Improvement of large-seeded common bean cultivars under sustainable cropping systems in Spain

Approaches are needed to broaden the genetic base and improve earliness and yield potential of large-seeded beans under sustainable cropping systems. The objective of this research was to develop adapted dwarf bean populations having a commercial seed quality and yield suitable for the production in the South of Europe. The original base populations were produced from crosses between genotypes within each Mesoamerica, Nueva Granada and Peru races, and between Peru and Nueva Granada, and Mesoamerica and Nueva Granada races. Visual mass selection for plant performance was practised in the F₂ and F₃ generations. In the F₄ and F₅, single plants were harvested under two cropping systems (sole cropping and intercropping with maize). From F₄, selection was based on precocity, combined with seed yield and seed commercial type. The F_4:7 selected lines from each original population were compared with their parents and five checks at four environments and two cropping systems. Differences among environments, populations, parents and checks were observed for all traits. Under intercropping with maize, there was a 50% reduction in seed yield. Yield of parents and checks belonging to Andean South American races, intraracial (Nueva Granada × Nueva Granada) and interracial (Nueva Granada × Peru) populations, was higher than that of those of Middle American origin. Intraracial crosses within large-seeded Andean South American (Peru race) and Middle American gene pools (Mesoamerica race) did not produce lines yielding more than the highest yielding parent. Only two large-seeded lines selected from crosses between small- and large-seeded gene pools out-yielded the best parent and check cultivar.     

Selection for seed yield in Andean intra-gene pool and Andean × Middle American inter-gene pool populations of common bean

Yields of large-seeded Andean (A) common bean (Phaseolus vulgaris L.) cultivars of Chile and Nueva Granada races are 40 to 60% lower compared to their Middle American (M) counterparts of small-seeded Mesoamerica and medium-seeded Durango races. Our objective was to use the concept of congruity backcrossing between Andean x Middle American inter-gene pool [AM 11833 = A 483 (A)///// A 686 (M) //// PVA 800A(A)/// ‘Carioca’ (M)// Carioca (M)/ G 19833 (A)] and between races within Andean gene pool [i.e., intra-gene pool Andean, AA 11834 = A483 (A)//// ‘Cardinal’(A) /// ‘Blanco Español’(A) // BlancoEspañol (A) / ‘Taylor’ (A)] to compare selection for seed yield improvement of large-seeded Andean beans. Seven hundred sixty seven F₂-derived F₃ (F_2:3) families were produced for each population. Visual appraisal for total plant performance, combined with seed yield from non-replicated plots was used for selection of 551 families in F_2:3, 182 families in F_2:4, and 91 families in F_2:5 in each population. Eight hundred twenty three F_5:6 lines were developed from the 91 F_2:5 families in each population. Visual selection, combined with seed yield in non-replicated plots was again used to select 294 lines in F_5:6 in each population. Similarly, 44 highest yielding F_5:7 lines were selected in AM 11833 and 39 F_5:7 lines in AA 11834. Thus, single plant selections were made in the F₂ and F₅, and plants within each plot were harvested in bulk in F₃, F₄, F₆, and F₇. Thirty nine F_5:8 lines from AA 11834 and 44 lines from AM 11833, parents, and checks were evaluated at Popayán and Quilichao, Colombia in 1998 and 1999. Selected lines in both populations, on average, out-yielded the mean of their large-seeded Andean parents. Mean yield of the lines selected from AM 11833 was 50% higher than AA 11834 lines. Twelve F_5:8 lines out-yielded the highest yielding Andean parents G 19833 and A 483 in AM 11833, whereas only one line yielded significantly higher (p < 0.05) than the highest yielding parent A 483 in AA 11834. However, none of selected lines out-yielded small-seeded Middle American parents used in AM 11833 (A 686 and ‘Carioca’). The mean 100 seed-weight of AA 11834 was 36 g compared to 28 G for AM 11833 F_5:8 lines. Selected lines had similar days to maturity as parents in AM 11833, and matured 3 d later in AA11834. Correlation coefficients between yield and 100 seed-weight were negative in both populations. Yield and days to maturity were positively correlated in AA 11834.     

