Selection for combining ability in Pyrethrum,Chrysanthemum cinerariaefolium Vis.

Summary Pyrethrum is grown for the insecticidal pyrethrins, present mainly in the flowers. It is a cross-fertilizing perennial which can be propagated vegetatively. The main selection criteria are fresh flower yield and pyrethrins content; important secondary criteria are flower size and resistance to lodging; dry matter content varies very little.For these traits, the breeding values (general combining ability) of 22 clones were determined by means of a polycross. The heritabilities were estimated from the regressions of the polycross progeny data on those of the female parent clones; these were: flower yield 0.31; pyrethrins content 0.70; flower size 0.80 and resistance to lodging 0.87. For dry matter content no correlation between parents and progenies could be measured. Independent heritability estimates, obtained from 33 single crosses between clones not represented in the polycross, were: flower yield 0.56; pyrethrins content 0.64. Of 18 single crosses, made between clones also represented in the polycross, the actual flower yields and pyrethrins contents were highly correlated with the estimated yields and contents which were calculated from the breeding values of the parent clones (r = +0.87 and +0.86, respectively).These data show that the performance of single crosses is caused predominantly by the general combining ability of the parent clones. Therefore, single crosses offer no fundamental advantages over synthetic varieties, so that, for practical reasons, the latter are preferred. At the same time it is recommended to practise reciprocal recurrent selection as this will exploit the general combining ability in the initial stages of selection and the specific component increasingly in later selection cycles.Published with the approval of the Director of Agriculture, Kenya.     

Recurrent selection as a breeding procedure for rye and other cross-fertilized plants

Summary The inadequacy of the established breeding methods (mass selection, family selection) and the fact that a breeding procedure based on heterosis is not feasible for all crops, have given the incentive to search for more effective breeding methods, in particular for cross-fertilized plants.On the basis of experiments with rye, the writer comes to the conclusion that a breeding system founded on the polycross test is somewhat unsatisfactory, because it leads too soon to a narrowing of the material, so that the degeneration caused by inbreeding cannot be avoided.Better possibilities are offered by the methods of recurrent selection developed for maize, particularly by those schemes initiating from two entirely unrelated populations (reciprocal recurrent selection). The two populations are subjected to a test cross system whereby individuals from one population are outcrossed on representative groups of individuals from the other one, and conversely. Tests of the progenies obtained in this way reveal which individuals from the population K combine well with population L, and vice versa. By following the line described in the article, a narrowed population K (indicated as K₁) and a narrowed population L (indicated as L₁) are obtained which more or less complement each other. This complementary character is enhanced by repeating the selection process. Crossing in mass of these two narrowed populations may result in a valuable hybrid which is best referred to as a varietal hybrid. It seems that here the danger of inbreeding is reduced to a minimum.This method has already yielded favourable results at the Plant Breeding Station of the Central Bureau at Hoofddorp c.f. Koopman (7). From the current experiments at the Institute of Agricultural Plant Breeding at Wageningen it also appears that by the breeding system described in this paper many fallacies of the old procedures are eliminated.     

Quantitative genetics in an apple breeding strategy

Summary Open-pollinated seed of 500 genotypes of commercial cultivars, breeding selections and wild species of apples (Malus × domestica) was collected from overseas mixed-clone repositories. This is to generate the first generation of the new Apple Breeding Population.Subsequent generations will be intercrossed in a pair-crossing design. Recurrent selection for general combining ability (GCA) is the breeding method used for genetic improvement of the Breeding Population. Every parent in the control-pollinated breeding population is tested for GCA using a polycross mating design.The best individuals of the Breeding Population, intensively selected between families using the polycross GCA estimates, and also within families are intermated to produce a Cultivar Production Population. Trees selected from this have then to be evaluated in grafted clonal tests before commercial release.Each generation, a new Breeding Population of 500 genotypes is selected from the pair-cross families, based on parental GCA values (from polycross tests), full-sib family means and individual performance and then intercrossed. Greatest emphasis will be on within-family selection not between-family, so as to avoid reducing the effective population size.     

The inheritance of resistance to Empoasca kraemeri Ross & Moore in the common bean,Phaseolus vulgaris L.

