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.     

Genetics of stem elongation ability in rice (Oryza sativa L.)

Summary The genetics of stem elongation ability in rice was studied in parents, F₁, F₂ and backcross generations of six crosses. Segregation analysis indicated dominance for stem elongation ability. Estimation of genetic parameters under epistatic model indicated more than one locus control stem elongation ability and both additive and nonadditive gene effects were important. Epistatic effects were predominant over additive and dominance effects with an important role of duplicate type of epistasis. The occurrence of significant additive and additive x additive types of genetic variation and the moderately high broad sense heritability indicated the possibility of selection for an increased manifestation of stem elongation ability.     

The Concept of Varietal Ability for Partially Allogamous Crops

General (GVA) and specific (SVA) varietal ability quantify the value of genetic stocks as components for synthetic varieties. Additionally, these parameters can be used to predict the performance of the synthetics. The concept of varietal ability is known for completely allogamous crops. It has not yet been worked out for partial allogamy with a genotype-dependent degree of cross-fertilization. We have developed formulae to calculate GVA and SVA referring to generation Syn 1 in this general situation. The concept of varietal ability considers the influence of the components' per se performance on the resulting synthetic variety. This influence increases with decreasing degree of cross-fertilization. Thus, our formulae should be of special benefit to partially allogamous crops. The formulae are illustrated by a numerical example taken from our research on Vicia faba L. To estimate the relevant parameters for Vicia fabe L., we propose to use two morphological markers (flowers and seed colour) simultaneously.     

Three-Way and Four-Way Recurrent Selection in Plant Breeding

Three or four-way recurrent selection is proposed for the development of three or four-way crosses. In term of genetic effects, it could be justified if there are interactions between more than three genes, whether alleles or not, if autopolyploids are also considered. For autotetraploids, in particular with tetragenic interaction, four-way recurrent selection could be more effective than reciprocal recurrent selection (two-way selection). According to the types of progenies used to develop the tests, different selection procedures are considered, the two extremes being called method 1 and method 3. With method 1 (selection for three or four-way general combining ability) a plant from a population is used to produce three-way or four-way progenies with the other populations used as testers. With method 3 (selection for three or four-way specific combining ability) three-way or four-way crosses are developed with one plant from each population. To study the genetic advance, the theory is developed to consider either diploidy or autotetraploidy. It is shown that method 3 will allow a greater genetic advance than method 1 for low heritability and low selection intensity. The way towards hybrid development with three and four-way pedigree selection is discussed: again selection can be for general or specific three or four-way combining ability. It is emphasized that an advantage of recurrent selection for three or four-way specific combining ability is that it allows direct varietal development with pedigree selection on three or four-way specific combining ability. Some other practical aspects are also considered.     

Genetic analysis of mechanisms associated with inheritance of resistance to sheath rot of rice

Understanding genetic mechanisms controlling inheritance of disease resistance traits is essential in breeding investigations targeting development of resistant genotypes. Using North Carolina design II, 32 F₁ hybrids were generated by crossing eight susceptible to four resistant parents and submitted for field evaluation. The analysis of general and specific combining ability (GCA and SCA) indicated involvement of additive and non‐additive gene action controlling inheritance of horizontal resistance to sheath rot of rice. High GCA/SCA ratio and high heritability estimates revealed additive effects and were more predominant than none additive ones. The level of dominance indicated dominant genes was more important than recessive genes. Estimates of GCA and SCA analysis suggested that crop improvement programmes should be directed towards selection of superior parents or good combiners, emphasizing on GCA. As far as source of resistance is concerned, most promising genotypes were Cyicaro, Yunertian and Yunkeng. The predominance of additive genetic effects together with the relevance of dominant genes suggested possibilities of improving the resistance by introgression of resistance genes through recurrent selection coupled with phenotypic selection.     

