Inheritance of flower and pod colour in cowpea (Vigna unguiculata L. Walp.)

Inheritance of flower colour and pod colour in cowpea (Vigna unguiculata L. Walp.) has followed a qualitative pattern. Purple flower colour is dominant over white flower colour, whereas black pod colour is partially dominant over white pod colour. A segregation ratio of 3 purple:1 white flowers in F₂ generations of two crosses indicated that white flower colour is controlled by a single recessive. Segregation ratio of F₂ 1 white:2 light black:1 black indicated that black pod colour is partially dominant over white pod colour and is governed by one gene. These results were further confirmed by backcross generations. White flower and pod colour are controlled by single recessive genes on separate chromosome. Gene symbols were assigned. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic analysis of yield and its components in vegetable cowpea (Vigna unguiculata L. Walp)

The F2 and backcrosses of a cross between two vegetable cowpea (Vigna unguiculata L. Walp) varieties with contrasting characteristics were evaluated for pod yield and its components, with the aim of understanding the genetic basis of these characteristics. A four-parameter model incorporating the additive, dominance and additive × additive genetic components fitted the data for pod yield and clusters per plant. The additive and additive × additive effects were positive and were larger than the dominance component. The relatively large additive and the predominantly positive dominant effects suggest that selection would be effective. Pod weight had high broad (84%) and narrow sense heritability (75%) and can be effectively selected for in the early generations. The study suggested that vegetable cowpea improvement programs should focus on selecting for clusters per plant and average pod weight in the early generations, while selection for dry pod yield could be delayed to later generations. It was concluded that pods per plant may be a useful selection criterion in multi-location trials aimed at selecting for stability of yield. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inheritance of seed size in cowpea (Vigna unguiculata (L.) Walp.)

Summary The inheritance of seed weight in cowpea was examined in a field planting of the parents, reciprocal F₁s, F₂s and backcrosses to both parents of a cross between TVu 1977-OD (small seeded) and ACC 70002 (large).Seed weight was inherited quantitatively and small seed was partially domiminant to large seed size. Gene action was predominantly additive but dominance and additive × additive epistatic effects were also significant.Broad and narrow sense heritabilities were 85.1±5.3% and 75.4±18.6% respectively. The minimum number of loci involved in the inheritance of seed size was eight, and each gene pair contributed up to 1.02 g increase to seed weight. The estimate of genetic advance from F₂ to F₃ generations with 5% selection intensity was 3.58 g.International Institute of Tropical Agriculture, Semi-Arid Food Grain Research and Development Project/National Cowpea Improvement Program, B.P. 1783, Ouagadougou, Upper Volta.Department of Primary Industries, Hermitage Research Station, Warwick 4370, Queensland, Australia.International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), ICRISAT Center, Patancheru, P.O. 502 324, A.P., India.     

Resistance to cowpea mottle carmovirus in Vigna vexillata

Cowpea mottle carmovirus (CPMoV) causes grain yield losses of up to 75% in cowpea (Vigna unguiculata [L.] Walp.). There is no resistance to this virus among cultivated cowpea lines, but a high level of resistance exists in Vigna vexillata, a wild Vigna species. Fifty‐four accessions of V. vexillata germplasm collection at IITA were tested for resistance to CPMoV. Seedlings were mechanically inoculated with the virus and susceptibility or resistance was assessed by visual scoring of disease symptoms and serological analysis using antigen‐coated plate enzyme‐linked immunosorbent assay (ACP‐ELISA). All but three V. vexillata lines belonging to the variety angustifolia were resistant to CPMoV. Crosses were made between two resistant V. vexillata lines and the three susceptible lines. Segregation patterns observed in the F₂ and the backcross populations of all the crosses showed that resistance to CPMoV in V. vexillata is controlled by a single dominant gene, and the level of resistance conferred by this gene in V. vexillata is very high.     

Adaptation and competition in mixtures of cowpea (Vigna unguiculata (L.) Walp.)

Summary Six pure lines and four mixtures were grown in six environments throughout lowland Papua New Guinea. There were no significant transgressive increases in the grain yield of mixtures above their pure line components, and mixture yields were adequately predicted by the mean of components. In yield stability it was found that the individual buffering of pure lines was of more importance than population buffering, and that the magnitude of population buffering varied with the particular combination of components. Competitive effects in all the mixtures were of the compensating type. Dramatic changes in mixture composition resulting from natural selection precluded their use in local agriculture. The outcome of competition in mixtures was strongly influenced by the growing environment, such that selective index was correlated to general fertility.     