Summary Genotypes of Phaseolus vulgaris differ in their level of resistance to Empoasca kraemeri, but the speed with which these differences can be accumulated into a resistant variety will depend on whether they can be detected in single F₂ plants, and on whether the genetic control of resistance is additive. Thirty-eight homozygous genotypes were therefore grown in small plots with and without insecticidal protection, and also in simulated F₂ populations containing one plant of each genotype. In addition, the F₂ and F₃ generations of crosses were studied. Both generations of a 13 parent diallel were grown with and without insecticidal protection. Six crosses, involving some of the parents in the diallel and two additional genotypes, were studied in more detail. Protected and unprotected F₂ plants from these crosses were harvested individually, and the resulting F₃ families were grown with and without protection. In all the expriments, the level of damage to the unprotected plants and the vigour of the protected plants was assessed visually by means of a numerical score, and the seed yield was measured.The performance of a single homozygous plant was a reasonable predictor of the performance of the same genotype in a small plot, but in the six crosses studies in detail, the correlations between F₂ and F₃ performance were low. In the analysis of the diallel cross, the significant differences among crosses were due to specific combining ability and not general combining ability. These results indicate that the genetic control of resistance is largely non-additive, and that F₂ selection will probably be ineffective. However, F₃ selection should be effective, and transgressive segregants for resistance should be obtainable.In some of the six crosses, the F₃ families differed both in general vigour and in narrow-sense resistance to E. kraemeri, as defined by Galwey & Evans (1982). The visual assessment of damage symptoms in each plot was related to the subsequent seed yield of the plot, but was more heritable than yield. The relationship between these two variables was investigated by the genetic correlation coefficient and by an alternative method, and was found to be partly genetically determined, but partly due to the influence of environmental factors on both variables. This indicates that screening for E. kraemeri resistance should be done by means of a visual damage score in a carefully chosen, uniform environment.     

Effect of Bradyrhizobium Strain on Combining Ability for Nitrogen Fixation in Peanut (Arachis hypogaea L.)*

Improving the host genotype is one method of increasing biological nitrogen fixation by Bradyrhizobium in symbiosis with peanut (Arachis hypogaea L.), but host-strain interactions also must be considered. The objective of this study was to estimate the effect of Bradyrhizobium strains on the inheritance of traits related to nitrogen fixation. Eight parents and the F₁ generation of a diallel cross were evaluated in symbiosis with two bacterial strains for combining ability in a factorial arrangement in the greenhouse. The parents represented three taxa of cultivated peanuts. The two Bradyrhizobium strains NC 92 and NC 123 produced different general and specific combining ability, maternal (reciprocal general) effects, and reciprocal specific effects for the host genotypes. With strain NC 92, the peanut cultivars NC 7 and NC Ac 2821 had the best general combining ability for all traits evaluated; while, with strain NC 123, Robut 33-1 exhibited a high general combining ability (GCA) for nitrogenase activity and specific activity. Genotype NC Ac 2821, when inoculated with strain NC 123, had a negative GCA effect for nodule number, while with strain NC 92 this cultivar had the highest GCA effect. The importance of strain selection in evaluating lines in a breeding program is discussed.     

Genetic combining ability for concentration of mineral elements in cabbage head (<Emphasis Type="Italic">Brassica oleracea</Emphasis> var<Emphasis Type="Italic">. capitata</Emphasis> L.)

Brassica vegetables are important source of dietary nutrition. However, information on the genetic combining ability of mineral elements such as iron, zinc, copper, manganese, potassium and calcium or their types of genetic effects (i.e. additive or non-additive) is scarce but important as it influences the selection of parents and breeding approaches to be adopted for improvement of nutritional quality of cabbage. Therefore, an attempt was made to estimate combining ability in a line × tester (5 × 11) mating design for minerals. Significant mean square for line × tester interaction was observed for all minerals under study indicating the prevalence of non-additive variance; while less than unity value of σ_gca² /σ_sca² ratio for iron, zinc, manganese, potassium and calcium accumulation indicate predominance of non-additive gene action. The parents 83-2, Pride of Asia, Pusa Mukta, Red Cabbage and MR-1 were found good general combiners for four mineral elements. The general combining ability effects of the parents for various minerals revealed that none of the parents excelled for all the minerals suggesting the need for multiple crossing approaches. The cross 83-1 × AC-1019 (Poor × Poor general combiner) exhibits desirable significant specific combining ability effects for all six minerals might be due to presence of high magnitude of non-additive especially complementary epistatic effects which can be utilize for commercial exploitation of heterosis. This study shows clearly that specific combining ability is more important than general combining ability for predicting hybrid combinations for high mineral content in cabbage head.     