Efficiency of Recurrent Selection Methods to Improve the Line Value of a Population

Five breedings methods have been studied for their efficiency in improving the line value of a population, i.e. the value of all lines that can be derived from the population. These methods are: half-sib family recurrent selection (HSF-RS), full-sib family recurrent selection (FSF-RS), half-sib progeny recurrent selection (HSP-RS), recurrent selection (S₁-RS) and the single-doubled-haploid descent recurrent selection (SDH-RS). All breeding methods are compared on the basis of the genetic advance per unit of time, with the constraint of the same effective size after the first cycle of selection. Two modalities of SDH-RS are considered for an annual plant without an off-season generation: one with a cycle in four years (SDH₁-RS) and the other with a cycle in five years (SDH₂-RS). For all methods the possibility of one off-season generation for intercrossing is considered. Fields of efficiency of the best methods are determined considering five main parameters, the heritability, the importance of additive variance, the correlation QL between the additive effect for line value and the additive effect for the per se value, the length of the cycle for SDH-RS and the selection intensities. SDH-RS is the best method when the heritability is low, with a low proportion of additive variance and with a relatively low correlation QL between additive effects for per se and line values. Without off-season generation, S₁-RS is the best method among the remaining realistic situations; with one off-season generation for intercrossing it is FSF-RS. The use of an off-season generation reduces the domain of efficiency of SDH-RS while an increase in the selection intensity increases this domain. With a selection intensity for SDH-RS equal to that for S₁-RS, SDH-RS is the best in a large range of realistic situations except for SDH₂ with one offseason generation and medium to high correlation QL.     

Diallel analysis of net blotch resistance in doubled haploid lines of Nordic spring barleys

A half-diallel was made between five six-rowed Nordic spring barleys to study the genetics of resistance to net blotch. Twenty-five doubled-haploid (DH) lines from each cross and the parents were sown in hill plots in Finland in 1997 and 1998. The plots were artificially inoculated with Pyrenophora teres Drechs. f. teres Smedeg. and assessed for resistance to net blotch. There were statistically significant differences in resistance of the five parents to net blotch. General combining ability (GCA) of the parents and specific combining ability (SCA) effects in the progeny were statistically significant in both years, but GCA effects predominated. Evidence for additive epistasis was minimal. Progeny of a particular cross were less resistant to net blotch than the better parent. The most resistant progeny were derived from the cross between the two most resistant parents, Pohto and WW7977, and resistance was governed by at least eleven effective factors. Narrow sense heritability estimates for resistance to net blotch were high during both years (0.84–0.99). It appears that net blotch resistance of progeny from crosses can be largely predicted from reactions of the parents. Quantitative resistance to net blotch can be further advanced by identification and incorporation of superior parents, from a screening such as reported here, into a recurrent selection breeding programme. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of Resistance to Mycosphaerella graminicola in Hexaploid Wheat

Septoria tritici blotch constitutes a major disease problem of wheat world-wide. To efficiently breed wheat for resistance to this disease, an understanding is required of the inheritance of resistance. Our objective was to study the quantitative inheritance of resistance under field conditions. A nine-parent diallel and a generation mean experiments were conducted at Toluca, México in 1986 and 1987, respectively, to investigate gene effects. General combining ability effects accounted for most of the variation although specific combining ability effects were detected in some crosses. Ias20*5/H567.71, Thornbird, and RPB709.71/Coc contributed the most to reduced disease severity. Reciprocal effects were detected in two of 36 crosses, where RPB709.71/Coc contributed additional reduced disease severity when used as female. The analysis of generation means confirmed results obtained from the diallel. Additive effects were also most important. Dominance effects and epistasis, mostly of the additive × additive type, were found in some crosses. Hence, substantial genetic progress for resistance can be expected among progeny from crosses with resistant parents. However, selection would be most effective if delayed to later generations because of dominance, and choice of the specific female parent may produce a higher level of resistance.     

