Genetic progress after four cycles of recurrent selection for yield and grain traits in common bean

The objective of this study was to evaluate the genetic progress after four cycles of recurrent selection in common bean. The base segregating population was obtained from 10 parents, and derived the S_0:1 and S_0:2 families that were evaluated. The S_0:3 families with higher grain yield and grain color, like the standard carioca were selected, and were intercrossed to generate the population of the following cycle. This process was repeated for four cycles. The best families were evaluated in each cycle by many generations and locations, and the five best lines of each cycle were identified. The 20 lines thus obtained were evaluated in two growing seasons, sown in July and November 2002. The grain yield (kg/ha) and grain type (scale of scores) were evaluated. Genetic progress was confirmed for both traits. The mean annual gain with selection for the grain type was 10.5% and 5.7% for grain yield, with no evidence of variability reduction in the population. These results show that recurrent selection is a good alternative for improving common bean quantitative traits.     

Recurrent selection in oat for adaptation to diverse environments

In order to test if selection can improve a population's adaptation to diverse environments simultaneously, three cycles of recurrent selection based on grain yield in Iowa, Idaho, and Norway were practiced in an oat (Avena sativaL.) population developed from North American, Scandinavian, and wild species (A. sterilis L.) germplasm sources. Specific objectives were to determine if selection: increased mean yields across environments and within all environments; changed the genetic correlation of yields in different environments; and changed genetic variation for yield within the population. We evaluated 100 to 210 randomly-chosen families from each cycle of selection at three Iowa locations, in Idaho, and in Norway for two years. Grain yield within each location and mean yields across locations increased significantly over cycles of selection. Mean yields across locations expressed as a percent of the original population mean increased at a rate of 2.6% per year. Several families from the third cycle population exhibited both high mean yields across locations and consistently high yields within all locations. Average genetic correlations of yield in different environments were higher in the second cycle than in the original population. A trend of reduced genetic variation and heritability was observed in Iowa only. These results suggest that we successfully improved mean population yield both within and across locations, and yield stability across environments, and in developing families with outstanding adaptation to diverse environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pedigree selection for Gibberella ear rot resistance in maize

The pedigree method is often used for developing inbred lines in maize (Zea mays L.). This study was conducted to assess the effectiveness of pedigree selection for improving resistance to Gibberella ear rot in four maize populations. Selection was based on the severity of ear rot symptoms after inoculation with macroconidial suspensions of Fusarium graminearum (Schwabe) into the silk channel (for two populations) and into the developing kernels (for two other populations). Samples of the selfed families (S₁ to S₅), recovered from remnant seed from the selection programs, were evaluated for disease resistance during three years, using inoculation and evaluation protocols similar to those used during selection. Among-family selection was effective in both of the populations selected after silk inoculation and in one of the populations selected after kernel inoculation. Responses to selection were more evident in later than in earlier generations for both types of inoculation. Changes in the estimated genetic gain over generations were consistent with changes in the variances among families, which tended to increase in early generations and to decrease in later generations. Selection after kernel inoculation seemed to have been more effective than selection after silk inoculation in developing families with more stable resistance. Based on the results obtained here, it seems that responses to family selection could be accelerated without increasing operational costs by increasing selection intensity in later generations and inoculating fewer plants per family.     

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

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

Recurrent selection for grain yield in early generations of two barley populations

Summary Two spring barley composites, one based on eight West-European two-rowed cultivars (A) and one, the predominantly six-rowed composite XXI, based on several thousands of barley cultivars (B), formed the starting point of a recurrent selection procedure. The aim was to study whether recurrent selection in early generations is an effective procedure to improve barley populations for agronomic characteristics, especially grain yield.After two cycles of recurrent selection in the two populations A and B separately, and consisting of single plant selection followed by line selection, one cycle of recurrent selection was applied to the population produced by intercrossing the selected A and selected B lines.The selection for grain yield in the F2 single plant stages and the F3 line stages was carried out by selecting the plants or lines with the largest mean head weight provided that they did not tiller too poorly. Also some selection was applied against extremes in heading date and against tall plants.After two cycles of recurrent selection the grain yield, tested over two years and two locations, had increased with 16.5% in population A and with 27.0% in population B. Both the single plant selection and the line selection had contributed significantly to this response.The population created by crossing the selected A with the selected B lines showed a significant reduction in yield compared to the mean yield of the A and B lines constituting this population. Single plant selection, followed by line selection did raise the yield level 5.7% above that of the mean of these A and B lines. Several lines produced from this intercross combined an excellent grain yield with outstanding resistance to barley leaf rust and powdery mildew. However, these lines like the entire intercross population suffered from susceptibility to lodging, a characteristic derived from population B.In five experimental situations mixtures of genotypes were compared with the mean of the monocultures of the constituting genotypes. The mixture yielded always more, the average mixture effect being 4.5%.     