Diallel analysis of combining ability and heterosis for yield and yield components in rice by using positive loci

Diallel crosses have been widely used for analysis of general combining ability (GCA), specific combining ability (SCA) and heterosis. In the present research, 12 lines were selected from 60 inbred rice bred by International Rice Research Institute with extremely-high or -low yielding records according to the previous three consecutive seasons of yield trial experiments, to construct a half diallel cross. The genetic distances (GDs) revealed by molecular markers for the 12 lines ranged from 0.2288 to 0.7169, averaging at 0.5882; clustering analysis showed the 12 lines were divided into four groups maintaining the original cluster structure of the 60 lines. The positive loci (PLs) including effective-increasing loci (ILs) and effective-decreasing loci (DLs) were screened. The results showed that hybrids derived from those parents with higher GCA effects had better performance for traits of yield and yield components. The SCA effects were strongly correlated to F₁ performance as well as heterosis; the GDs based on ILs were significantly positive correlated to SCA effects and heterosis for yield and yield components while the GDs based on DLs showed the significant negative correlations. Based on this research, a new conception, i.e. general sum of combining ability (GSCA) was conceived, which is defined as the sum of GCAs for two parents of a hybrid. The GSCA and SCA showed similar correlations with traits of yield and yield components. The results illustrated that ILs could be used for further study on prediction of heterosis for traits of yield and yield components; and GSCA may be another considerable parameter combined with ILs for breeders in selecting elite hybrid.     

Combining ability of soybean (Glycine max) for low phosphorus tolerance on acidic soils of Western Ethiopia

Determining the gene actions governing the inheritance of traits of interest has paramount importance in designing a breeding approach to improve the progeny populations. This study was undertaken to determine the combining ability of nine selected parental lines in the F₂/F₃ segregating populations for low P tolerance. The experiment was laid out in an alpha lattice design in two locations, that is Mettu and Assossa of Western Ethiopia that are characterized by soils with low P and pH. General combining ability (GCA) effects were highly significant for grain yield, pod length, days to maturity and plant height, while specific combining ability (SCA) effects were highly significant for grain yield, 100-seed weight, pod length and plant height. The parent Hardee-1 was identified as the best general combiner for yield, number of seeds per pod, pod length, plant height and pod number. The results suggest that additive gene action was important for several of the studied traits, implied by significant GCA effect, which might indicate selection for these traits could be effective in later segregating generations.     

The effect of accidental selfing on the analysis of a diallel cross with sugar cane

Summary The diallel cross was found to be an unsuitable design for quantitative genetic analyses with sugar cane.Accidental selfing caused substantial inbreeding depression for most characters. Consequently, analysis of a diallel cross without regard to the effect of accidental selfing would be subject to bias, if accidental selfing were common. A selfing model proposed by Dudley (1963) was extended to include differential selfing, and was tested in an experiment in which the percentage of selfing in each cross was estimated visually.Variance components had large standard errors, which prevented firm conclusions from being drawn, but selfing appeared to cause substantial bias in the estimates. Estimates of genetic variance components from a p(p-1) diallel set, using the extended selfing model, were compared with estimates obtained from the standard analyses in which reciprocal or maternal effects should account for the effect of selfing. The estimates were comparable, but the standard analyses were simpler and produced estimates with lower standard errors.When the effects of selfing were minimized, general combining ability (g.c.a.) effects appeared to be much more important than specific combining ability (s.c.a.) effects for most characters.     