Genetic analysis of rapeseed self-incompatibility lines reveals significant heterosis of different patterns for yield and oil content traits

Self‐incompatibility is one of the most effective approaches to utilizing heterosis in oilseed rape around the world. To evaluate the heterosis of double low self‐incompatibility, the possibility of combining seed yield and oil content, and the genetic effects of parents on their hybrid progenies, a 2‐year field trial using a 3 × 22 NC II mating design was conducted during the 1999‐2001 growing seasons in Wuhan, China. Significant differences in seed yield per plant and seed oil content were observed among the F1 hybrids and between F1 progenies and their parents. However, the heterosis for seed yield per plant was much greater than that for seed oil content. Mid‐parent heterosis and high‐parent heterosis of seed yield per plant ranged from 5.50 to 64.11% and from –2.81 to 46.02%, while those of seed oil content ranged from –1.55 to 7.44% and –3.61 to 6.55%, respectively. Non‐additive genetic effects were a major mechanism that accounted for the yield heterosis in addition to additive effects. In contrast, seed oil content heterosis was mainly dependent on an additive genetic effect. General combining ability (GCA) determined the stability of hybrid cultivars. In hybrid breeding, parental materials might be selected by the sum of GCAs and variances of special combining abilities (SCAs) of female and male parents for traits affected by both additive and non‐additive effects, and by the sum of GCAs of two parents for traits controlled mainly by additive effects. Primary branches and their siliques were the most important yield traits.     

Genetic diversity, hybrid performance, and combining ability for yield in Musa germplasm

Genome size variation occurs within and across generations in Musa spp., which reduces the predictive accuracy of parental performance on progeny value for yield and other traits with complex inheritance. Parental selection through progeny testing of prospective parents is required to achieve further genetic gains. This was carried out in this study, using a factorial mating design involving five 4x females and five 2x males. Genetic differences among offspring families were essentially due to differences in additive effects of the parents. Thus, little recombinative heterosis can be expected upon 4x-2x cross-breeding, and breeding strategies should target the development of 4x and 2x cultivars by accumulation of favorable alleles through recurrent selection within each ploidy pool. Offspring yield was positively correlated with parental GCAs but not with mid-parent values. Hybrid performance was also associated but not significantly correlated with genetic similarity indices based on both pedigree and molecular data. This study further suggests that current genetic models may not be adequate for populations with intergeneration genome size polymorphism. This revised version was published online in July 2006 with corrections to the Cover Date.     

不同玉米穗部性状的配合力分析

摘要：本实验采用双列杂交的方法按照P（P-1）/2对9个不同的亲本材料进行配合力分析,结果表明：EGM、云油199、沈137等亲本材料的一般配合力高,EGM×云油199、沈137×云油199、T8×EGM等组合的特殊配合力高.从对各性状遗传参数的分析结果可知,各性状主要受加性基因遗传影响,而受环境条件的影响较小,均能较真实地将亲本的这4个性状遗传给杂交一代。     

Inheritance of Carbon Isotope Discrimination in Bread Wheat (Triticum Aestivum L.)

Summary Reliable selection of families with increased grain yield is difficult in breeding programs targeting water-limited environments. Carbon isotope discrimination (Δ) is negatively correlated with transpiration efficiency, and low Δ is being used for indirect selection of high wheat yield in rainfed environments. Yet little is known of genetic control and opportunities for improving selection efficiency of Δ in wheat. Half-diallel and generation means mating designs were undertaken to provide estimates of the size and nature of gene action for Δ in a range of wheat genotypes varying for this trait. Significant (P < 0.01) differences were observed for leaf tissue Δ among parents (19.3 to 20.7‰) and F₁ progeny (19.4 to 20.9‰) in the half-diallel. General (GCA) and specific combining ability (SCA) effects were significant (P < 0.05), while Baker's GCA/SCA variance ratio of 0.89 was close to unity, indicating largely additive gene effects. GCA effects varied from −0.38 to + 0.34‰ for low and high Δ genotypes `Quarrion' and `Gutha', respectively. GCA effects and parental means were strongly correlated (r = 0.95, P < 0.01) while directional dominance and epistasis contributed to small, non-additive gene action for Δ. Smaller Δ in F₁ progeny was associated with accumulation of recessive alleles from the low Δ parent. Narrow-sense heritability was high (0.86) on a single-plant basis. Generation means analysis was undertaken on crosses between low Δ genotype Quarrion and two higher Δ genotypes `Genaro M81' and `Hartog'. The F₁, F₂ and midparent means were not statistically (P > 0.05) different, whereas backcrossing significantly changed Δ toward the mean of the recurrent parent. Gene action was largely additive with evidence for additive × additive epistasis in one cross. Narrow-sense heritabilities were moderate in size (0.29 to 0.43) on a single-plant basis. Genetic gain for Δ in wheat should be readily achieved in selection among inbred or partially inbred families during the later stages of population development.     