An allelic series at the Co-1 locus conditioning resistance to anthracnose in common bean of Andean origin

In this study, we characterized the genetic resistance of the Andean bean cultivars Kaboon and Perry Marrow and their relation to other sources of anthracnose resistance in common bean. Based on the segregation ratio (3R:1S) observed in two F₂ populations we demonstrated that Kaboon carries one major dominant gene conferring resistance to races 7 and 73 of Colletotrichum lindemuthianum. This gene in Kaboon is independent from the Co-2 gene and is an allele of the Co-1 gene present in Michigan Dark Red Kidney (MDRK) cultivar. Therefore, we propose the symbol CO-1 ² for the major dominant gene in Kaboon. The Co-1 is the only gene of Andean origin among the Co anthracnose resistance genes characterized in common bean. When inoculated with the less virulent Andean race 5, the segregation ratio in the F₂ progeny of Cardinal and Kaboon was 57R:7S (p = 0.38). These data indicate that Kaboon must possess other weaker dominant resistance genes with a complementary mode of action, since Cardinal is not known to possess genes for anthracnose resistance. Perry Marrow, a second Andean cultivar with resistance to a different group of races, was shown to possess another resistant allele at the Co-1 locus and the gene symbol Co-1 ³ was assigned. In R × R crosses between Perry Marrow and MDRK or Kaboon, no susceptible F₂ plants were found when inoculated with race 73. These findings support the presence of a multiple allelic series at the Andean Co-1 locus, and have major implications in breeding for durable anthracnose resistance in common bean. This revised version was published online in July 2006 with corrections to the Cover Date.     

Growth habit and gene pool effects on inheritance of yield in common bean

Summary To satisfy farmer and consumer preferences, breeding efforts to increase yield potential in common bean must take into account the interrelated effects of growth habit, seed size, maturity, and gene pool on yield expression in segregating populations. To examine the relationships among these traits, a genetic study was conducted to determine the effect of growth habit on yield and seed size in crosses among five bean lines from diverse gene pools. Two parental bean lines had determinate, type I growth habits and large seed size typical of the Neuva Granada-Andean gene pool. Two other lines were tropical Mesoamerican types with type II growth habits and small seed size; and the fifth line, G13625, a landrace of the Jalisco gene pool from the Mexican highlands, had a type IV climbing growth habit and medium seed size. Individual F₂ plants from each cross and parental lines were evaluated for growth habit and yield component traits under high input field conditions. The following season, the evaluations were repeated on random F₃ plants. Of the five parental lines, only G13625 showed significant GCA effects for yield in both the F₂ and F₃ generations. Improved yielding ability of G13625 progeny was associated with an increased expression of climbing bean growth habit traits: guide length, climbing ability, node number on main stem, and plant height. Crosses between Andean x Mesoamerican and Andean x Jalisco genotypes, as well between growth habit type I (Andean x Andean) and between type II (Mesoamerican x Mesoamerican) had very low parent-offspring heritability values for yield. Yield heritability was only significant for crosses between Mesomerican x Jalisco gene pools. An apparent simple genetic control of growth habit modification towards semi-climbing and climbing types is proposed as the major reason for increased yields in these crosses. No genetic linkage between genes controlling growth habit and seed size was detected which might restrict the development of high yielding large-seeded type II lines.     

Combining ability for nodulation in common bean (Phaseolus vulgaris L.) genotypes from Andean and Middle American gene pools

Ten F₁'s obtained from crosses among five common bean genotypes of Andean (WAF 15, Mineiro Precoce and Batatinha) and Middle American (BAT 304 and Ouro) gene pools were assessed for their combining abilities for root nodulation with Rhizobium tropici strain CIAT 899. The plants were grown under controlled conditions and evaluated for number of nodules per plant (NN), nodule dry weight (NDW), mean nodule weight (MNW) and plant fresh weight (PFW). The subdivision of the treatment effects on the general (GCA) and specific combining effects (SCA) were performed according to Griffing's diallel analysis method 2, model 1. The analyses of variance and estimates of quadratic components showed that non-additive gene effects were more important in the expression of NN and PFW, whereas additive gene effect was predominant for NDW and MNW. A close association was observed between high number of nodules and GCA. Generally, crosses involving parents of different gene pools yielded hybrids with high positive estimates of SCA for all assessed traits. The genotypes of Andean origin WAF 15 and Mineiro Precoce are the most promising parents for breeding programs to increase NN and NDW in common beans. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heterosis in crisphead lettuce (Lactuca sativa L.) hybrids

Summary Six of 16 F₁ hybrids from diverse crosses of crisphead lettuce with butterhead, cos or other crispheads significantly outyielded their respective better parent. Four were crosses between crisphead and cos parents, one was a butterhead × crisphead cross and one, a winter crisphead × winter crisphead cross. This latter hybrid plus one other also proved heterotic in a subsequent trial of ten crosses involving five winter crisphead parents, with yield excesses over better parent of 16 and 19%. It appeared significant that the heterotic crosses were given by parents of dissimilar origin. Trials of F₂ segregants from one of these heterotic hybrids enabled predictions to be made for 10.2 to 24.1% yield increases over better parent by the retention and clonal multiplication of the 10% highest yielding selections. Heterosis demonstrated in these trials may constitute the first reported cases in lettuce.     