Simultaneous selection of segregating two-row winter barley material in England and Italy

Summary Independent but simultaneous selection by two breeding teams of the same segregating two-row winter barley material at Cambridge, England and Fiorenzuola, Italy was carried out over the years 1978–1982 with the objective of comparing the individual selection of the two breeding teams and assessing the adaptation of the surviving selections. Visual selection in the generations F₃ to F₅ did not result in any clear cut compartmentalisation of the material and both teams were as likely to select from families previously selected only by the other team as from families they had themselves previously selected. Lines with specific adaptation to each site as well as lines well adapted to both sites were identified from yield trials carried out in F₅ and F₆. The former showed only a very small yield advantage over lines with general adaptation to both sites.     

Transgressive segregation for yield traits in Oryza sativa IR58025B X Oryza meridionalis Ng. Bc2F3population under irrigated and aerobic conditions

Wild species of the genus Oryza are a good source of beneficial alleles for enhancing rice yield under normal and adverse conditions. BC₂F₃ population was derived from a cross between Oryza sativa IR58025B and Oryza meridionalis Ng. (2n = 24, AA) a heat tolerant wild species to evaluate 12 yield traits under irrigated and aerobic conditions. Analysis of variance and genetic estimates indicated there is substantial genetic variation among progenies under both conditions. Grain yield had high heritability (61.9%) and genetic advance (36.4%) under irrigated conditions but moderate heritability (49.6%) and genetic advance (13.3%) under aerobic conditions indicating that selection for yield will be effective under both conditions. Panicle number, grain number, spikelet fertility, and test weight showed significant positive correlation with grain yield under both conditions. Families out-performing IR58025B for yield under both conditions were obtained providing evidence that phenotypically inferior O. meridionalis contributed to yield increase. This species can be a novel source of natural genetic variation for the improvement of rice under irrigated as well as under aerobic condition.     

Breeding two base populations of perennial ryegrass

Summary A program designed to develop two base populations of perennial ryegrass was initiated in 1985 in Galicia. Multitrait mass selection by a base index was practised in two genetic resources groups of wild Galician populations. Plants selected were intercrossed and the two resulting base populations (F and G) were evaluated in two locations at the spaced plant level and in plot trials with the aim of estimating the genetic parameters and the possibilities for future breeding. The two populations did not show significant differences from commercial cultivars. Production characters had low heritabilities (0.01–0.32) in both locations. Seasonal yields had medium to high family heritabilities (0.36–0.73). For the economic traits chosen, the expected genetic advances were important in both populations for summer and autumn growth in Mabegondo and spring and summer-autumn dry matter yield (1^st and 3^rd year) in Puebla de Brollón. Expected genetic advance for early spring growth was 0.62 in population F. In the autumn of 1991 equal numbers of plants, selected at each location by combined family and mass selection from each of these populations, were planted in two polycrosses with the aim of improving the genetic basis and adaptation of these populations.     

Development of an appropriate breeding scheme for tolerance to Empoasca kraemeri in common bean

Summary Significant advances in increasing tolerance to the leafhopper Empoasca kraemeri Ross & Moore in common bean have been obtained using a new breeding scheme where yield under leafhopper attack is the principal selection criterion in the evaluation of progenies. However, to further refine this breeding scheme, a study was conducted to determine whether selection for nonprotected yield would be more effective in early versus late generations. Two selection strategies were compared. In Strategy I, early generation selections in the F₂ and F₃ were compared to Strategy II where late generation selections were made in the F₄ and F₅, with the F₂ and F₃ generations advanced using single pod descent and bulk practices, respectively. Yield trials of the F₆ lines from both selection strategies were conducted under nonprotected and insecticide protected treatments. No significant differences were detected between the two selection strategies. However, Strategy II did produce advanced lines with greater nonprotected yields than did Strategy I, with the best F₆ line, in three out of four crosses, coming out of Strategy II. Late generation selection is recommended over early generation selection. Results of the new leafhopper breeding scheme, based on yield, are compared to the old breeding scheme where selections were made using visual selection practices in early generations.     