Evaluation of Pedigree and Single Seed Descent Selection Methods for Cultivar Development in Cowpea (Vigna unguiculata L. Walp)

Three crosses of cowpea (Vigna unguiculata [L.] Walp) were used to assess the effectiveness of two selection procedures in cowpea. The selection procedures were pedigree selection (PD) and single seed descent selection (SSD). Lines developed through each procedure were yield tested and selected at F₆ and F₇ for PD and SSD, respectively. The selected lines for both procedures were compared at F₈ for grain yield per plant (YLD), number of pods per plant (NPO), and pod development period (PDP), in two locations. Lines developed through both procedures differed more in their population means than in the performance of their top 10% or highest yielding lines. There was no consistent relationship between magnitude of genetic variability and percentage of superior lines obtained from each procedure. Both procedures have been effective in producing superior genotypes for yield and number of pods. The PD method produced superior transgressive segregants in two out of the three crosses for PDP. SSD allowed a more rapid generation than PD.     

Variation in seed proteins from mutagen-treated cultivars and selected lines of Vigna unguiculata L. Walp

The storage protein profiles of the seeds of two IITA cowpea cultivars (‘IT84E–124’ and ‘Vita 7’) exposed to three mutagens—sodium azide (NaN₃), ethyl methane sulphonate (EMS) and ⁶⁰Co gamma rays—and those of 18 selected M₃ lines were investigated by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis (PAGE). The total seed protein, globulin and albumin fractions showed differences in number and intensity of subunit bands, but the least differences were found in the albumin fraction. Both high and low molecular weight bands were observed, the highest being 94.5 kDa and the lowest 12.0 kDa.‘Vita T showed less variability compared with ‘IT84E–124’, as indicated by the relative similarity indices (S.I.) of the two cultivars. The lowest S.I. of 0.200 was found among ‘IT84E–124’ lines while the lowest S.I. among ‘Vita T lines was 0.4706. A number of lines with particular traits were found to be characterized by the presence of specific polypeptide bands. This study demonstrates that induced mutation could create additional variability to supplement existing germplasm and that SDS-PAGE is a useful tool for discriminating and estimating genetic similarities among selections.     

Segregation Pattern of Decumbent, Climbing and Bushy Growth Habits in Vigna unguiculata (L.) Walp.

F₁ and F₂ data from crosses of cowpea accessions with three different growth forms showed that the inheritance of growth habit is governed by two allelic pairs, AA and BB. The genetic ratio obtained implicated a modifier gene interaction in which the genotype A-B- has the decumbent growth habit; the genotype aaB- has the climbing and aabb has the bushy habit. While the decumbent growth habit was dominant and epistatic over the climbing habit, the decumbent and the climbing growth forms were each monogenically dominant over the bushy habit.     

Linkage relationships among loci controlling morphological traits in cowpea (Vigna unguiculata (L.) Walp.)

Summary Linkage among loci controlling various morphological traits in cowpea were determined using F₂ progenies. Data were collected on individual plants of four crosses segregating for several loci. Recombination estimates between the following pairs of loci were as follows: Sw (swollen vs normal stem base)-Fbc (cream vs green flower buds) (41±4.8%), Pu^s(purple vs green stems)-Cbr (cocoa-brown vs straw-yellow dry pods) (31±5.7%), Pu^p(purple vs green immature pods)-Cbr (30±5.7%), Pu^s-Pu^p (4±1.5%), Ndt (non-determinate vs determinate)-Pd (peduncle colour) (26±2.8%), Ndt-Hg (semi-erect vs erect plant type) (26±2.8%), P^t(purple vs green pod tips)-Bk (greyish-black vs straw-yellow dry pods) (19±2.4%) and Hg-Bpd (normal vs branching peduncle (24±9.5%). Four linkage groups (LG) were identified in these studies. The proposed LG I contained loci Sw and Fbc; LG II loci Pu^s, Pu^p, and Cbr; LG III loci Pd, Ndt, Gh, and Bpd; and LG IV loci P^tand Bk.     

RFLP mapping of an aphid resistance gene in cowpea (Vigna unguiculata L. Walp)

Summary Restriction fragment length polymorphism (RFLP) analysis has several advantages over traditional methods of genetic linkage mapping, one of these being the starting point for map-based cloning. The recent development of an RFLP map of cowpea (Vigna unguiculata L. Walp) has allowed the investigation of associations between genes of interest and RFLP markers. A cross between an aphid (Aphis craccivora Koch) resistant cultivated cowpea, TT84S-2246-4, and an aphid susceptible wild cowpea, NI 963, was screened for both aphid phenotype and RFLP marker segregation. One RFLP marker, bg4D9b, was found to be tightly linked to the aphid resistance gene (Rac ₁) and several flanking markers in the same linkage group (linkage group 1) were also identified. The close association of Rac ₁ and RFLP bg4D9b presents a real potential for cloning this insect resistance gene.     