Introgression of resistance to Columbia and Northern root-knot nematodes fromSolanum bulbocastanum into cultivated potato

Summary Resistance toMeliodogyne chitwoodi races 1 (MC1) and 2 (MC2) andM. hapla (MH) derived fromSolanum bulbocastanum was introduced into the cultivated potato gene pool through somatic fusion. The initial F₁ hybrids showed resistance to the three nematodes. Resistance to reproduction on roots by MC1 was accompanied by resistance to tuber damage in F₁ clones. Tuber damage sometimes occurred, however, in hybrids of BC₁ progeny resistant to reproduction on roots when MC2 and MH were the challenging nematodes. Resistance to reproduction was transferred into BC₁ individuals, but a greater proportion of BC₁ progeny was resistant to MC1 than to MC2 or MH. Resistance to MC1 appears to be dominant and discretely inherited. F₁ and BC₁ progeny were pollen sterile, but seed were produced from crosses using cultivated tetraploid pollen sources. Approximately 11 and 33 per cent of pollinations produced berries on F₁ and BC₁ pistillate parents, respectively. Seed yield increased fourfold overall in crosses with F₁ compared to BC₁ individuals.Abbreviations MC1 Meloidogyne chitwoodi race 1 - MC2 Meloidogyne chitwoodi race 2 - MH Meloidogyne hapla - Rf Reproductive factor     

Indica-Japonica differentiation of paddy rice and its relationship with heterosis

The Indica‐Japonica differentiation of three photoperiod‐sensitive and/or thermosensitive genetic male‐sterile rice (PGMS or TGMS, respectively) lines and 47 male parental lines from seven ecotypes were studied for their restriction fragment length polymorphism marker data to determine which ecotype crosses with the three PGMS and/or TGMS lines could lead to higher yield potential, and to estimate the relationship between the Indica‐Japonica differentiation of parents and heterosis in grain yield and its components. The results indicated that hybrids derived from ‘N422s’ and the early‐middle ripening Indica varieties from southern China, and hybrids between ‘Pei'ai64s’ and three Japonica ecotypes, including North‐eastern Japonica varieties, restoring lines of Japonica hybrid rice and north China Japonica varieties, showed the highest grain yields. There was less variation of yield among the F1s between ‘108s’ and the seven ecotypes than among the other F₁s. Highly significant positive correlations between heterosis of the F₁ yield and genetic distance of the parents were detected, although the correlation between F₁ yield performance and genetic distance did not reach a significant level. Considerable variation of correlation between heterosis and genetic distance was also detected in the Indica × Indica crosses and Indica × Japonica crosses. There was much higher correlation (r = 0.63) between the F₁ yield performance and the genetic distance of parents in the Indica × Indica crosses than in the others. It is proposed that a genetic distance of 0.4‐0.8 between the two parents of hybrid rice might be appropriate not only for F₁ performance, but also for heterosis.     

Partial resistance of barley to leaf rust,Puccinia hordei. III. The inheritance of the host plant effect on latent period in four cultivars

Summary The latent period (LP) in the barley-leaf rust relationship is an important component of the partial resistance complex. The inheritance of the host plant effect on LP was studied in five crosses between four cultivars. The LP, effectuated by the susceptible cultivars L94 and L92, were 8.0 and 8.6 days resp., those of the resistant cultivars Minerva (Mi) and Vada (Va) 16.9 and 17.1 days resp. The mean F₁ and F₂ values of the crosses L92×L94 and Mi x Va were intermediate between the parental ones. The variances of the F₂'s were slightly larger than those of the parents and the F₁'s indicating some segregation. In the crosses between a susceptible and a resistant cultivar the F₁ value was half way between the mid-parent and susceptible parent value. The F₂ mean lay approximately half way between the mid-parent and F₁ value, with a distribution positively skewed and slightly bimodal. There was no transgression, in fact not even the parental values were recovered among nearly 500 F₂ plants. The F₃-lines of the crosses between susceptible and resistant cultivars showed within line variances from as low as the parental values to as high as or higher than those of the F₂. In hte F₃'s the parental values could be recovered although no transgression occurred.L94 is supposed to carry no genes effecting a longer LP. The long LP of Mi and Va, assuming no linkage, is thought to be effectuated by the cumulative action of a recessive gene with a fairly large effect and some four to five minor genes with additive inheritance. One of these minor genes is supposed to be carried by L92, while Mi and Va are thought to differ for one minor gene only. In case linkage exists, the number of minor genes involved could be higher.     