Genetic parameters for yield in macadamia

Broad-sense heritability, and genetic and phenotypic correlations were estimated for yield (nut-in-shell; kernel yield; and net present value of crop) per tree in macadamia to 10 years after planting from a trial of 40 cultivars replicated over 4locations. In addition, canopy width at 10years of age was measured and used to calculate yield efficiency as yield per square metre of projected canopy area. Stem girth above the graft union was also measured. There was a low broad-sense heritability (0.06 < H <0.22) and correlation of cultivars across locations (r _gloc = 0.14–0.52) for yield per tree. However, the genetic correlation of cumulative yield was high among 3 of the 4locations (g ≥ 0.75), suggesting the higher genetic variance at Rockhampton was the main cause of the genotype by environmental interaction. Heritability was higher for canopy width (H = 0.28) and yield efficiency (H ≈ 0.47) and cultivar performance was highly correlated across locations (r _gloc ≈ 0.70) for these traits. There was a strong genetic correlation (r _g ≥ 0.90) between cumulative yield per tree of cultivars to 7 years and cumulative yield per tree to 10 years. Genetic correlation among all measures of yield per tree were high (r _g ≥ 0.73), but there was no genetic correlation with canopy width. On the other hand, small cultivars tended to have higher yield efficiency (r _g <–0.62). There was also a slight negative correlation between nut-in-shell yield per tree and kernel recovery (r _g = –0.37). These results suggest the use of family information and index selection may improve the efficiency of selection and breeding programs in macadamia. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of predicted and observed response to selection in two breeding populations of perennial ryegrass

To predict any genetic response to selection, two breeding populations of perennial ryegrass, consisting of half-sib families, were evaluated as spaced plants from 1987 to 1989 for several agronomical traits. In each population, the best plants selected through a multitrait Smith and Hazel's index were planted in a polycross to produce the second generation half-sib families. Both generations were evaluated together from 1991 to 1993 as spaced plants. The difference between generations was the observed genetic response, which was compared to that predicted. Rust tolerance, aftermath heading and persistence were significantly improved in both populations. In trial plots used to assess the yield, no effect of mass selection was noticed. Thus, it is concluded that mass selection failed to increase productivity but produced some improvements in disease tolerance and aftermath heading.     

Genetic effects determining rice grain weight and grain density

Summary Increasing grain weight is one means of increasing rice (Oryza sativa L.) grain yields. Selection for increased density of filled grains may offer an approach to increasing rice grain weight. Before rice breeding programs can begin effectively selecting for higher grain density, the nature and amount of the genetic variation present must be evaluated. A Design II mating plant with two sets was constructed using 16 parents. The 16 parents were representative of cultivars and elite breeding material available to breeders of long-grain rice in the southern U.S. The parents and 32 F₁ hybrids were evaluated in 1990 at two Arkansas locations: Stuttgart and Marianna. Additive variation was essentially zero for grain density as indicated by nonsignificant general combining ability (GCA) and specific combining ability (SCA) mean squares and a narrowsense heritability estimate –0.04 (±0.07). Increasing rice grain weight through increased grain density would not be feasible in U.S. southern long-grain rice unless new germplasm with higher filled grain densities is introduced. Genetic variation for grain weight was predominately additive and complemented with additive x additive epistatic variation, indicating that selection for increased grain weight could be practiced in segregating generations. The small proportion of epistatic variation to additive variation would not justify delaying selection until epistatic combinations are fixed in the homozygous line. Midparent grain weight means would serve as an adequate indicator of progeny performance for cross appraisal.Abbreviations GCA General Combining Ability - SCA Specific Combining Ability     

A recurrent selection programme for grain yield in winter barley

Summary The effect of a recurrent selection procedure was evaluated in a winter barley (Hordeum vulgare L.) population. Cycle zero (C₀) was initiated by crossing six high yielding winter barley cultivars with the short straw cv Onice. The F₁'s were crossed according to a diallel scheme without reciprocals. A total of 750 S₀ plants were derived and evaluated; 329 S₀ plants were selected and their progenies (S₁ lines) tested. Fifteen S₁ lines were chosen and used as parents of cycle 1 (C₁), by producing 105 F₁ hybrids which simulated a random mating offspring. One hundred and three randomly chosen S₁ lines belonging to C₀, and 103 S₁ lines belonging to C₁, were evaluated at two locations.For grain yield a significant difference between cycles was observed. From C₀ to C₁ the grain yield increased with 307 g/m². This increase was due to a higher number of seeds per m². For plant height, heading date and 100-kernel weight no differences between cycles were observed.The positive results obtained in this study indicate the potential usefulness of recurrent selection for developing parents or lines superior for grain yield, with little change in other important agronomic traits.     