轮回选择在烤烟品种改良中的应用探讨

笔者在国外学者研究文献的基础上,归纳了轮回选择在烤烟品种改良中的应用价值,并结合烤烟作物自身特点,分析了烤烟育种应用轮回选择的优势,最后结合育种实践经验,指出创建烤烟轮回选择基础群体应注意亲本的选配、亲本的数目及基础群体的合成方式,提出多目标烤烟育种采用改良混合选择法结合S1选择法的轮回选择程序,同时建议采用开放式的轮回选择方案、适时引入适当遗传比率的优异种质或基因,根据基础群体所包含的种质状况、选择性状的遗传模型及遗传复杂性确定轮回选择后代种植群体规模,依据轮回选择群体的规模、轮回选择实际进程和目标性状遗传方式确定对后代的选择强度,采用约束指数对多个目标性状同时进行选择。     

Diallel Analysis of Grain Yield in Barley Using Doubled-Haploid Lines*

Grain yields of seven barley cultivars (Hordeum vulgare L.) and 398 doubled-haploid lines derived from their diallel crosses were evaluated in hill plots at two locations in Ontario. Additive × additive epistasis and linkage disequilibrium were detected in the seven cultivars. Additive × additive genetic variance was significantly greater than zero but additive genetic variance was not significantly greater than zero. The narrow-sense heritability on the basis of line means was intermediate (0.33). It was predicted that 13.1% yield improvement could be realized in the next recurrent selection cycle using the doubled-haploid method.     

Continued response through seven cycles of recurrent selection for grain yield in oat (Avena sativa L.)

Long-term selection experiments provide germplasm to study the effects of selection in a closed population. Recurrent selection to enhance grain yield in oat has been ongoing at the University of Minnesota since 1968. The objectives of this study were: (i) estimate the GCA and SCA effects for three agronomic traits in the seventh cycle of selection, (ii) assess the effect of the current methods of selection on parental contribution and unselected traits, and (iii) determine the direct and indirect responses to seven cycles of recurrent selection for grain yield. Progeny of the Cycle 6 parents and parents for Cycles 0 through 7 were grown in two separate tests. Grain yield, heading date and plant height were evaluated in each test. Grain yield was increased by 21.7% after seven cycles of selection. Evaluation of Cycle 6 progeny showed that GCA effects were significant for all three traits studied, and SCA effects were significant only for grain yield. Four Cycle 6 parents did not have any progeny selected as Cycle 7 parents. Results from this study indicate that long term recurrent selection has continued to increase grain yield. Alternative selection strategies may be necessary to maintain the genetic variability in this population, particularly when improvement of secondary traits is required. This revised version was published online in August 2006 with corrections to the Cover Date.     

Combining ability of common bean (Phaseolus vulgaris) genotypes for root traits across diverse environments

Root system architecture is important for common bean (Phaseolus vulgaris) adaptability to diverse environments. Beans employ complex adaptive root mechanisms for coping with multiple stresses in production environments. Understanding genetic control of root traits is central to improvement of common bean for adaptation to marginal environments. The objectives of this study were to (i) determine combining ability of root and agronomic traits and (ii) estimate the heritability and genetic correlation of root and agronomic traits in common bean. Four bean lines with superior root traits were crossed with four locally adapted varieties in a North Carolina II mating scheme to generate 16 crosses. The 16 F₁s were selfed and advanced to F₂ generation. Eight parents and their F₂ progenies were evaluated in an alpha-Lattice design with two replications. General and specific combing ability mean squares were significant (p ≤ .05) for all traits measured. General predictability ratios ranged from .47 to .68 across locations suggesting that both additive and non-additive gene action modulate root traits and seed yield. Positive and significant (p ≤ .05) phenotypic and genetic correlations revealed significant association between root traits and yield. Moderate to high heritability estimates of between .43 and .67 were realized. Such estimates point to possible deployment of a successful selection programme. Genotype AFR398 displayed significant positive GCA effects among its crosses for both root and agronomic traits hence a potential candidate genotype for inclusion in a bean genetic improvement programme for marginal environments.     