Derivation of superior F5 lines from heterotic hybrids in pea

Summary F₅ seeds from six highly heterotic F₁ hybrids were produced to determine whether the superior performance of F1 hyrids could be fixed in pure-line derivatives of pea (Pisum sativum L.). For each cross, 24–31 F₅ lines derived from single seed descent were compared with F₁ hybrids and their parents in two environments. The F₁ hybrids out-yielded the best parent by up to 11%. All crosses produced F₅ lines which were as high in yield as the F₁ hybrids indicating that pure-line derivatives equivalent in yield to the heterotic F₁ hybrid could be developed by conventional breeding. Heterosis of the F₁ hybrids over the best commercial cultivar was thus a useful indicator of the future performance of the pure-line derivatives. These results indicate that overdominance was not an important component of heterosis in peas.     

Heterosis and inbreeding depression in forage rape (Brassica napus L.)

Summary Heterosis and inbreeding depression for total fresh and dry weights were studied over two seasons in 15 crosses of six cultivars of forage rape (Brassica napus L.). Some of the hybrids exhibited considerable heterosis over both mid-parent and better parent; followed by depression in the F₂. The cross Windal × Canard had 62% and 25% greater dry matter yield over the best cultivar used in this experiment in 1978 and 1979, respectively. Possibilities of the utilization of this hybrid superiority are discussed.     

Agronomic and qualitative characteristics of ten bread wheat hybrids produced using a chemical hybridizing agent

Summary Ten experimental hybrids derived from the crossing of representative Italian pure-line varieties were grown at three locations in 1984–85. The amount of F₁ seed produced from the crossings ranged from 1.5 up to 4 kg per plot of 20 m². Hybrid purity was on average 88% when estimated through the growout test and 82% when estimated through acid polyacrylamide gel electrophoresis (A-PAGE) of gliadin.All the hybrids showed high-parent heterosis for plant height. Grain yield was higher in the hybrids (71.1 q/ha) than in the female parents (66.5 q/ha); the range of mid-parent heterosis being-10% to +17%. Four hybrids showed standard heterosis (yield advantage). The highest value (+6%) was obtained with the combinations Chiarano x Leopardo and Claudia x Leopardo. Among yield components, the number of spikelets/spike proved to be an important trait and may explain the superiority of the hybrids over the traditional varieties.The bread-making quality appeared intermediate to that of the parents or closer to that of the poorest parental variety.     

Fertility improvement in tetraploid pearl millet

Summary Autotetraploid (2n=4x=28) pearl millet inbred lines, Tift 23BE and Tift 239DB, have been developed for use in crosses with other polyploid Pennisetum species. Each line set less than 1% and 3% selfed and open-pollinated seed, respectively. Seed germination was usually less than 17%. First generation (F₁) hybrids between the two inbreds set up to 61% seed while up to 40% of the seed from hybrids germinated.Seed weight per inflorescence for two planting dates averaged 0.32 g and 3.06 g for the two inbreds and the second generation F₂ progeny, respectively. One hundred seed weight was also significantly higher (0.78 g vs 0.48 g) in the F₂ progeny, probably due better endosperm development. Chromosome behavior and pollen stainability were similar in the inbred parents and hybrids. However, the hybrids shed more pollen than the inbred parents.Heterosis was evident in the F₁ hybrids and F₂ progeny which showed significant increases in plant height, leaf length, leaf width, and inflorescence length in addition to fertility improvement.     

Interspecific hybridization between rice bean (Vigna umbellata) and its wild relative (V. minima): Fertility-sterility relationships

Summary The pre-and post-fertilization barriers in the interspecific crosses between Vigna umbellata and V. minima were investigated. In the reciprocal crosses (V. minima as the parent) the entry of pollen tubes into the ovary was delayed by about 4 h, and no seed set was observed. However, no pre-fertilization barriers were encountered in crosses involving V. minima as the parent and V. umbellata as the parent (normal cross). The delay/absence of divisions in the endosperm and the failure of embryo to divide were the post-fertilization barriers responsible for somatoplastic sterility in normal crosses which yielded a few hybrid seeds. The hybrid seeds showed poor germinability. The F₁ hybrids were intermediate between the parents in most morphological characters, and are completely sterile for pollen. Backcrossing of F₁ hybrid with both the parents did not restore fertility in the progenies. V. minima is considered as the tertiary gene pool of the rice bean.     