Genetic variation, heritability and progeny testing in meadow bromegrass

Meadow bromegrass is a recently introduced pasture grass in western Canada. Its leafy production and rapid regrowth have made it a major grass species for pasturing beef animals in this region. Because relatively little breeding work has been done on this species, there is little information on its breeding behaviour. The main objective of this study was to estimate genetic parameters and assess breeding methodologies for meadow bromegrass. Forty half‐sib (polycross and open‐pollinated) and full‐sib (selfed) progenies were evaluated for dry matter and seed yield, fertility index, harvest index, plant height, plant spread, crude protein, neutral detergent fibre and acid detergent fibre. Genetic variation for seed yield, harvest index, and plant height was significant in the open‐pollinated and selfed tests. Genetic variation estimates for dry matter yield were negative (polycross), not significant (open‐pollinated), or significant (selfed). Estimates of genetic variation for quality traits were not significant, except for acid detergent fibre (selfed). Correlations among characters indicated that it is possible to simultaneously improve seed and forage yield. Rankings of progenies by the half‐sib tests for forage and seed yield were not altered by the use of the synthetic parental value (SV_i), which includes information from selfed progeny. The polycross progeny test did not discriminate parents as well as the open‐pollinated and selfed tests; this may have been a result of non‐random pollination in the polycross. Correlation among the progeny tests, showed that open‐pollinated and selfed progeny tests agreed for all characters, except for harvest index, while correlation between polycross and selfed tests were significant for six characters but not dry matter yield, and acid and neutral detergent fibres. The most dissimilar tests were polycross and open‐pollinated, with significant correlations found only for fertility index, height, spread and neutral detergent fibre. It was concluded that there was significant variation and moderate heritability for most traits in the meadow bromegrass populations evaluated, and that the open‐pollinated progeny test is the method of choice for selecting parents for synthetics of this species.     

Application of half-sib mating for genetic analysis of forage yield and related traits in Bromus inermis

Despite efforts made, forage yield of smooth bromegrass has increased slowly over the last 50 years of breeding. It therefore seems necessary to investigate more on the genetic basis of agro-morphological traits in this cool-season, highly drought resistant grass. The present study was aimed at estimation of total genetic variance, narrow-sense heritability, general combining ability, phenotypic and genotypic correlation among different quantitative traits in half-sib (HS) families derived from polycross of 25 smooth bromegrass genotypes that were mainly originated from Iran. Families differed significantly for all of the agro-morphological and quality traits measured. Narrow sense heritability ( ${\text{h}}_{\text{n}}^{2}$ ) ranged from about 0.2 (plant height) to 0.7 (day to inflorescence emergence and day to anthesis). Moderate to high heritability for forage dry matter yield (0.42) indicates that phenotypic selection for this trait can be successful. A wide range of general combining ability was observed for most of the studied traits, especially those related to forage yield. Relatively low genetic variation and heritability for crude protein and also negative correlation of this trait with forage yield, indicates a low probability of improving forage yield and quality simultaneously.     

A polycross pattern formula

Summary A formula defining a polycross pattern is introduced. The pattern appears to be a special latin square in which every clone has any one other clone as a neighbour and just once in each of the four directions North, South, East, and West, presenting an ideal polycross pattern. The formula holds good for n clones when n+1 is a prime.     

杂交粳稻亲本米质性状优异配合力的标记基因型鉴定

杂交粳稻米质整体水平不如常规粳稻也是限制杂交粳稻广泛种植的原因之一。本研究选用115个SSR引物扩增6个粳稻BT型不育系和12个恢复系的标记基因型,并分析72个F₁组合谷粒长、谷粒宽、糙米率、精米率、整精米率、垩白米率、垩白度、糊化温度、胶稠度和直链淀粉含量10个米质性状的配合力,结合亲本SSR分子标记数据和性状配合力数据筛选了10个米质性状优异配合力的标记基因型。结果共鉴定出30个SSR标记基因型与亲本10个米质性状配合力显著相关,其中25个与亲本米质性状不良配合力相关,5个与优异配合力相关。标记基因型RM263-175/180和RM444-230/240可以使F₁整精米率分别提高3.2%和2.5%。RM3-120/150可以使F₁谷粒长缩短2.4%,RM444-180/240可以使F₁谷粒宽增加2.1%。RM428-273/294可以使F₁植株上的杂交稻米直链淀粉含量减少7.0%。有8个标记基因型同时也影响产量性状配合力。RM3-120/150同时可以使F₁的每穗总粒数和每穗实粒数分别增加15.9%和10.9%。RM1211-150/160可使F₁的糙米率和精米率分别减少0.9%和1.1%,同时使F₁的每穗总粒数和每穗实粒数分别增加21.8.%和20.4%。RM23-150/160可使F₁的垩白米粒率和垩白度分别增加44.1%和45.7%,同时使F₁的单株日产量和每穗总粒数分别增加11.2%和11.6%。这些结果可用于指导亲本米质性状和产量性状配合力的分子标记辅助改良以及未来杂交粳稻组合配置中的亲本选配。     