Genetic analysis of postharvest flower longevity in Asiatic hybrid lilies

To investigate the genetic regulation of postharvest flower longevity in Asiatic hybrid lilies (Lilium L.), 10 cultivars and 45 progenies were forced, harvested and evaluated under standardised conditions in growth chambers. Analysis of variance for individual flower longevity indicated highly significant (p < 0.001) variation among parents, among progenies and among descendants within progenies. High broad-sense heritability (0.79) calculated at the individual plant level indicated that selection for long individual flower longevity can be expected to be very effective. General combining ability (GCA) effects were highly significant ( p < 0.001), and the estimated narrow-sense heritability was high (0.74). Therefore, individual flower longevity of a genotype can be used as an indication for its breeding value. Although deviating results can be expected as specific combining ability (SCA) effects were also significant ( p = 0.046). Small, but significant correlations between individual flower longevity and other plant characters were found. The impact of these correlations on the selection efficiency for improved postharvest performance of lily inflorescences is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic variation and heritability of forage yield in Mediterranean tall fescue

The genetic control of tall fescue forage yield has been poorly investigated. Full‐sib families from diallel crosses of Mediterranean germplasm were evaluated for forage yield over 34 months in a Mediterranean environment with severe drought stress (diallel 1, with 20 parents) and over 16 months under irrigation in a heated greenhouse simulating the Mediterranean temperature pattern (diallel 2, with 15 parents). Genetic parameters were estimated for fresh biomass in diallel 1 and dry‐matter yield in diallel 2. Additive genetic variance was always larger than non‐additive (dominance) variance. Narrow‐sense heritability was fairly high (h² = 0.61) in diallel 1 and moderate (h² = 0.45) in diallel 2. Predicted yield gains from one selection cycle were larger in the former diallel (23.9%) than in the latter (10.5%), suggesting that gains can be enhanced by selection under severe drought stress and over a time span sufficient to allow the variation in persistence to fully emerge. General combining ability effects of eight parents that were common to both diallel crosses were highly correlated (r = 0.94) across the contrasting evaluation environments. The extent and consistency of additive genetic effects across environments suggest that rapid improvement of forage yield is possible.     

夏大豆产量性状的遗传力和配合力分析

用６个夏大豆品种进行双列杂交试验，估算了主要产量性状的遗传力、配合力效应。结果表明：大豆有效分枝数，每荚粒数，百粒重的狭义遗传力较高，可以进行早代选择；单株数、单株产量的狭义遗传力较低，在中高世代进行选择效果较好。大豆产量性状的加性遗传效应都是主要的，并以单株产量表现较大的特殊配合力效应。新品系８５２１２．８４２４０Ｂ１、７７２４９都是高产育种较理想的亲本材料，尤以品系７７２４９育种的效果最佳。     

Genetic analysis of yield and quantitative traits in pigeonpea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> L. Millsp.)

Basic information on genetics and inheritance of quantitative characters, which is necessary to develop future breeding programme, is not widely studied in pigeonpea. Hence, present study was conducted among 5 generations in four pigeonpea crosses to know significance of additive-dominance model, gene action involved in inheritance of quantitative characters, heritability and genetic advance. “Scaling” and “joint scaling test” was significant for most characters indicating that additive-dominance model alone is not enough to explain the inheritance of a character. Though additive variance was more, dominance variance also played important role for most of the traits. Positive and negative alleles were found to be distributed between parents. Additive gene effect (d) was significant for pods per plant and seeds per pod whereas dominance gene effect (h) was more predominant among pod yield and seed yield. Dominance × Dominance inter-allelic interactions (l) was more important than Additive × Additive type (i) for most of the traits studied which could be exploited by selecting individuals based on their performance in recurrent selection. Complementary gene action was observed among many traits with few exhibiting duplicate gene action. Heritability and genetic advance was high indicating the effectiveness of selection. Since dominance effects is also present along with additive effects selection could be practised in later generations to identify high yielding genotypes.