Inheritance of resistance to Striga gesnerioides biotypes from Mali and Niger in cowpea (Vigna unguiculata (L.) Walp.)

Pot and laboratory experiments were performed in order to elucidate the genetics of resistance of three cowpea (Vigna unguiculata (L.) Walp.) resistant sources to two biotypes of the parasitic weed Striga gesnerioides (Willd.) Vatke (witchweed). B301, Suvita-2 and IT82D-849 were crossed to susceptible cultivars. Roots of each plant of parental, F₁, F₂, BC₁ and BC₂ populations were washed free of soil and examined closely for S. gesnerioides attachment. Data on resistant and susceptible cowpea plants were analyzed using the chi-square test to ascertain the goodness of fit to different genetic ratios. Segregation patterns indicate monogenic dominant inheritance of resistance to both S. gesnerioides biotypes in B301. Suvita-2, which is susceptible to the biotype from Maradi, Niger, showed segregation patterns that indicate monogenic dominant inheritance of resistance to S. gesnerioides biotype from Cinzana, Mali. Results also indicate monogenic dominant inheritance of resistance to S. gesnerioides biotype from Cinzana in IT82D-849. However, the inheritance of resistance to the biotype from Maradi would be monogenic and recessive in this line. These results suggest that the resistance of all three sources would be easy to transfer to promising breeding lines. This revised version was published online in July 2006 with corrections to the Cover Date.     

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

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

Effect of Water Stress on the Yield of Cowpea (Vigna unguiculata L. Walp.) Genotypes in the Delmarva Region of the United States

Drought is an important yield-reducing factor for corn and soya bean which are the two major crops in the Delaware, Maryland and Virginia (Delmarva) region of the United States. Cowpea (Vigna unguiculata L. Walp.) is primarily grown in drier regions of the world where it is one of the most drought-resistant food legumes. Field experiments were conducted in which 10 genetically diverse cowpea genotypes were evaluated for adaptability to the Delmarva area. The cowpea genotypes were grown in rain-out shelters under non-water-stressed and water-stressed conditions. The results showed that under non-water-stressed conditions cowpea genotypes California Blackeye 5, Champion and Mississippi Silver gave higher seed yields, while genotypes White Acre, Six Week Browneye and Texas Cream 8 provided lower seed yields. Genotypes California Blackeye 5 and Champion gave comparatively better seed yields under water-stressed conditions. California Blackeye 5 was the highest seed-yielding genotype under both water-stressed and non-water-stressed conditions. The highest biological yield under non-water-stressed conditions was given by genotypes Two Crop Brown, White Acre and Elite, whereas under the water-stressed condition genotypes Texas Cream 8, California Blackeye 5, and Mississippi Silver gave higher biological yield. Genotypes Quickpick Pinkeye and Elite were identified as early maturing genotypes. The harvest index (HI) varied significantly among genotypes, with Texas Cream 8 having the lowest HI. Cowpea genotypes which gave higher seed yield under water-stressed conditions could play an important role in sustaining crop production in the Delmarva region.     

Inheritance of Resistance to Yellow Mosaic Virus in some Vigna Species

Inheritance of resistance to Yellow Mosaic Virus (YMV) was studied in crosses of mungbean, black-gram and their interspecific crosses with Vigna sub-lobata. Resistance to YMV was recessive in the three Vigna species. The segregation ratios in F₂ and back crosses indicated that the resistance was digenic recessive in the crosses of mungbean and in interspecific crosses of mungbean with blackgram and Vigna subiobata but YMV resistance was monogenic recessive in blackgram crosses.     

Isolation, sequence analysis, and linkage mapping of resistance-gene analogs in cowpea (Vigna unguiculata L. Walp.)

Degenerate oligonucleotides designed to recognize conserved coding regions within the nucleotide binding site (NBS) and hydrophobic region of known resistance (R)genes from various plant species were used to target PCR to amplify resistance gene analogs (RGAs) from a cowpea (Vigna unguiculata L. Walp.) cultivar resistant to Striga gesnerioides. PCR products consisted of a group of fragments approximately 500 bp in length that migrated as a single band during agarose gel electrophoresis. The nucleotide sequence of fifty different cloned fragments was determined and their predicted amino acid sequences compared to each other and to the amino acid sequence encoded by known resistance genes, and RGAs from other plant species. Cluster analysis identified five different classes of RGAs in cowpea. Gel blot analysis revealed that each class recognized a different subset of loci in the cowpea genome. Several of the RGAs were associated with restriction fragment length polymorphisms, which allowed them to be placed on the cowpea genomic map. The potential for using these sequences to isolate R genes, and subsequent direct manipulation of disease and pest resistance using genetic engineering is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Identification and marker‐assisted introgression of QTL conferring resistance to bacterial leaf blight in cowpea (Vigna unguiculata (L.) Walp.)