Use of molecular marker diversity to increase forage yield in timothy (Phleum pratense L.)

Molecular marker technologies may provide a tool to overcome the forage yield plateau in timothy (Phleum pratense L.). Therefore, a study was designed to investigate the relationship between marker‐based genetic distance (GD) estimates and yield. Genetic distances were estimated using Dice coefficients framed by 28 simple sequence repeat markers. In Experiment 1, parents with high general combining ability (GCA) from two contrasting first‐generation synthetic strains (exhibiting high and low yields) were compared. Average GDs of each strain were 0.74 (high) and 0.68 (low). These differences indicated that GD may be partially the basis for contrasting yields. In Experiment 2, GDs among 40 parents of a polycross were used for calculating general genetic distance (GGD), which is indicative of their allelic complementation. Analyses revealed a significant (P < 0.01) moderate correlation (r) between GGD and GCA for yield (r = 0.45) and a significant (P < 0.01) residual mean square for the regression of yield on GGD, suggesting that considerable non‐additive effects were associated with GCA. The results are indicative of the potential use of GD estimation for yield improvement in timothy.     

Combining ability of resistance to head blight caused by Fusarium culmorum (W.G. Smith) in the F1 of a seven parent diallel of winter wheat (Triticum aestivum L.)

Fusarium head blight (FHB, scab) caused by Fusarium spp. is a widespread disease of cereals causing relevant yield and quality losses and contaminating cereal products with mycotoxins. Breeding resistant cultivars is the method of choice for controlling the disease. Resistance to FHB is a quantitative trait and is most likely governed by several genes. We present the results of an F₁ diallel analysis of FHB resistance involving six resistant and one susceptible European winter wheat genotypes of diverse origin in order to identify promising combinations for the selection of improved cultivars. Parents and F₁s including reciprocals were evaluated for FHB resistance in an artificially inoculated field trial. Two traits were assessed: visual disease symptoms on the heads and the percentage of Fusarium damaged kernels in a harvested sample. General combining ability (GCA) and specific combining ability (SCA) effects were statistically significant for visual symptoms and kernel damage, whereas reciprocal effects were small or not significant. Heterosis for resistance was common, indicating that the parental genotypes possess different resistance genes. Selection of transgressive segregates should be feasible from such heterotic combinations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phenotypic recurrent selection for oil content in East Indian lemongrass

Summary Phenotypic recurrent selection was carried out for oil content in East Indian lemongrass, Cymbopogon flexuosus (Steud) Wats. In each cycle, the top 5% of plants for oil content were selected. Their ramets were used for the establishment of replicated polycross blocks in isolation. Realized gains from selection were determined by evaluating the populations and the best clones from each population in separate two-year replicated trials. Three cycles of recurrent selection increased oil content by 32% per cycle at the population level. Realized gains from selection were slightly smaller or similar to predicted gains. Mean realized heritability for oil content was 0.56. At the clonal level, the best C₁ and the best C₂ clones had, respectively, a 22% and 73% higher oil content than the best clone from the base population (C₀). Selection for oil content did not adversely affect leaf yield, dry matter content or citral content in oil. Probable reasons for the high response to selection are mentioned.     

On bacterial wilt of alfalfa (Medicago sativa L.) caused by Corynebacterium insidiosum (McCull.) Jensen,in Italy

Bacterial wilt of alfalfa (Medicago sativa L.) caused by Corynebacterium insidiosum (McCull) Jensen was unknown in Italy until 1957. The authors established in that year in the province of Bologna this dangerous disease in 2–3 year-old stands of alfalfa which appeared uniformly and extensively thinned.They started a breeding programme with the use of bacterial wilt resistant material from the U.S.A., while also studies on methods of testing for resistance under Italian conditions are in progress.     

