCS were positively correlated (r=0.59). One or more of the PMR genes probably causes LCS as a pleiotropic effect or is closely linked to LCS, but also other factors, not genetically linked to PMR can cause LCS. Five F₆ lines combined a significantly higher level of PMR than Corona with a significantly lower LCS than Profito. Three of these lines could hardly be distinguished from Corona for the level of LCS.Abbreviations LCS leaf chlorosis sensitivity - PMR powdery mildew resistance - SSD Single Seed Descent     

A comparison of traditional and haploid-derived breeding populations of oilseed rape (Brassica napus L.) for fatty acid composition of the seed oil

Summary Microspore embryogenesis technology allows plant breeders to efficiently generate homozygous micros-pore-derived breeding populations of oilseed rape (Brassica napus L.) without traditional generations of inbreeding. This study was conducted to compare the frequency distribution of microspore-derived population and single seed descent populations with respect to fatty acids of seed oil. Both microspore-derived populations and single seed descent populations were produced from each of three crosses made between selected parents containing contrasting amount of erucic, oleic, linoleic and linolenic acids. The fatty acid content of F₃ plants derived lines (F₅ seed) developed by single seed descent was compared to that of microspore-derived populations. The means, ranges and distribution pattern of seed fatty acid contents were similar in both populations for each fatty acid studied, although a few heterozygous lines were observed in the single seed descent populations. The results indicated that microspore-derived population form random, homozygous F₁ plant derived gametic arrays for all fatty acids evaluated. Selection for altered fatty acid composition in microspore-derived and single seed descent homozygous populations should be equally efficient, in the absence of linkage of traits investigated.