Heterosis and inheritance of drought tolerance in faba bean, Vicia faba L.

Twelve parental lines of faba beans and 27 F₁ hybrids derived from them were field-tested under a dry and a well-watered treatment, at two German locations (Göttingen, Hohenheim) in two years (1995, 1996). Drought was artificially induced using rain shelters from anthesis until maturity. Data were collected on yield under dry (Yd) and well-watered (Yw) conditions. Drought tolerance was assessed as Yd/Yw. The mid-parent heterosis was significant for Yd and Yw, but not for Yd/Yw. Relative heterosis for Yd (52.0%) was greater than for Yw (39.3%). Parental heritability was greater for Yw (0.86) than for Yd (0.61). However, it was similar for both traits in F₁ hybrids (0.65 and 0.66). Principal component analysis showed that F₁ hybrids exhibited the same pattern of adaptation as their parents. The heritability of drought tolerance was 0.48 in F₁ hybrids and 0.70 in parents. Attention should be paid to its negative correlation with yield. Several hybrids combined high tolerance, a favourable pattern of adaptation and appropriate performance.     

Selecting superior spring wheat crosses in early generations

Summary Three methods were evaluated as potential aids in identifying superior crosses early in the breeding program: (1) F₁ yield, (2) F₂ yield, and (3) midparental yield. Two sets of spring wheat (Triticum aestivum L. em. Thell.) crosses and parents were evaluated in replicated single row plots in F₁ and in replicated multi-row plots in F₂ over a period of five years. On the basis of F₁ yields, two of the highest yielding and two of the lowest yielding crosses in each set were chosen for a more detailed selection study in subsequent generations.In Set 1, random F₂ head selections for each of the selected four crosses were evaluated in 3-row yield plots in F₄. In Set 2, F₂ plant selections were made on the basis of head weight and the highest yielding 10% evaluated for yield in F₄.Lines of crosses identified as high yielding in F₁ had significantly greater mean yields in F₄ than those of crosses that were low yielding in F₁. The high yielding crosses had three to four times as many lines yielding in the top 10% in F₄ than did the low yielding crosses.Correlations for yield were obtained between F₁ and F₂ in different years, F₂ and midparents in the same year, lower or no correlations between F₁ and midparents in the same year, and no correlations between F₁ and midparent and F₂ and midparent in different years. It is suggested that larger plot sizes rather than single rows be used to evaluate parents.Midparental yield, F₁ yield and F₂ yield tests are recommended as a progressive set of screening tests for a given set of crosses to effectively maintain the superior crosses in the breeding program.     

Introgressing resistance to bacterial and viral diseases from the middle American to Andean common bean

The genetic base of cultivars within market classes of common bean (Phaseolus vulgaris L.) is narrow. Moreover, small- and medium-seeded Middle American cultivars often possess higher yield and resistance to abiotic and biotic stresses than their large-seeded Andean counterparts. Thus, for broadening the genetic base and breeding for higher yielding multiple stress resistant Andean cultivars use of inter-gene pool populations is essential. Our objective was to determine the feasibility of introgressing resistance to Been common mosaic virus (BCMV, a potyvirus), and the common [caused by Xanthomonas campestris pv. phaseoli (Xcp) and X. campestris pv. phaseoli var. fuscans (Xcpf)] and halo [caused by Pseudomonas syringae pv. phaseolicola (Psp)] bacterial blights from the Middle American to Andean bean, using gamete selection. Also, we investigated the relative importance of the use of a landrace cultivar versus elite breeding line as the last parent making maximum genetic contribution in multiple-parent inter-gene pool crosses for breeding for resistance to diseases. Two multiple-parent crosses, namely ZARA I = Wilkinson 2 /// ‘ICA Tundama’ / ‘Edmund’ // VAX 3 / PVA 773 and ZARA II = ‘Moradillo’ /// ICA Tundama / Edmund // VAX 3 / PVA 773 were made. From the F₁ to F₅ single plant selection was practiced for resistance to the common and halo bacterial blights in both populations at Valladolid, Spain. The parents and F₅-derived F₆ breeding lines were evaluated separately for BCMV, and common and halo bacterial blights in the greenhouse at Filer and Kimberly, Idaho in 2001. They were also evaluated for the two bacterial blights, growth habit, seed color and 100-seed weight at Valladolid in 2002. All 20 F₁ plants of ZARA I were resistant or intermediate to common and halo bacterial blights in the greenhouse, but their F₂ and subsequent families segregated for both bacterial blights. Segregation for resistant, intermediate, and susceptible plants for common bacterial blight occurred in the F₁ of ZARA II. Simple correlation coefficient for common bacterial blight between the F₁ and F₁-derived F₂ families was positive (r = 0.54 P < 0.05) for ZARA II. From the F₂ to F₅ the number of families resistant to both bacterial blights decreased in both populations. Only four of 20 F₁ plants in ZARA I resulted in seven F₆ breeding lines, and only one of 32 F₁ plants in ZARA II resulted in one F₆ breeding line resistant to the three diseases. None of the selected breeding lines had seed size as large as the largest Andean parent. The use of elite breeding line or cultivar as the last parent making maximum genetic contribution to the multiple-parent inter-gene pool crosses, relatively large population size in the F₁, and simultaneous selection for plant type, seed traits as well as resistance to diseases would be crucial for introgression and pyramiding of favorable alleles and quantitative trait loci (QTL) of interest between the Andean and Middle American beans.     