Bacterial leaf blight (BLB), caused by Xanthomonas axonopodis pv. vignicola (Xav), is widespread in major cowpea [Vigna unguiculata (L.) Walp.] growing regions of the world. Considering the resource poor nature of cowpea farmers, development and introduction of cultivars resistant to the disease is the best option. Identification of DNA markers and marker‐assisted selection will increase precision of breeding for resistance to diseases like bacterial leaf blight. Hence, an attempt was made to detect QTL for resistance to BLB using 194 F_2 : 3 progeny derived from the cross ‘C‐152’ (susceptible parent) × ‘V‐16’ (resistant parent). These progeny were screened for resistance to bacterial blight by the leaf inoculation method. Platykurtic distribution of per cent disease index scores indicated quantitative inheritance of resistance to bacterial leaf blight. A genetic map with 96 markers (79 SSR and 17 CISP) constructed from the 194 F₂ individuals was used to perform QTL analysis. Out of three major QTL identified, one was on LG 8 (qtlblb‐1) and two on LG 11 (qtlblb‐2 and qtlblb‐3). The PCR product generated by the primer VuMt337 encoded for RIN2‐like mRNA that positively regulate RPM1‐ and RPS2‐dependent hypersensitive response. The QTL qtlblb‐1 explained 30.58% phenotypic variation followed by qtlblb‐2 and qtlblb‐3 with 10.77% and 10.63%, respectively. The major QTL region on LG 8 was introgressed from cultivar V‐16 into the bacterial leaf blight susceptible variety C‐152 through marker‐assisted backcrossing (MABC).     

Resistance of Cucurbita moschata to watermelon mosaic virus type 2 and its genetic relation to resistance to zucchini yellow mosaic virus

Summary Cucurbita moschata Menina originating from Portugal is known to carry a single dominant gene, Zym, for zucchini yellow mosaic virus (ZYMV) resistance. Resistance to watermelon mosaic virus type 2 (WMV2) was found in the same cultivar. In resistant plants, WMV2 migration from inoculated leaves or cotyledons seems to be limited and blocked. Resistance to WMV2 is conferred by one dominant gene and is effective against eight strains from different geographic origins. Results of linkage studies, based on co-inoculation of plants with WMV2 and ZYMV, indicate that resistance to these two viruses is conferred by the same gene, probably Zym, or perhaps by two closely linked genes.     

Inheritance of zucchini yellow mosaic virus resistance in Cucumis melo L.

Summary Resistance to zucchini yellow mosaic virus has been found in the muskmelon line PI 414723 from India. This resistance is effective against the ZYMV strains E15 and 1318 belonging respectively to the NF and F pathotypes. Resistance to E15 (no vein clearing and yellowing symptoms) is governed by one dominant gene (symbol Zym) according to segregations observed in F₁, F₂ and BC₁ progenies. This gene is epistatic dominant over Fn, which induces wilting and necrosis after inoculation with F pathotype. Linkage studies suggest that Zym inherits independently from Fom-1, Fom-2, Vat, Wmv and Fn but is linked with a (13.1 ±2.4 units).     

Genetics of resistance to bean golden mosaic virus in Phaseolus vulgaris L.

Summary The genetics of resistance to bean golden mosaic virus (BGMV) was studied in an 8×8 complete diallel cross of common bean, Phaseolus vulgaris L. The 28F₁ hybrids, their reciprocals, and eight parents were artificially inoculated with BGMV under glasshouse conditions. Data were recorded and analyzed for foliar yellowing, plant dwarfing, flower abortion, and pod formation, using a randomized complete block design with two replications.General combining ability (GCA) mean squares were highly significant (P<0.01) and larger than values for specific combining ability for all traits. Reciprocal and maternal effects were nonsignificant. Cultivars Royal Red and Alubia Cerrillos possessed significant negative and positive GCA for all traits, respectively. Porrillo Sintetico also had negative GCA for all traits except plant dwarfing. None of the GCA effects were significant for Great Northern 31 and PVA 1111. Positive associations existed among all traits studied.