Heritability and gain from selection for quantitative resistance to Xanthomonas campestris pv. vesicatoria in Capsicum annuum L.

Summary Breeding for disease resistance in peppers (Capsicum spp.) to the bacterial spot pathogen (Xanthomonas campestris pv. vesicatoria (Doidge) Dye) has been based on either qualitative or quantitative evaluation methods. Quantitative evaluation of components of resistance, lesion number and lesion diameter, has been useful for determining quantitative resistance, but few breeders have applied these methods in routine selection programs. This study was aimed at determining the heritability and gain from selection for resistance to the bacterial spot pathogen based on three components of resistance. Random selections from a diverse intermated population of Capsicum annuum L. were self-pollinated for two generations to create S₁ and S₂ families. Thirty S₁ families, corresponding S₂ families and four homozygous check lines were evaluated. At forty-two days after seeding, two different leaves of each plant were inoculated by leaf infiltration with low concentrations (5×10³ colony forming units) of Group 2 (XCV PT, race 1) and 4 (XCV P, race 1) of the bacterium, respectively. After 15 days, lesion number cm^-2 and lesion diameter were measured. Total lesion area was calculated. Narrow-sense heritabilities for lesion number, lesion diameter, and total lesion area were 0, 43, and 31%, respectively, with Group 2, and 26, 43, and 33%, respectively, with Group 4. Actual S₂ gain from 20% selection pressure in the S₁ was approximately 50%, when selection towards resistance was based on total lesion area.     

Increasing biodiversity of irrigated rice in Africa by interspecific crossing of Oryza glaberrima (Steud.) × O. sativa indica (L.)

The African rice Oryza glaberrima, traditionally cultivated since more than 3.500 years, is of poor agronomic performance but resistant/tolerant to various stresses and diseases. The introduction of these characters into O. sativa cultivars is difficult since crossing barriers cause spikelet sterility in F1. Backcrossing can restore fertility and recently facilitated the development of fertile O. glaberrima × O. sativa ssp. japonica hybrid progenies for rain fed systems. With the objective to gain access to African rice germplasm for improvement of irrigated rice, crosses were performed with eighteen O. glaberrima and twenty O. sativa ssp. indica accessions. In total about one hundred F1-hybrid grains were obtained. The F1 plants were all completely sterile and backcrossing (BC) to O. sativa was performed in order to restore spikelet fertility. Monitoring of Tog5681 × IR64 hybrid progenies under field conditions revealed a broad genetic diversity within the BC₁ and BC₂ populations. Some BC₁ and BC₂ progeny plants outperformed the O. sativa parent, indicating that the heterozygocity level and complementary gene action after two backcrosses are still sufficient to positively influence plant vigor. Spikelet fertility of progenies was highly variable, but almost complete fertility was already observed within the BC₁F₂ population. High spikelet fertility was preserved in one out of two analyzed BC₁F₃ families and inmost of the BC₂F₃ families. The ability to restore spikelet fertility within few generations and the potential of the genetic diversity present in interspecific progenies facilitates the development of plant types specifically designed for the African irrigated and lowland environment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Recurrent selection for yield in Glycine max using genetic male-sterility

Summary Recurrent selection, using genetic male sterility to facilitate intermating in Glycine max, was evaluated when selecting among individual space-planted S₀ soybean plants for three yield-related traits: 1) seed yield per se (YLD), 2) apparent harvest index (AHI), and 3) a selection index in which seed yield was regressed on maturity (REG). The original intermating population was a combination of the cultivar Century and two F₂ populations segregating for male sterility. The selection intensity through three cycles of selection was 20%. The YLD population increased in mean yield by 2.8±2.2 g plant^-1 cycle^-1 buf shifted significantly toward late maturity by 3.9±0.6 days cycle^-1. The AHI population decreased in yield by 4.4±2.6 g cycle^-1 while maturity shifted slightly earlier; there was very little change in AHI. The REG population increased in yield by 5.7±2.4 g plant^-1 cycle^-1 while shifting toward later maturity by only 0.8±0.5 day cycle^-1. We concluded that selection among S₀ plants in these populations, using the selection index of yield regressed on maturity, was affective in increasing single plant yields without causing a significant shift in maturity. Selecting for yield per se caused an unacceptable shift toward late maturity and selection for AHI was ineffective for increasing yield or AHI.     