Selection for seed yield in Middle American versus Andean × Middle American interracial common-bean populations

The objective of this study was to compare the seed yield, seed weight, and maturity of random lines derived from two contrasting populations of common bean ( Phaseolus vulgaris L.): Middle American (TR 7790) and Andean × Middle American (TR 7791). TR 7790 involved crosses between common-bean races of Middle American origin only (Mesoamerica and Durango), whereas TR 7791 involved crosses between races of Middle American and Andean origin (Mesoamerica and Nueva Granada). A total of 85 random lines from each of these two double-cross, interracial populations, a bulk sample of both populations, and all parents were evaluated in a replications-in-set design in eight environments in Colombia between 1991 and 1993. The mean yields of parents, population bulk, and lines from TR 7790 were significantly higher (P < 0.05) than those of TR 7791. Six lines in TR 7790 and nine lines in TR 7791 yielded significantly higher (P < 0.05) than the mean of their parents. Four lines from TR 7790 out-yielded the highest-yielding line from TR 7791. Moreover, while three lines from TR 7790 out-yielded its best parent, none of the lines from TR 7791 out-yielded its highest yielding parent. Thus, substantial yield gains should be expected from interracial populations within the Middle American group of common beans, whereas selection for higher seed yield from Andean × Middle American populations would seem problematic.     

Selection potential for seed yield from intra- and inter-racial populations in common bean

Common bean populations from crosses between lines of different races are thought to be more promising for selection of high yield potential than those from intra-racial crosses. Three distinct diallel crosses were made to test this hypothesis and to determine the possibility of substituting diallel crosses for multivariate techniques that estimate genetic divergence. The crosses were between races Mesoamerica × Mesoamerica, Mesoamerica × Durango and Jalisco, and Mesoamerica × Nueva Granada. The parents and the resulting F₄ populations were evaluated at Lavras-MG, Lambari-MG and Patos de Minas-MG, Brazil. The diallel analysis of seed yield was done and the genetic divergence estimated by Mahalanobis distance. Estimates of general and specific combining ability indicated that some inter-racial populations were more promising for selection to increase seed yield than intra-racial populations. However, due to their undesirable seed color and size, and growth habit, especially in a short term breeding program, the chances of obtaining high yielding lines with an acceptable bean is reduced. Genetic divergence was not a good measure to choose the parents because usually, the most divergent groups included were not adapted lines. This revised version was published online in July 2006 with corrections to the Cover Date.     

Potentialities of heterosis breeding in wheat

Summary Heterosis for yield and yield components were studied in F₁, F₂ and F₃ generations in ten crosses of wheat, Triticum aestivum L., under normal spacing. Eighty and 60% F₁ hybrids exceeded the high parent and standard variety to the extent of 44.3 and 36.4 per cent. respectively. The two best crosses excelled the high parent and the standard variety by a significant margin. The result showed that seed of advanced generations cannot be used for commercial wheat production, Heterosis in yield seems to be reflected through heterosis in spike number and 1000-kernel weight. In general it was found that the greater the genetic difference of the parents with a high yield capacity the better are their hybrids.