Bread wheat: F1 hybrid performance and parental diversity estimates using molecular markers

In wheat, the possibility of introducing F₁ seed into practical agriculture has been greatly enhanced by the discovery of effective chemical hybridising agents (CHAs). Although some technical and economic problems concerning the use of CHAs for large-scale production of F₁ seed remain to be solved, a first group of F₁ hybrids has been submitted for registration in several European countries i.e., France, England and Italy. Combining ability for grain yield and several agronomic and quality traits was studied in an eight-parent diallel cross. Highly significant combining ability effects were observed for all the traits while specific combining ability effects were statistically significant for grain yield, plant height, heading time and Chopin alveograph parameter P. The level of genetic diversity between parents as estimated using molecular markers is considered a tool for predicting the hybrid performance and heterosis of crosses. To explore this possibility, RFLP and RAPD markers were used to predict the performance of hybrids obtained from diallel and top crosses. The performance of the hybrids was determined in replicated plot trials sown at normal seed density in several locations. Coefficient of parentage (rp), based on pedigree information for all the pairwise combinations of the parents ranged from 0.01 to 0.34. The parents were assayed for random amplified polymorphic DNA (RAPD) with 87 primers which generated 304 polymorphic bands. Genetic similarity between parents, estimated on the basis of common bands using the Jaccard's similarity coefficient (J), ranged from 0.25 to 0.57. Correlation between parental diversity and hybrid performance was generally weak. A positive trend is observed in the yield potential of the hybrids produced in Italy in the last 10 years. In fact among the first set of hybrids produced by random crossing of the available cultivars, none produced 10% more than the checks whereas the last generation of hybrids includes combinations yielding 15% more than the best standards. Our results clearly indicate the need to develop specific strategies in order to identify and/or to select parental lines with a high level of general combining ability (GCA) and specific combining ability (SCA). The information regarding the genetic diversity of the parental lines do not appear helpful for predicting F₁ performance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Studies on the breeding of self-pollinating cereals

The behaviour of 19 varieties of spring wheat under different environmental conditions is discussed in terms of the following five characters: weight per grain, grains per spikelet, spikelets per ear, ears per plant and yield of grain per plant. The first four of these characters (the components of yield) are shown by correlation analyses to be largely independent of each other.Four varieties, chosen because they each showed extreme values of one or more of the components of yield, were crossed in all possible combinations. The F₁ and F₂ of this diallel set of crosses was analysed for each character according to the method of Jinks and Hayman. The results are discussed in relation to crop improvement and predictions are made for each character as to which cross or crosses are likely to be the most profitable for the breeder. The future handling of this material in order to test these predictions is outlined.     

The problem of induced sterility: A comparison between EMS and X-rays in Arabidopsis thaliana

Summary The problem of mutagen-induced sterility has been especially studied in barley by Gaul, and in Arabidopsis by Müller and by van der Veen. Their results and conclusions are essentially confirmed and extended by the present Arabidopsis experiment, which compares EMS with X-rays.For a given treatment, M₂-fertility is correlated with M₁-fertility, but mutant frequency in M₂ is equal for the different M₁-fertility classes. It follows that efficiency in mutation breeding can be increased by pre-selection for fertility in M₁, and by using doses which do not make fully fertile M₁-sectors too rare.EMS is more efficient than X-rays by a factor 4 at equal M₁-fertility, and a factor 3 at equal M₂-fertility. As a result of difference in fertility spectrum, fertility improvement from M₁ to M₂ is better for X-rays than for EMS.A considerable load of diplontic sterility and of embryonic lethals (non-germinating seeds) is simply the toll one has to pay for the higher mutant frequency after EMS.Fortunately, part of the diplontic sterility can be pre-selected against in M₁, and maternally conditioned embryonic lethals can be selected against by discarding seedlots from individual M₁-sectors which show reduced germination. (M₂-line selection may also be practised against recessive embryonic lethals and seedling lethals.)