Selection for resistance to Plasmodiophora brassicae Wor. in oriental subspecies of Brassica rapa L.

Summary Selection for resistance to Plasmodiophora brassicae Wor. in oriental groups of Brassica rapa L.Two hundred and sixty-five cultivars of leafy, oriental bassicas were tested for resistance to 18 collections of Plasmodiophora brassicae, the causal agent of clubroot. The tests were conducted in the greenhouse at low and high level inoculum concentrations. Eleven cultivars of B. rapa pe-tsai, five cultivars of B. rapa pak-choy and three cultivars of B. rapa choy-sum consistently segregated for resistance at the lower concentration of inoculum (1000 spores/ml). All 265 cultivars were susceptible at the higher concentration (1 000 000 spores/ml). Three cultivars were used in pedigree and recurrent selection schemes for increased resistance. After three cycles of selfing resistant individuals, significantly more resistant S₃ lines were derived from each cultivar. Lines derived from two cultivars. Chinese White and PI 257236, continued to improve with each cycle of selection and demonstrated increased resistance to higher levels of inoculum (up to 1 000 000 spores/ml) New cultivars based on intercrosses of S₂ resistant individuals also had significantly better resistance than the original cultivar. After two cycles of selection in the third cultivar, PI 419007, resistance did not increase and its S₂ mass did not differ significantly from the original cultivar. Evidence that indicates resistance is pathotype-non-differential and offers an alternative to major gene, pathotype-differential types of resistance currently being introduced to the leafy oriental brassicas from other Brassica rapa groups.     

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.     

Genetics of Seed Yield and its Components in Common Bean (Phaseolus vulgaris L.) of Middle-American Origin

Genetic variance, heritability, and expected response from selection arc useful in devising alternative methods and criteria of: selection. The objectives of this study were to estimate these for seed yield and its components from 200 F₂: populations involving 80 cultivars and lines of mostly small-seeded dry bush bean (Phaseolus vulgaris L.) of habits growth I, II, and III of Middle-American origin. All cultivars and lines were crossed in eight sets of ten parents each in a Design II mating system. The F₂ populations, without parents, were evaluated in the field in a replicates-in-sets design at two locations in Colombia in 1983. Estimates of additive genetic variance were significant for yield, pods/m², seeds/pod, and seed weight. Interaction with environments was also significant. Values for nonadditive genetic variance were not significant for either yield or yield components. The estimates of narrow sense heritability, based on the F₂ population mean and unbiased by genotype x environment interaction, were 0.21 ± 0.13 for yield. 20 ± 0.13 for pods/m², 0.57 ± 0.13 for seeds/pod, and 0.74 ± 0.15 for seed weight. The expected direct response from selection of the top 20 % of F₂ populations for yield per se would result in a 4.30 % increase in yield with a correlated response of 0.21 % in seed weight. In contrast, the expected gain from direct selection for seed weight would result in a 11.76 % increase in seed weight with a, correlated gain of 0.28 % for yield. Direct selection for pods/m² would decrease yield, seeds/pod and seed weight, while direct selection for seeds/pod would reduce pods/m² and seed weight but increase seed yield by 0.37 %. Data on yield from replicated trials in the early segregating generations could be utilized for identification and selection of promising crosses and families or lines with crosses for dry bean yield improvement.     

Inbreeding depression and single cross hybrids in leeks (Allium ampeloprasum ssp. porrum)

Summary All current commercial cultivars of leeks are open-pollinated and one of the major problems with the crop is poor uniformity with much of the variation being genetic in origin. Inbred lines and single cross hybrids were produced to try to reduce the genetic variation. Inbreds were generated by single seed descent from 5400 plants taken from six commercial cultivars and performance data for uniformity, yield and quality are presented for the S₁-S₃ generations. A few relatively vigorous inbred lines were obtained but overall, inbreeding depression was very severe with no compensating increase in uniformity as measured by coefficients of variation. In contrast, the experimental hybrids gave significant uniformity, yield and quality benefits compared to open-pollinated commercial cultivars and can be used as the basis for developing a range of commercial hybrid cultivars.Abbreviations SSD Single Seed Descent - HRI Horticulture Research International